The "Calories In, Calories Out" Confusion: A Comprehensive
Guide to Understanding Energy Balance
In filgma Statements (htt[!s://sigmanutrition.com/catego[Y/blog:[!ost/statements/). by Danny Lennon / November 4,
2020 I 26 Comments (htt[!s://sigmanutrition.com/cico/#comments).
Estimated Reading Time = - 40 minutes
What's The Confusion?
Unfortunately nutrition is a subject that gives rise to many emotive debates.
One passionately debated concept is that of "calories-in, calories-out" {CICO).
This is colloquial phrasing for how energy balance relates to bodily energy stores. And this gets translated as
shorthand for indicating how energy balance influences gain/loss of body mass.
Although one may suspect that CICO is something that seems simple, it is in fact a concept with a lot of
nuance buried within it. Unfortunately this nuance is often overlooked, leading to misunderstandings and
misleading characterizations being commonplace. At best, this results in people talking past each other
when "debating" the idea, whilst at worst, it serves as fertile ground for misinformation or "evidence" to
support fringe pseudoscientific ideas that centre on the rhetoric of calories being irrelevant.
When looking at some of the commentary related to CICO, there are two opposing positions that are both
incorrect. On one hand you have people claiming that "C/CO is wrong" and that looking at energy balance as
the main driver of changes in body mass is misguided. As we'll discuss later, this is often based on a
caricatured respresentation of the concept.
On another hand, there is a clear error in putting too much focus solely on energy balance. You don't have to
look far to some corners of the fitness industry where people just shout "calorie deficit" as the answer to
every problem, without acknowledging the pragmatic reality of what drives food intake and energy
expenditure.
Both of these positions illustrate a fundamental misunderstanding about what the energy balance equation
is, how that relates to human metabolism and body composition or the interacting variables within human
diet.
The hope is that this Sigma Statement will provide sufficient context and detail to allow for more accurate
understanding. And at least to prevent people talking past each other when the topic arises.
Understanding Calories-in Calories-out [CICO)
Energy intake (calories in) is simply the caloric value of the food we consume. Energy expenditure (calories
out) has four main components:
1. Resting metabolic rate (RMR)
2. Thermic effect of feeding (TEF), or diet-induced thermogenesis (DIT)
3. Physical activity (PA) thermogenesis
4. Non-exercise activity thermogenesis (NEAT)
Of these sub-components, those driven by activity (i.e. PA and NEAT) are the most variable. And unless there
are conscious changes in planned exercise, most of that variation is that of NEAT. Resting metabolic rate is
primarily determined by body size and lean body mass, whilst TEF stays relatively stable unless there are
drastic changes to calorie intake and/or macronutrient composition.
As already stated CICO is really just shorthand for how energy balance influences energy stores in the body.
The energy balance equation simply states that the difference between the energy coming into the body
and leaving the body is equal to the energy stored/lost in/from the body:
Energy in - energy out= energy stored/lost
Calories (kcal) are simply a unit of energy. So as an example:
If we take in 2,000 kcal in the form of food but expend 2,500 kcal, then there is 500 kcal of energy "lost" from
the system (i.e. the body). This shortfall in energy will then translate to a loss of some stored form of energy:
.(httRs://Rubmed.ncbi.nlm.nih.gov/17848938/)., for example energy stored as fat within a fat cell (adipocyte).
But energy can also be stored in other forms, for example as glycogen (stored carbohydrate) in muscle and
liver, or as protein that makes up muscle tissue. Breaking down any of these stored forms of energy can
contribute to making up that energy shortfall.
Conversely, if we take in 3,000 kcal from food but expend 2,500 kcal, then there is 500 kcal stored in the
system. Again, this can be stored in different forms, but the storage of fat (in the form of triglycerides) within
adipocytes tends to be the primary focus of these conversations.
It is for this reason that we draw the parallel between energy balance and body composition changes; i.e. it is
why a calorie deficit is discussed in relation to fat loss or a calorie surplus in relation to fat gain. A calorie
deficit or calorie surplus provides conditions suitable for loss or gain of body tissue respectively.
Crucially however, the energy balance equation is technically only telling us about differences in energy, not
about amounts of tissue change, and not about body weight change. Energy balance alone doesn't tell us
what the change in body weight will be, what the change in fat mass will be or what the change in muscle
tissue will be. To give some examples:
1. If someone eats at energy balance (calorie intake matches calorie expenditure) but dehydrates
themselves, they will lose body weight (mass as measured on a scale).
2. If someone eats at energy balance and starts taking creatine, they can see their weight increase due to an
increase in body water stores.
3. If someone eats in a 30% calorie deficit for a month, then the amount of fat mass and lean body mass
lost will depend on factors other than calories; it will depend on protein intake, physical activity
(stimulus on the muscle), etc.
4. If someone eats in a calorie deficit, but starts resistance training for the first time, they will likely gain
muscle mass and lose fat mass at the same time. So their energy balance isn't directly a measure of
changes in fat, muscle or body weight.
Change in Body Mass
Changes in Body Tissue
b
Fat
Fat balance
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Muscle
Muscle Protein
Balance
Other Changes in Mass
t
.
Body Water
.
.
Hydration
Sweating
Sodium
I
•
•
Glycogen Gut Residue
Carb intake
Exercise
.
Fibre
Undigested
food
However, let's not throw the baby out with the bathwater here. With all this said, this certainly does not
nullify the legitimacy of the energy balance equation, nor does it mean that considering calorie intake and
expenditure is unuseful as a proxy measure to predict body composition changes. Altering energy balance
through modifying calories in and/or calories out is still the primary tool at our disposal when it comes
to maximizing the rate of fat loss. Which in turn has knock-on effects on health.
As a heuristic, in most practical circumstances it holds true that a calorie deficit predicts weight/fat loss.
Insofar as, a sustained caloric deficit over time will lead to a decrease in fat mass. With other factors
dicate the amount of change in body tissues or the ease at which it is achieved? Absolutely. But this does
nothing to take away from the fact that a sustained caloric deficit is required for fat loss.
And similarly, with properly structured training and diet, a sustained caloric surplus will create the best
environment for muscle growth. However, energy balance does not perfectly describe changes in actual
body tissues.
Differences in Changing Fat Mass vs. Muscle Mass
The impact of energy balance on fat mass and muscle mass is not the same.
For loss of fat mass to occur, a calorie deficit is both necessary and sufficient:
• Necessary: Without a calorie deficit, fat mass cannot be lost.
• Sufficient: Regardless of other aspects of the diet, if there is a sustained caloric deficit, loss of fat mass
will result.
For muscle mass gain to occur, whilst a calorie surplus provides the best conditions for muscle growth,
a calorie suplus is neither necessary nor sufficient:
• Not necessary: With an appropriate training stimulus and protein intake, growth of new muscle tissue is
possible for most people (with this possibility decreasing as someone approaches their genetic potential
of muscle mass).
• Not sufficient: Simply having a calorie surplus in place does not necessarily mean muscle mass will be
gained. Take the example of someone on a low-protein diet who does no resistance training; in such a
case there would likely be no increase in muscle mass despite a period of a caloric surplus.
Additionally, the greater the calorie deficit, the faster the rate of fat loss will be. However, a greater calorie
surplus does not necessarliy drive a faster rate of muscle gain. There is a limit to how fast one can build
muscle. And as already noted, the main driver is exercise stimulus, not calories. An appropriate amount of
calories is permissive to maximal rate of muscle growth and increasing beyond that will not translate to
additional muscle gains. Hence why in most cases a slight surplus is beneficial, but aggressive overfeeding is
usually counterproductive.
The CICO Strawman
Eat less,
move more!
SIGMA
NUTIRITIO
-COffl
As already discussed, CICO is not an explanation of specifc changes in body tissue. Remember, all it does is
explain differences in energy, not mass of different tissues. However, knowing whether there is a calorie
deficit or surplus present over a specified period of time will let us know the directionality of likely changes
in mass; i.e. whether a loss or gain of stored energy, and thus mass, is going to result. So whilst the energy
balance equation alone can't account for specific changes in body composition, it still holds true. A calorie
deficit is still a requirement for fat mass.
And this is why it is frustrating to see the pseudoscientific gurus on the internet tell people that CICO is
"wrong" based on a false definition of what it means. They give a strawman (or sometimes just flat out false}
depiction of what the CICO concept means.
It's common to hear people dismiss the energy balance equation in favour of narratives that talk about how
" it's hormones, not calories" or similar rhetoric. Such claims usually emanate from the realms of
pseudoscience peddlers and diet cults. In dismissing the relevance of energy balance, a common move is to
cariacture CICO as a simplistic input-output idea that claims "calories are all that matter" and that "all
calories are created equal". The process such pseudoscientists follow is always the same:
• Step One: The guru attempts to win over their audience by painting "the opposition" as those who claim
a calorie surplus is what leads to fat gain or that calorie deficits are required for fat loss.
• Step Two: The guru then tells their audience that these people who accept CICO holds true are simply
going around telling people to eat less, demanding they count calories, or stating that food quality is
irrelevant.
• Step Three: Now that the audience seems how stupid CICO is, the guru can proceed to weaving a
narrative about how it's really hormones that matter, not calories. The audience then views this as a
much more elegant, scientific narrative and so it surely must be correct.
• Step Four: Members of that audience are now equipped with a lovely story to tell others and are able to
confidently call true nutrition experts idiots for being brainwashed by outdated notions about calories.
This would be a fine narrative. If only it were true.
So just to be clear, CICO is not synonymous with:
1. Advice to "eat less, move more" -That advice is largely unactionable and unhelpful, as is discussed in
the final section of this statement. CICO is not advice, it's just a descriptor of energy storage/release.
2. Tracking/counting calories -Again, CICO is just a descriptive concept, not a strategy or intervention. You
can track calories if you want or you can use an intervention that doesn't track calories. No matter what
the intervention, if fat mass is lost then the mechanism of action is a calorie deficit, regardless of
whether the individual is aware of their calorie intake or not.
3. Basing diet decisions solely on calorie values of food -Another weak, strawman argument utilised by
the 'hormones, not calories' crew is that of ridiculing the notion of diet decisions being based solely on
calories. And correctly, such a notion should be ridiculed, it's absurd. The problem is: no one is saying
the only factor one should consider is calories. Where are these people who say only calories matter? It's
fighting against a position no one is taking.
4. The "calorie is a calorie" meme - Similar to the last point, another pushback to CICO is that it dismissed
the impact of different foods and macronutrient profiles of the diet. And correct, different foods and
different macronutrient breakdowns don't have the same metabolic effects. But this isn't the claim of
CICO. It's another arugment against a caricatured version of the concept. This will be discussed more
below.
Other "arguments" you may hear against CICO include variations of:
• "The Calorie (kcal} is a unit of heat/energy. And we care about mass!"
•
•
•
•
•
"Your body isn't a bomb calorimeter!"
"Advising people to "eat less, move more" doesn't work"
"Eating less calories just means you expend less calories"
"Your metabolism slows when you eat less calories"
"CICO doesn't account for the role of hormones, which are the real determinants of body fat
storage/release"
However, none of these points illustrate that the energy balance equation doesn't hold true.
To get to the core of the misunderstandings, there are three primary topics I feel need to be understood:
1. Drivers of energy intake and energy expenditure
2. Fat oxidation vs. loss of fat mass
3. 'Calories in' and 'calories out' are not independent
Drivers of Energy Intake 8. Expenditure
There are both homeostatic and non-homeostatic drivers (httgs://P.ubmed.ncbi.nlm.nih.gov/25896063/). of
energy intake and expenditure.
Homeostatic Drivers
Various hormones orchestrate a suite of responses to our intake and expenditure in an attempt to exert
homeostatic control over our body composition. In response to both undereating and overeating, hormones
will drive us to want to consume more food or to stop eating, or to burn more energy or to rest and use up
less energy.
It is for this reason, that when things are working as they should (metabolically speaking), we have evolved
to maintain body weight and body fat levels within a range that is neither too low nor too high. There are a
number of different models of bodY. weight regulation (htt12s://dmm.biologists.org/content/4/6/733). that
won't be discussed here, but are worthy of deep discussion another time. However, this general premise that
there are homeostatic attempts by the body to avoid underweight/overweight is part of each model.
It is because of this homeostatic control of intake/expenditure in humans that we see examples of people
who, without ever monitoring their food intake or calculating how much to exercise, remain within a
relatively stable body weight range for years and decades.
"Energy intake and energy expenditure clearly do not correlate over a short period oftime such as a day
or t wo. Equally clearly, however, their correlation over an extended period of time, such as a week to a
fe w months to years, is excellent"
- Gro1212er, Smith & Groff
_{htt12s://books.google.ie/books/about/Advanced Nutrition and Human Metabolism.html?
id= 88a5DQAAQBAJ&source=kR book descri12tion&redir esc=y)_
But clearly this is not everyone and in modern society it is becoming increasingly difficult for humans to
regulate body mass without concious restraint. Why? Because intake and expenditure are not only driven by
internal homeostatic controls but also by external non-homeostatic controls.
Nan-homeostatic Drivers
However, as has become increasingly central to discussions around nutrition and health, our modern
environment and behaviour patterns have led to the ability to override our internal homeostatic hormonal
controls of intake and expenditure. Now it is easy to continue to consume calories long after there is any
hormonal drive to eat or experience hunger:
• Hyperpalatable, calorie-dense food allows large calorie consumption beyond the point of nourishment.
• Such foods are often low in protein and fibre, leading to less satiation and satiety.
• Many people consume calories in the form of sugar-sweetened beverages, which are known to have a
much lower satiety response than an equal amount of calorie and sugar from solid food.
• Convenience and the low cost of such foods makes them an easy option.
• Marketing messages nudge our behaviour, even subconsciously, towards being more likely to over
consume such products.
On the energy expenditure side, it is incredibly easy for modern humans in the developed world to spend
entire days with close to zero physical activity. One can wake up, drive to work, sit at a desk, order food via
apps on their phone, talk with friends over social media, and do many of their leisure activities online or
without leaving home.
Peer groups influence our energy intake and expenditure. Despite what our hormones might say about our
current hunger levels or what our current drive to move around is, if we are spending time in a social circle
that makes inactivity and consumption of calorie-dense meals/snacks at the centre of the time shared
together, then our behaviour is likely going to be different than if our social time was centred around
physical activity or with people who hold health conscious worldviews.
So one can acknowledge that hormones, environment, food availability, socioeconomics, peer groups,
habits and behaviour patterns all influence body composition. But this doesn't undermine the fundamental
claim about energy balance driving changes in body composition. Those factors all exert impacts on our
caloric intake and energy expenditure. These are not reasons "against" the CICO concept, they are factors
that modify intake (calories in) or expenditure (calories out).
So how does all this talk of energy balance actually relate to our fat mass? How does it influence fat storage
or fat usage? What puts fat into a fat cell and what causes it to be released and "burned"?
Fat Oxidation vs. Loss of Body Fat
When thinking about the movement of fat (stored energy) in and out of a fat cell, there are three main
processes to consider:
1. De novo lipogenesis (DNL}: conversion of glucose to fat. Accounts for a relatively small part of the
equation.
2. Re-esterification (RE}: re-assembling fatty acids on a glycerol backbone inside fat cell depends on how
much fat is consumed and the metabolic demands for fatty acids.
3. Lipolysis (L}: the breakdown of stored fat. Involves hydrolysis of triglycerides into glycerol and free fatty
acids.
•
Glucose
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Regulation of Fat Stores
De nova lipogenesis
Lipolysis
Fat Cell
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,
Fatty cids
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siGMA
NUTRITION
- Wtrr,s,,nu IIHtns -
The first two of these processes (DNL & RE) relate to storing energy in a fat cell. Whilst lipolysis is the process
of releasing stored fat from the fat cell. So we are in a state of fat balance when the amount of fat going into
the cell is equal to the amount of fat leaving the cell.
Consider an instance where you have just consumed a meal. You take in food, it's broken down into its basic
components (e.g. protein into amino acids, etc.). And therefore you now have a large amount of these energy
substrates (fatty acids, glucose, amino acids) that can be used for many processes. These substrates are
available to go one of three ways:
1. Immediate energy demands- The substrates are oxidized (or "burned") to produce energy that the
body needs right now.
2. Metabolic processes- For example, providing amino acids for building/repairing muscle tissue (process
of muscle protein synthesis).
3. Storage for later usage - For substrates that don't need to be used immediately, the body will store
them for later use.
So if one doesn't use up these substrates immediately (for either option 1 or 2 above), does that mean one is
gaining body fat? Well, certainly the excess energy may be stored in fat cells. But just because we store this
energy as fat right now, that is not synonymous with what we usually define as gaining body fat (i.e. a
practically meaningful amount of fat storage that persists, leading us to have more body fat than previously).
But rather, over the course of the rest of the day, as we go through hours of not eating, moving, and
expending energy, we will need to use our stores of energy to do this. At these times, we can tap into these
reserves of energy that we stored in fat cells after meals earlier in the day.
So the storage of excess energy after a meal in fat cells can be transient. This storing and releasing of fat
in/from fat cells is happening every day, regardless of if we are in a calorie deficit, calorie surplus, or at
energy balance. What dictates whether one actually gains fat mass that is sustained beyond those few hours
between meals relates to net energy balance over 24 hours.
Energy balance
over 24 hours
Negative energy
balance over
24 hours
Energy
Stora e
Energy
Release/Use
Energy
Re/eaie/Uie
Energy
Storage
Energy
Release/Use
Positive energy
balance over
24 hours
SfGMA
NUTRITION-
In the above graphic, three scenarios are shown for a hypothetical case where someone is eating three meals
across a 24 hour period:
1. At energy balance over 24 hours (blue) - Net energy storage is equal to net energy release/use
2. Negative energy balance over 24 hours (green) - Net energy storage is less than net energy release/use
3. Positive energy balance over 24 hours (red) - Net energy storage is greater than net energy release/use
In all three cases we can see that after consuming a meal, there will be net energy storage at that moment in
time. And this should make total sense; if I eat 500 kcal at a meal, there is rarely a case where the body would
need to burn all those calories immediately (in such a case, we'd have to eat constantly all day to survive). So
if I don't need to burn all those calories now, I will store some of them for later parts of the day when I won't
have energy coming in, but will still need to expend energy to move, have my organs function, etc.
In between each of these energy storage periods, we can see periods of energy release/use. These are
periods where we are not eating food (no calories coming in) but continue to expend calories (we expend
energy to maintain normal function to survive, plus more for physical activity). In these periods, this is where
we call upon the stored energy in the body. One example would be some energy from our earlier meal which
we stored as triglycerides in a fat cell (or as glycogen in muscle and liver cells).
But what dictates whether one is gaining body fat or losing body over time is not whether we ever store fat
(as shown here, we do this everyday regardless of what we eat), but rather what is the net balance between
this fat storage and fat release/use over an extended period (in this case 24 hours). And what dictates the
net amount of fat storage or release (i.e. the size of the curve above or below the line respectively)? The net
amount of calories consumed or expended in that time.
And in practice we should consider this extended period beyond 24 hours; i.e. what is the net difference
between fat storage and fat release over ]one week, two weeks, a month, etc.? That is what dictates
increases/decreases in body fat stores.
And so focusing on whether an acute elevation in insulin promotes fat storage processes (which it does) is
irrelevant, because that is looking at a snapshot in time, not the net effect on fat stores over longer periods.
Fat Oxidation
When fat is stored within a fat cell (adipocyte), it is stored as triglyceride (three fatty acids attached to a
glycerol molecule). So in order to decrease fat stores, these triglycerides must first be broken down into their
constituent parts (free fatty acids and glycerol), which are then released into the bloodstream (via lipolysis).
Once the free fatty acids are circulating in the bloodstream, in order to "burn" that fat for energy, the free
fatty acids need to undergo a process called beta-oxidation. This fat oxidation is what is often colloquially
called "fat burning". However, it's important to note that this "fat burning" is not the same as what most
people in the population would assume "fat burning" to be. It is not the same as losing practically
meaningful amounts of body fat stores. In other words, there are many ways in which fat oxidation can be
increased acutely, without having any impact on body fat stores.
For example, if you eat a high-fat meal, fat oxidation increases. This is how a "Bulletproof Coffee" (coffee
blended with tablespoons of butter and MCT oil) gets spun as a "fat-burning" meal to start your day. The
marketing uses the confusion around fat oxidation as a means of painting this as a good choice for weight
loss and health. If marketing were more honest, perhaps the concoction would be renamed "Atherosclerosis
Accelerant Coffee"?
And as we've seen reducing carbohydrate intake and replacing those calories with dietary fat leads to
increased fat oxidation. However, it also leads to a reduced carbohydrate oxidation, that leaves net energy
balance being the same.
Finally, we need a discussion of how calorie intake and calorie expenditure are not independent, i.e. if you
change one, you will cause a change in the other. The energy balance model is not at odds with this fact. And
in fact, this is how CICO aligns with issues like metabolic adaptations to dieting, influence of hormones on
food intake or activity, and many other of the concepts some people erroneously believe are examples of "it
not being about calories".
'Calories In' and 'Calories Out' Are Not
Independent
1.
Calories In Impacts Calories Out
There are several ways in which our food intake influences energy expenditure. There are obvious and direct
mechanisms, for example diet-induced thermogenesis (DIT), or as it's also called: the thermic effect of
feeding (TEF), where we expend energy to digest and metabolise food. And different nutrients have different
values. So a high-P-rotein meal will lead to greater DIT/TEF (htt[2s://[2ubmed.ncbi.nlm.nih.gov/12499328/).
than a calorie-matched low-protein meal. So clearly meals and diets of differing macronutrient composition
have different metabolic effects, with just one of those effects being the impact on energy expenditure via
DIT/TEF.
But this is accounted for within the energy balance model. It's why the often heard rebuttals by calorie­
denilists go along the lines of "oh, so you're saying 2,000 kcal of broccoli is the same as 2,000 kcal from beef?"
(or "a calorie of brownies" and "a calorie of kale" as some guack doctors might say_(httP-s://Y.outu.be/Ulju Y.Za9k).). Again it's attacking a position nobody holds as a means of elevated some other fringe idea about
how calories are irrelevant.
There are a suite of metabolic adaptations to both overfeeding and underfeeding, that the body coordinates
in an attempt to maintain stasis. In overfeeding situations, the body increases energy expenditure in an
attempt to account for this increased energy intake. The primary adaptation is an increase in non-exercise
energy thermogenesis {NEAT). NEAT can be thought of as those movements we make throughout the day but
that are not actually planned physical activity per se. So with increased NEAT levels, that means increased
subconscious movements (e.g. fidgeting) and an increased drive to move about.
To underscore the dynamic interplay between 'calories in' and 'calories out', it is worth exploring the
metabolic adaptations to dieting in a bit more detail. In the below graphic, scenario A represents the
situation at energy balance; i.e. average energy intake is ~2,500 kcal and expenditure is matching this
amount {2,500 kcal).
Now let's take a situation where this person purposely restricts their calorie intake by 700 kcal, leading their
intake to now be 1,800 kcal. Scenario B represents what some people assume is now the case; i.e. there is a
700 kcal deficit created. However, scenario B is what would be the case if there was no adaptation in energy
expenditure. But we know that calorie expenditure will decrease in response to decreased calorie intake
{with stark differences between individuals). This decrease will also increase the longer the deficit is
maintained. So just for illustrative purposes, imagine a situation where the combined effect of all the
metabolic adaptations to dieting leads to an individual's energy expenditure to decrease by ~ 300 kcal. This
is scenario C. Because of this decrease, the net calorie deficit is now 400 kcal. So in this hypothetical
example, a decrease of 700 kcal in calorie intake does not equate to a 700 kcal deficit, but rather a 400 kcal
deficit.
Again, this doesn't violate the energy balance equation. It fits perfectly within it. So anecdotes of reducing
calorie intake but weight loss slowing down aren't evidence against CICO at all. In fact, it's exactly what one
would predict based on the physiology of dieting and weight loss.
An overfeeding study....QY. Levine {httP-s://P-ubmed.ncbi.nlm.nih.gov/9880251/l illustrates nicely how 'calories
in' influences 'calories out'. Healthy participants were overfed by 1,000 kcal/d for 8 weeks. However the
resulting gain in fat mass wasn't what would be "predicted" by a 1,000 kcal surplus. Instead some of that
surplus was offset by an increase in energy expenditure, with two-thirds of this coming from increased NEAT.
Whilst there was wide variation in how much their NEAT increased {and hence how much fat each person
gained), in one of the participants NEAT alone was seen to be able to increase by almost 700 kcal per day
{average was 328 kcal/d). So for this person the total increase in energy expenditure almost completely offset
the increase in calorie intake, which was reflected in the very small amount of fat this person gained {0.36
kg), despite the 1,000 extra calories per day over the eight week period. {Strangely, at the opposite end,
another individual actually saw their NEAT decrease by ~ 100 kcal/d, and had a fat mass gain of over 4 kg
across the study).
This inter-individual variation seems to be at least partly explained by genetics, as similar overfeeding
studies done on identical twins would suggest. In one such study by Bouchard et al.
.{httP-s://P-ubmed.ncbi.nlm.nih.gov/2336074{). the researchers had 12 pairs of twins complete a 100-day
overfeeding phase, where they ate 1,000 kcal above baseline for six days of the week, every week for the
duration of the trial. And whilst there was a large inter-individual variation among all the participants in a
similar fashion to that in the Levine study, there was three times more variance among twin pairs than within
twin pairs; i.e. a pair of twins showed a more similar response, than compared to others. This suggests that
genetics is driving much of the adaptive response to overfeeding. And the same happens with underfeeding.
And so we could take two individuals with the same "maintenance calories", and increase their intake by the
same amount but the net calorie surplus may be different.
By way of example, let's consider two individuals with very similar baseline characteristics who maintain
their current weight on an average intake of 2,500 kcal per day {image below). Now we increase their intake
to 3,200 kcal per day {i.e. a 700 kcal increase). But as has been discussed, metabolic adaptations will lead to
an increase in energy expenditure. In person A, their expenditure increases by 100 kcal/d {from 2,500 to 2,600
kcal/d). Whilst person B experiences a 300 kcal increase in energy expenditure {up to 2,800 kcal/d). So
despite increasing both individuals intake by 700 kcal, we have a situation where person A has a net calorie
surplus of 600 kcal, whilst person B has a net calorie surplus of 400 kcal. The important point here is that an
increase in calorie intake is not equal to the actual net calorie surplus.
And likewise, a decrease in calorie intake is not equal to the net calorie deficit. Hence why simplistic
calculations are erroneous. For example, consider someone trying to calculate expected fat loss over an 8week dieting period. They decrease their calorie intake by 300 kcal/d, and accurately do this for the full 8
weeks. The amount of weight/fat loss can not be estimated on the basis of presuming this is a 300 kcal deficit
running consistently for 8 weeks. There will be metabolic adaptations to decrease energy expenditure, and
such a decrease will only increase over the duration of the diet, thus meaning there is no 300 kcal deficit
{unless energy intake or expenditure is purposefully modified to re-attain this level of deficit).
Exercise exerts a role on body composition in both direct and indirect ways. There is of course a contribution
to total energy expenditure through the energy burned during exercise bouts. There is also the influence of
resistance exercise stimulating muscle growth or retention of muscle mass. And clearly what we eat and how
much we eat can have impacts on exercise performance. Taking the resistance training example, if we go into
a hard training session having already consumed 1,500 kcal earlier in the day versus if we hadn't eaten at all,
is it plausible that how much volume one could do in that session is different? Or how intense one could
work for a set amount of volume? In either case, it's at least hypothetically possible that we are changing
energy expended during the training session. Calories in influencing calories out.
And to further explore some of the ways in which exercise can exert an indirect role on body composition,
one can consider how exercise impacts our food intake. Which is an idea that sets the stage for our next
section; how 'calories out' impacts 'calories in'.
2. 'Calories Out' Impacts 'Calories In '
Work done by the team at the University of Leeds {which includes Mark Hoi;Mns
.{httP-s://sigmanutrition.com/eP-isode299/) and John Blundell) has shown how P-hY.sical activitY. and aP-P-etite
control are not indeP-endent of one another (httP-s://www.ncbi.nlm.nih.grudP-mc/articles/PMC5097075/), but
rather are interconnected. At very low levels of physical activity there seems to be an inabilitY. to regulate
gP-P-etite and energY. intake {httP-s://pubmed.ncbi.nlm.nih.gov/27503946/) appropriately. Whereas at higher
levels of physical activity there seems to be an ability for us to appropriately match our calorie intake to our
energy demands. They have referred to these as "unregulated" and "regulated" zones, respectively.
So changes in energy expenditure therefore can have knock-on effects on calorie intake. Again highlighting
that these are not two independent variables, but rather that they are inextricably tied.
Common Claims "Disproving " CICO
There are a number of justifications that people give as to why 'calories-in, calories-out' is nonsense. Much of
these are built around personal anecdotes, followed by an interpretation of what said anecdote must mean
for CICO. For example:
1. " I am eating more now and my body composition is better. CICO is nonsense"
2. " I reduced my intake but I stopped losing weight. CICO is nonsense"
3. "Tracking calories never worked for me, but when I went low-carb weight just dropped off. CICO is
nonsense"
4. "I developed hypothyroidism and gained weight on the same diet, see it's hormones. CICO is
nonsense"
5. "Tracking calories isn't psychologically healthy and isn't a sustainable way to live. CICO is
nonsense"
6. "There is more to diet and body composition than calories. CICO is nonsense"
Of course, in each of these cases, the initial observations can be correct but the interpretation that "CICO is
nonsense" is incorrect. Each of those observations can align perfectly with CICO holding true:
"I am eating more now and my body composition is better."
• There are absolutely cases where someone can be eating "more" and see their body composition
improve. For example:
o Ex. 1 : When the person says "more" they are referring to more food {i.e. in terms of mass or volume)
but because of changes in food choices they are actually eating less calories. As a result, they drop
into a calorie deficit and lose body fat.
o Ex. 2 : Someone has increased their calorie intake but has also recently started resistance training,
and so lean body mass increases, thus leading to better body composition.
• So there are cases where either subjectively or objectively someone eats more food and experiences
either decreases in fat mass or increases in lean mass. All of this can be explained without any violation
of CICO.
"I reduced my intake but I stopped losing weight."
• One can reduce their calorie intake but eventually stop losing weight. As discussed already, there are
metabolic adaptations to dieting that occur to cause a decrease in energy expenditure. This, in addition
to having a lower body mass, means over time the deficit shrinks more and more, even if you stay eating
the lower amount of calories.
• Therefore, even when stalls occur in weight loss {and presuming the person has adhered to a lower
calorie intake), this doesn't violate CICO. In fact, this is to be expected given our previous discussion of
'calories in' influencing 'calories out'.
"Tracking calories never worked for me, but when I went low-carb weightjust dropped off."
• Many people don't find tracking calories a useful strategy. But not succeeding in losing weight with such
a strategy isn't an indication that CICO is wrong, but rather that tracking calories is a strategy that the
individual finds difficult to adhere to, is tracking incorrectly, or hasn't accounted for reductions in energy
expenditure over time. CICO and tracking calories are not synonymous.
• For some people, going on a low-carbohydrate diet leads to spontaneous reduction in calorie intake due
to a combination of factors {reduced food options, reduced processed food intake, increased protein,
greater satiety, etc.) that allows that person to reduce body weight. In the early week or so of embarking
on such a diet there may also be losses of water and glycogen which would lead to a lower body weight
measurement.
"I developed hypothyroidism and gained weight on the same diet."
• Hypothyroidism can lead to weight gain, but this is due to a decrease in metabolic rate and therefore
decreased energy expenditure. So yes, a hormonal change is resulting in weight gain, but the
mechanism is still via the creation of a calorie surplus, thus fitting within the energy balance model.
Appropriate drug intervention can offset this decreased energy expenditure.
"Tracking calories isn't psychologically healthy and isn't a sustainable way to live."
• Correct, tracking calories isn't psychologically healthy for many people. And a strong case can be made
that in the long-term people should aim to move away from it, at least for the majority of the time. But
again, tracking calories and CICO are not synonymous !
• CICO simply is a recognition that energy imbalance can cause alterations in body tissue stores. It says
nothing about having to use any specific intervention or strategy, whether that be tracking/counting
calories or otherwise.
"There is more to diet and body composition than calories."
• Of course there is more to diet and body composition than calories. I know of no reasonable person who
says otherwise. This is perhaps the weakest argument that someone can give in arguing "against" CICO.
Problems Calculating Calories
Another point of discussion relates to problems in accurately assessing calorie intake and energy
expenditure. And unless we are in a metabolic ward, it is absolutely true that we can't assess these with
precision. For calorie intake we can get relatively close (or at least a decent approximation that works for
practical purposes). If someone weighs all the food they consume and logs that into an app that tracks
calories, then a calculation of total energy intake for the day can be obtained. But as someone may
(correctly) outline, this is based on estimates and averages of the calories in the foods measured. There may
also be inconsistencies with the precise amounts of ingredients used. And for processed foods, food labels
come with a margin of error. So yes, even meticulously tracking calorie intake doesn't likely give the exact
correct number.
But the more important question is: how much does that matter? Even for calorie estimates that are a bit off,
comparisons can still be made between the calculated intake and the average habitual intake for that
person. Comparisons can still tell us if someone is likely in a calorie deficit or not. It doesn't really matter if
one's calculated 350 kcal deficit is in reality a 327 kcal or 386 kcal deficit.
For energy expenditure, any estimation we make in real-world settings is just a rough approximation. But
again, how much does this matter? What any estimation of energy expenditure is doing is giving a starting
point to help determine what an appropriate calorie intake for this individual may be.
In cases where an estimated calorie intake is calculated with the goal of fat loss, but no change in body
composition occurs, it does not invalidate CICO. Rather it simply signifies that the calculation was inaccurate
and that this individual is not in a calorie deficit, for one or more of the reasons already discussed. So a
further reduction in intake (and/or increase in expenditure) may be required.
Influence of Macronutrients g Food Quality
Even between isocaloric diets, impact on body composition will differ based on the macronutrient profile of
the diets. This fact is often misconstrued as undermining CICO. But such a conclusion is based on the false
premise that CICO is synonymous with diets matched for calories will have the exact same impact. This leads
to the whole "a calorie is not a calorie" rhetoric, which attempts to justify the conclusion that CICO is
nonsense by comparing 2,000 calories of lean beef to 2,000 calories of sugar and claiming CICO-proponents
are suggesting these are equal in their impact on body composition. This is of course ridiculous and a
fallacious argument.
Of most importance in this respect is the protein content of the diet. A metabolic ward study:
.{htt12s://www.ncbi.nlm.nih.gilld12mc/articles/PMC3777747/) out of George Bray's lab showed that when
people were put in a 40% calorie surplus {an extra ~ 950 kcal/d) for eight weeks, whilst fat-free mass
increases on moderate and high-protein diets {15% and 25% of calories from protein respectively), fat-free
mass actually slightly decreased on a low-protein {5% of kcal) diet.
So differing amounts of protein in the diet have differing impacts on muscle mass. But who is disputing this?
The "calorie is not a calorie", beef vs. sugar comparisons are fighting against a position that doesn't exist. A
tactic that does well to embolden followers of certain ideologies, but nothing to help advance honest
discourse.
Similarly, it is well established that the different macronutrients have different thermogenic potential (i.e.
different levels of thermic effect of feeding, TEF, which is a measure of how much energy is expended in
digesting food). Protein has by far the highest TEF value of the macronutrients. This is yet another example of
how calories in affects calories out. As you change the amount of protein in the diet, there will be a change in
TEF, and therefore potential in total energy expenditure.
The different macronutrients can also influence satiety differently. Protein exerts a strong influence on
satiety, and so low protein intake can mean an increased likelihood of eating more calories, compared to a
similar diet with more protein consumed. It is also well understood that dietary fibre increases satiation and
therefore isocaloric diets with a high vs low fibre intake can have differing effects on satiation, and therefore
calorie intake.
It is known that what foods are selected in the diet can have implications on appetite, satiety and drive to
consume more food later in the day. Hall and colleagues showed that when food is available as ultra­
processed food (e.g. breakfast cereal, muffins, white bread, sausage, etc.) it leads to greater ab libitum
calorie intake (httRs://www.cell.com/cell-metabolism/fulltext/S1550-4 131(19)30248-7?
returnU RL=httRs%3A%2 F%2 Flinkinghub.elsevier.com%2 Fretrieve%2 FRii%2 FS15504131 19302487%3 Fshowall%3 Dtrue).
compared to unprocessed foods.
All of this is to say that the macronutrient profile of the diet does absolutely matter. And beyond that, food
quality also matters. But this doesn't undermine CICO, they both co-exist without any conflict.
Influence of Other Factors
There are a number of other factors that influence body composition, that deserve longer commentaries at
another time, but are worth highlighting here. Three of particular significance are resistance training, sleep
and hormones.
Resistance training
When engaging in resistance training of sufficient stimulus over a sufficient period of time, there will be
creation of new muscle and therefore an increase in body mass, even without a caloric surplus. In fact, even
in a calorie deficit it is possible to gain lean mass, given the right conditions. The stimulus placed on the
muscle is by far the most important variable. And from a nutrition perspective, if an appropriately high
supply of protein is provided, ideally distributed across the day in 3+ high-protein meals, then there should
be accrual of new muscle tissue. Of course, other factors dictate how likely this is and the extent of the
muscle gain (genetics, training experience, proximity to genetic potential, returning from a phase of de­
training, etc.). However, to maximize the amount of muscle gained in a certain period of time, a slight calorie
surplus provides the best environment, all else being equal.
But to circle back to what was discussed towards the beginning of this statement, CICO is telling us about the
amount of energy to be stored in, or released from, the body. It is not an explanation of amounts of muscle
tissue that will result. And in the case of resistance training and gaining muscle, calories are not driving the
gain in muscle, training is. A surplus of calories is just the most conducive environment for building new
muscle tissue, which is an energy intensive process.
Sleep
Some experimental evidence exists suggesting that body composition changes differ between calorie­
matched and macronutrient-matched diets, based on differences in sleeP- duration
.{httP-s://www.ncbi.nlm.nih.gm£LP-mc/articles/PMC295 l287/). In one particular crossover study, participants
spent two 14-day periods (with at least three months washout period between them), having to spend either
8.5 hours or 5.5 hours per night in bed. They consumed the same diet, with calories set to 90% of their
resting metabolic rate (so they were in negative energy balance). The study was carried out in the lab, with
all food being weighed before and after each meal to determine actual consumption.
As shown in the graphic below, there were significant differences in body composition between the
conditions. Although weight change was the same, in the sleep-restricted condition less fat was lost and
more lean mass was lost. In the 5.5 hours/night condition, fat oxidation seemed to be impaired.
In more real world situations, sleep restriction can influence body composition through altering food
choices. As was discussed in a previous Sigma Statement, sleeR curtailment has significant effects on hunger
and aRRetite regulation {httRs://sigmanutrition.com/sleeR-nutrition/). therefore promoting increased calorie
intake. Sleep curtailment also results in reduced dietary restraint and increased disinhibited eating. And in
cases of chronic sleep curtailment, there may be a reduced drive for physical activity and greater feelings of
tiredness, resulting in lower energy expenditure. So sleep can exert an influence on both 'calories in' and
'calories out', and thus changes in mass.
Hormones
Are hormones important when it comes to body composition? Of course. It is well-known and universally
accepted that a long list of hormones play vital roles; from ghrelin driving appetite, to leptin impacting both
energy intake and expenditure, to cortisol playing a role in fat release from cells. However, just because
hormones have important functions, this doesn't somehow make calories irrelevant. In fact the two are
linked.
When someone fasts for 10 hours, and their ghrelin levels rise throughout, their appetite increases. However,
we wouldn't say that this has nothing to do with them not ingesting any calories and it's simply a hormone
issue. But rather it is the lack of nutrient ingestion that is driving the hormonal change.
Stating that CICO holds true does not oppose any of the well-known mechanisms by which hormones
influence body composition. But hormones are exerting this influence via driving energy intake, energy
expenditure, food choices, stimulating muscle growth/repair, etc.
Much of the other "hormones, not calories" rhetoric comes from people citing the carbohydrate-insulin
model of obesity (CIM), which has been promoted by a number of figures, particularly those within the low­
carbohydrate community. In recent times, Ludwig and Ebbeling
.(htt12s://12ubmed.ncbi.nlm.nih.gov/29971406/). have been two of the most prominent advocates for the
model within academia. Whilst there have been various iterations and interpretations of the model, Ludwig
& Ebbeling state that according to the CIM, obesity is a result of the fact that increased "consumption of
processed, high-glycemic-load carbohydrates produce hormonal changes that promote calorie deposition in
adipose tissue, exacerbate hunger, and lower energy expenditure." Essentially, the problem is claimed to be
elevated insulin, driven by increased intake of carbohydrates. The elevated insulin "traps" substrates in the
fat cells and decreases circulating levels of substrates (e.g. glucose, free fatty acids, etc.), which in turn leads
to weight gain, increased calorie intake, and decreased energy expenditure. However, this seems to be at
odds with much of the extant evidence. For example, it is well documented that individuals with obesity
have at least normal, and often increased, levels of free fattY. acids
.(htt12s://12ubmed.ncbi.nlm.nih.gov/5806343/) and glucose in circulation. Additionally, their fat cells release
more, not less, free fattY. acids,_(htt12s://pubmed.ncbi.nlm.nih.gov/26495 12/). compared to those without
obesity, and this is often in the presence of hyperinsulinemia.
And in metabolic ward conditions (htt12s:// pubmed.ncbi.nlm.nih.gov/26278052/), despite a carbohydrate­
restricted diet resulting in reduced insulin and increased fat oxidation, this does not result in greater total
energy expenditure or greater body fat loss compared to a fat-restricted diet that is matched for calories and
protein intake. The increased fat oxidation in the carb-restricted diet is in parallel to an increased intake of
fat. So those who restrict carbohydrates will see an increase in fat oxidation. But they also have a greater
proportion of the energy in their diet coming from fat. And so in conditions where calories and protein are
matched, the net impact on body fat stores is the same, because energy balance is the same. Those oxidising
more fat, are also oxidising less carbohydrate. And as shown in the Hall studY.
.(htt12s://12ubmed.ncbi.nlm.nih.gov/26278052/)., the increased fat oxidation in carbohydrate-restricted group
was offset by their decreased carbohydrate oxidation.
Overall, yes hormones are important. But this notion that hormones and calories are independent concepts
with only one answer being "right" is wholly misleading.
"Eat Less, Move More" is Unhelpful Advice
It is unfortunately common to see the conflation of CICO with the advice to "eat less, move more". Whilst
understanding energy balance leads us to conclude that a sufficient reduction in calorie intake and/or
sufficient increase in energy expenditure would lead to a calorie deficit, and thus fat loss, simply advising
people to eat less is at least unhelpful, and in some instances harmful.
As has been alluded to earlier, a modest reduction in intake will lead to some decrease in energy
expenditure. Additionally, feedback mechanisms lead to an increased drive to consume food, with the drive
increasing the greater the duration and extent of the caloric restriction.
After creating a calorie deficit, that net deficit will decrease over time, thus requiring future modifications to
calorie intake and expenditure in order to keep that deficit at the same level. This closing of the deficit
happens both physiologically and/or behaviourally.
Physiologically, as the diet persists, metabolic adaptations to the calorie deficit and the weight loss will lead
to decrease in energy expenditure. So if energy expenditure decreases by 150 kcal over a number of months
dieting, but average calorie intake stays the same, then what started out as a 400 kcal deficit is now a 250
kcal deficit, thus resulting in a slowing in the rate of fat loss, despite eating the same amount of calories.
But beyond the physiology, given that most people who diet will not be precisely controlling calorie intake,
even when trying to keep the same habits, it seems that there is a gradual increase in calorie intake over
time. In one paper by Hall et al., they referred to it as an "exponential decay in adherence".
For fat loss, the answer is a calorie deficit. But is that actionable or useful information for the individual
asking the question or embarking on a specific diet?
In order to actually make change it is much more beneficial to counsel on the factors that in turn influence
calorie intake and physical activity:
•
•
•
•
•
•
Generally increasing diet quality
Learning how to prepare healthy meals
Modifying the food environment
Building a habit of regular exercise
Getting an adequate amount of sleep
Creating a consistent structure with diet
There also needs to be an acknowledgement of the many barriers that exist to someone consistently making
good food and lifestyle choices or achieving a certain body composition. Some individuals have a greater
genetic P-redisP-osition towards obesity_(httP-s://www.ncbi.nlm.nih.gQYLP-mc/articles/ PMCS663l24/).. There is
a large body of evidence on social determinants (httP-s:// pubmed.ncbi.nlm.nih.gov/22784581/) (the
conditions in which people are born, grow, live, work and age, and the fundamental drivers of these
conditions: the distribution of power; money; and resources) on health and obesity. Social inequalities also
mean there is a difference in access to healthy food markets, health centres, safe areas to exercise, and social
supports. Within these social determinants of health, socioeconomic conditions specifically (e.g., poverty
and the associated stresses) act as a significant barrier to implement what is often thought of as "simple"
advice.
The advice to "eat less, move more" isn't just ineffective, it can often cause harm. On receipt of such advice,
many individuals may restrict food intake in an unsustainable or unplanned manner. The likely result is that
any initial weight loss will eventually be regained, after the individual is driven to increase calorie intake in
response to the hormonal response to undereating, as was discussed earlier. Without appropriate planning,
guidance and/or education, simply trying to restrict food intake will be unlikely to work. But even more
problematically, this oscillation between weight loss attempts and weight regain can cause feelings of
shame, guilt and negative self-appraisal. People are made to feel like they are at fault or that they simply lack
willpower. This is one of core issues at the centre of weight-neutral approaches (with Health At Every Size
being perhaps the most well-known movement). Advocates for weight-neutral approaches quite correctly
outline how damaging it can be psychologically to feel that one is "failing" at weight loss. And such negative
psychological effects are potentially worse than physical effects of a certain degree of adiposity. A full
discussion of the potential harm of weight loss interventions is beyond the scope of this particular statement
but is acknowledged as an incredibly important issue.
The conflation between "eat less, move more" as advice with simply acknowledging the role of energy
balance in body composition is both erroneous and problematic. Energy balance is the primary driver of
body fat stores, and ClCO is not "wrong". However, advice to "eat less, move more", whilst technically the
basis for creating a calorie deficit, is incomplete advice at best, and can potentially lead to problems.
Summary of Key Points
1. The energy ba lance eq u ation si m p ly states that the d ifference between the energy com i n g into
the body and leaving the body is eq u a l to the energy stored in, or lost from, the body.
2. The energy ba lance eq u ation descri bes differences i n energy, not a bout specific amou nts of
tissue change.
3. A ca loric deficit is both necessa ry and sufficient for a decrease in fat mass.
4. A ca lorie su rplus provides the best envi ron ment for opti mal muscle growth, however a calorie
su plus is neither necessa ry nor sufficient for m u scle growth to occu r.
5. CICO is not synonymous with:
0 Advice to "eat less, move more"
o Tracki ng/cou nti ng ca lories
o Basing diet decisions solely on ca lorie va lues of food
o The "ca lorie is a ca lorie" meme
6. There a re both homeostatic and non-homeostatic d rivers of energy inta ke and expenditu re.
7. What d ictates whether one is ga i n i ng or losing fat mass is the net ba lance between fat storage
a n d fat release/use over a n extended period.
8. 'Calories in' and 'ca lories out' a re not i ndependent: Calorie i nta ke i nfluences energy
expend itu re, and vice versa.
9. Even between isoca loric diets, i m pact on body com position wi ll d iffer based on the
m acron utrient profi le of the diets.
10. Other factors such as resista nce tra i n i ng, sleep and hormones exert i nfluences on body
com position. However this still doesn't u nderm i ne the fu ndamenta l i nfluence of energy
bala nce on body mass.
11. " Eat less, move more" is u n helpfu l advice and li kely i neffective. In o rder to achieve changes i n
body mass, advice a round d rivers o f i nta ke and expend itu re need to b e d iscussed .
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Statement Author: Danny Lennon