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CARB TOLERANCE
Before delving into the topic of carb tolerance, it is fundamentally important to understand how insulin
works and that it’s not ‘the evil fat storage hormone’. To start, you are recommended to read this series
about the myths surrounding insulin.
Depending on who you ask, ‘carb tolerance’ is seen as either broscience or an important
advanced nutrition topic. The main problem with discussions about carb tolerance is, like with
many discussions, that the term has no commonly agreed upon definition. From a bodybuilding
perspective, it makes sense to define carb tolerance as responsiveness to carbohydrate intake
for body recomposition. People with a good carb tolerance lose more fat and build more
muscle with a higher proportion of carbs in their diet; people with poor carb tolerance lose
more fat on higher fat diets on a calorie and protein equated basis.
Diet customization
In aggregate, most research concludes that the fat to carb ratio of a diet is inconsequential for
weight loss success, as you’ve learned. Energy balance is after all the prime determinant of
weight loss. (Note: A caveat will be discussed for long term results in the topic on fat.)
What is true in the population on average is not necessarily true for each individual. If a study
sample contains people with good and people with bad carb tolerance, on average a study may
show no effect of the carb:fat ratio on weight loss success. It would take very high statistical
power and a type of segmental/cluster analysis to find that there is individual variability. Since
there is already marked interindividual variability in metabolic rate, diet compliance, activity
level, etc. it is very difficult to find evidence of differences in carb tolerance in studies that are
not specifically looking for this.
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Fortunately, carb tolerance has been researched in many studies. Studies that control for the
amount of insulin resistance and the insulinogenic strength of the diet find that different people
respond differently to high and low carb diets.
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The determinants of carb tolerance
Insulin sensitivity, how strongly your body reacts to insulin, is often touted as the mechanism
behind carb tolerance: good carb tolerance is equated with good insulin sensitivity.
Insulin sensitivity
Yet insulin sensitivity is only one part of the puzzle. There is a rough correlation between
insulin sensitivity and carb tolerance and in general people with poor insulin sensitivity lose
more fat on low glycemic load diets [2, 3], adhere better to low carb diets and achieve better
health outcomes on a low carb diet, even when calories and protein are the same between
groups.
In contrast, insulin sensitivive women have been found to lose more fat on higher carb than a
lower carb diet [2]. And sometimes insulin resistant individuals actually lose more fat on a
highly glycemic weight loss diet. These are the carb hyper-responders that do best on a high
carb diet.
This is not an artifact of the diet. Researchers have already identified some of the genes
responsible for carb hyper-responsiveness. The notorious ‘fat mass and obesity–associated gene
(FTO)’ may also play a role in carb tolerance.
Moreover, insulin sensitivity does not correlate with perilipin activity, a marker of fat
mobilization. This shows that there is no direct effect of insulin sensitivity on fat loss or gain.
Insulin sensitivity also doesn’t predict the ratio of muscle to fat gain during overfeeding
(‘bulking’). I’ve talked to Michael Jensen from the Mayo Clinic, where the role of insulin
sensitivity on regional body composition changes has been studied, and although the findings
weren’t published (who cares about muscle growth, right?), they didn’t find any effect of
baseline insulin sensitivity on the subsequent amount of muscle built during overfeedings trials.
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In identical twins insulin sensitivity also didn’t predict the ratio of fat to muscle growth during
long term overfeeding.
Insulin release
Why does insulin sensitivity provide an incomplete picture? There’s another determinant of
carb tolerance: insulin secretion. In the 6 month CALERIE trial, insulin sensitivity did not predict
weight loss in response to higher vs. lower carb diets in healthy men and women, but insulin
production in response to an oral glucose challenge did. So insulin resistant individuals didn’t
necessarily lose more weight on a lower carb diet, but individuals with high insulin production
did.
By itself, insulin secretion is not related to fat gain. Diets that vary wildly in their degree of
insulin stimulation (insulinogenicity) produce the same rate of fat loss, as you’ve learned.
What really matters is if an individual’s insulin secretion is appropriate given the individual’s
level of insulin sensitivity. You can be severely insulin resistant, but if your body cannot produce
enough insulin to compensate for this, you may still do better on a highly insulinogenic diet.
This scenario essentially constitutes a subclinical form of type 1 diabetes. This type of carb
hyperresonsiveness is rare, however.
The interaction between insulin production and insulin sensitivity may explain the anecdotal
synergy between growth hormone and insulin usage reported by steroid users. The insulin
compensates for the insulin resistance caused by growth hormone. Remember that insulin is a
fundamentally anabolic hormone that can benefit muscle growth by increasing protein balance
and the rate of glucose uptake in muscles (though supraphysiological dosages, i.e. insulin
injections, are required to promote muscle growth).
On the other hand, if you are very insulin sensitive, but your pancreas oversecretes insulin,
you’ll still do better on a low carb diet, because too much insulin without a great stimulus for
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muscle hypertrophy can result in poor nutrient partitioning, excessive fat storage and
hypoglycemia (low blood sugar).
Thus, normal carb tolerance is associated with an appropriate insulin release given the
individual’s insulin sensitivity. Most people fall into this category. Both poor and high carb
tolerance are associated with an inappropriate release of insulin.
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The effects of carb tolerance
Not eating in line with your carb tolerance has various detrimental effects for your body
composition even beside the direct effects of insulin and tissue glucose uptake.
Poor carb tolerance is relatively easy to understand and quite common in overweight
individuals. It is effectively (pre-) type II diabetes. It is often characterized by insulin resistance
and high (but not always high enough) insulin production. A high carbohydrate intake here
results in pathological flucutuations in blood glucose levels with the traditional ‘peaks and
crashes’ after meals, a high fasting blood glucose level and often high fasting insulin levels in the
long run.
Since blood sugar is inherently inflammatory, excessively elevated blood glucose levels can
impair nutrient partitioning by interfering with the inflammatory signal for muscle repair
(discussed in greater detail in the topic on fats). If we combine this with high insulin levels, we get
easy fat storage and difficulty gaining muscle. And that’s exactly what we see in many
overweight individuals (discussed in greater detail in the topic on deciding whether to cut or bulk).
As you’ve seen, poor carb tolerance can also result from normal or even good insulin sensitivity
but excessive insulin production. This is much less common, however.
High carb tolerance can be characterized into 3 types.
1. The classic bodybuilding carb hyperresonder is the individual with extremely good
insulin sensitivity (often as a result of androgenic-anabolic steroids), particularly in
muscle tissue, so that nutrient partitioning is very favorable for muscle growth.
2. The rarer form of carb hyperresponsiveness is the kind analogous to type I diabetes,
where individuals with insulin resistance effectively need an insulinogenic diet to produce
enough insulin. This is the rarer type, especially in strength training circles.
3. Type 3 carb hyperresponsiveness is not really good carb tolerance but rather poor fat
tolerance. As we’ll discuss on the topic on the thermic effect of food, certain individuals,
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especially overweight individuals with poor insulin sensitivity, have a very low thermic
effect of fat, causing almost no increase in energy expenditure after the consumption of
fat.
It is also plausible that carb tolerance correlates with the thermic effect of carbohydrates, i.e.
individuals with poor carb tolerance have a lesser increase in energy expenditure after
consuming carbohydrates. Individuals with poor carb tolerance often have a lower energy
expenditure than healthy individuals with a similar body composition.
Other than the physiological effects, not eating in line with your carb tolerance arguably has
even stronger psychological effects that affect diet adherence. Since insulin has an appetite
suppressing effect, fluctuations in insulin levels can cause similar flucutuations in hunger levels.
Blood sugar fluctuations outside of the normal range can notably impair mental functioning,
causing lethargy and fatigue when blood sugar levels are low.
Carb tolerance also has a behavioral aspect. As we discussed in the topic on carbohydrates,
carbohydrates can stimulate both dopamine and serotonin production. These
neurotransmitters are normally well regulated by the body, but if an individual has abnormal
levels of either, carbohydrates can significantly affect mental performance and motivation. This
can impact spontaneous physical activity and the amount of effort put into someone’s training
program. In support of this, large variation in the metabolic response to high and low carb diets
have been found between individuals.
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Measuring carb tolerance
So how do you find out if someone’s carb tolerant? Many coaches simply ask the client if they
respond better to high or low carb diets.
Intuitively as this may be, it’s not a good approach, because you do not want to test a person’s
subjective preference for either carbs or fats or the person’s mood state. As explained in the
lecture on carbohydrates, carbs have strong psychological effects even if serotonin and
dopamine are well regulated and these effects can be wholly distinct from the physiological
effect they have. As a result, allowing people to choose themselves if they should go on a high
or low carbohydrate diet does not improve diet adherence, quality of life or weight loss
success.
It is common for people to erroneously believe they are carb hyperresponders. The truth is
they just like eating carbs, because they have become entrained to glucose and carbs therefore
make them feel good. Menno has had many clients that believed they did better on high carb
diets but after trying both high and low carb diets, they performed noticeably better on the low
carb diet. Women in particular seem susceptible to this, as women express more emotion and
are more likely to weigh their feelings into their decision making, though this stereotypical
gender difference is vastly overrated.
The opposite also occurs. Particularly in paleo and keto dieting communities, many overweight
individuals are taught that carbs are the devil and insulin is ‘the fat storage hormone’, so they
believe fat loss is correlated primarily with insulin levels rather than energy balance, while in fact
they have perfectly normal carb tolerance or they actually have fat intolerance. (Energy balance
is discussed further in the topic on energy metabolism.)
So how then do you measure someone’s carb tolerance?
Without extensive bloodwork, the test Menno has found most useful is assessing someone’s
energy level and hunger after eating a high vs. low carb meal. After a high carb meal, carb
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tolerant individuals should experience a stable dose-response appetite suppression and
increase/maintenance of mental energy level after a high carb meal. Carb intolerant individuals
generally get a strong ‘carb knock-out’ after which they get hungry again not long after.
Everything needs to be the same during the testing, including the satiety index of the high fat and
high carb foods (Menno likes to use bread and eggs; the satiety index will be discussed in greater
detail in the topic on ad libitum dieting), but also the time of day, the timing in relation to
exercise, etc.
Since this is a subjective test with many confounders, you want to be wary of reading too much
into the data. Carb hyper-responsiveness is rare and generally accompanied by a very clear
result in favor of carbs. For example, carb hyperresponders may complain they can’t sleep after
eating a high carb meal because they’re too energized. Carb intolerance is much more common,
but most people fall in neither category and will not show a clear trend in their results in favor
of either macronutrient. Based on Menno’s experience and the literature, approximately 80% of
people have a neutral carb response, 15% has carb intolerance and 5% responds favorably to
high carb diets. It is worth noting that in sedentary, overweight individuals, carb intolerance is
much more common, since exercise greatly improves insulin sensitivity.
Due to the subjectivity of the test and the strict lifestyle control required to meaningfully test
this, you probably don’t want to test all your clients. During a cut, there is often little point in
testing anyway, as you don’t have the calories to make large adjustments to the carb:fat ratio
without either going unhealthily low in carbohydrates or going into ketosis (reviewed in its own
course topic).
As a concrete example, here’s a case study of someone’s carb tolerance testing results. This is a
result typical of someone with normal carb tolerance.
Instead, you can roughly estimate someone’s carb tolerance based on the following guidelines.
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
The higher someone’s body fat percentage, the lower their carb tolerance, generally. If
someone’s overweight, it’s unlikely they’ll respond very well to a high carb diet.

Women are unlikely to be carb intolerant, due to their superior glucose and insulin
metabolism. As mentioned above, note that this does not imply they are carb
hyperresponders, which is still a very rare trait in women. (See the topic on gender
differences for further details and references.)
o Women with polycystic ovary syndrome or similar conditions that result in
amenorrhea are an exception to the rule and almost always have very poor carb
tolerance.

Older individuals tend to be more prone to carb intolerance, though differences
generally only become significant at a truly elderly age (65+), depending on how healthy,
lean and active they are. (See the course topic on the effects of age.)

Carb hyperresponsiveness is often noticeable from someone’s dietary records, food
preferences and activity level, the latter of which fluctuates strongly based on their
previous carb intake. For example, carb hyperresponders may complain they can’t sleep
after a high carb meal eaten at night because they’re too energetic.

People on androgenic-anabolic steroids are far more likely to be carb hyperresponders.
(See the course topic on AAS.)
Recommended reading

Dietary composition and weight loss: can we individualize dietary
prescriptions according to insulin sensitivity or secretion status?

Tailoring dietary approaches for weight loss
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