Obesity and Energy Balance
Key Skills
• Define obesity and body mass index (BMI), and explain the health
consequences of obesity and underweight.
• Discuss contributing factors that have led to an increase in obesity in the U.S.
• Use the “calories in/calories out” model to explain how positive and negative
energy balance lead to weight increase.
• Distinguish between hunger and appetite, and explain how various hormones
affect these sensations.
• Discuss the metabolic set point hypothesis of obesity, and how it relates to
both hunger and satiety, as well as the growth/division of fat cells.
• Understand factors that affect caloric input, and how we harvest energy from
sugar and fat.
• Explain how multiple factors affect a person’s calorie “output,” including
their basal metabolic rate, the thermic effect of food, and their physical
activity.
• Explain the major components of a sustainable weight maintenance program.
Comparing Weights
Suppose one person weighs 175 pounds and
another weighs 120 pounds.
Is this enough information to tell whether
either person is overweight?
Does directly comparing the weights of two
people tell you which one is “fatter?”
What other information might help a doctor
work out what a person’s healthy weight is?
Many factors determine a healthy
weight, such as sex, age, ethnicity,
and height.
BMI
BMI takes height into account to
estimate how heavy a person is for
their size. BMI < 18.5 =
underweight; BMI > 25 =
overweight; BMI > 30 = obese
BMI is not for making a
diagnosis! Muscular athlete with
compact frame may have BMI over
25 (“overweight”).
(Img source:
vertex42.com)
Underweight
Being underweight (BMI < 19) can
be dangerous, especially in age.
Factors that cause underweight
include genetics, eating disorders,
depression, digestive problems, and
thyroid conditions.
As people get older, it becomes
harder to heal if the body doesn’t
have some fat reserves. Especially in
older women, being very thin can be
associated with osteoporosis and
brittle bones.
(Img source:
Medguidance.com)
Obesity in the US
The rate of obesity in the United
States has nearly tripled since the
1960’s.
Between 1980 and 2012, the
percentage of adolescents (age 1219) who are obese increased from
5% to 21%!
(Img src: Centers for Disease Control)
Even children as young as 6-11 years
old are now showing obesity (18% of
children in this age group in 2012).
Hypothesize some factors that could be worsening this epidemic of obesity.
Causes of Obesity
Obesity is caused by caloric imbalance. The body takes in more dietary
calories than it releases through exercise and body functions, and stores
the excess energy by making fat.
A simplified model:
Calories in > calories out = Weight gain
Calories in < calories out = Weight loss
(Img credit: Centers for Disease Control)
Be careful! Losing weight safely is NOT as simple as just reducing your
caloric intake or increasing your caloric output with exercise. As we’ll
see, a combination of calorie in/calorie out/appetite-based approaches
with healthy meals is needed to lose weight in a maintainable way!
Factors in the Obesity Epidemic
Factors raising the Calorie Input of Americans:
• Soda and juice consumption.
• Snacking
• Bigger portion sizes
• “Value-sized” empty calories
• Advertising
• Fast food
Factors lowering the Calorie Output of Americans:
• Limited time for exercise
• Sedentary lifestyle/jobs
• Increased use of TV and video games
• Lack of safe parks and walking areas
Portion Sizes
The average portion sizes for a burger
and fries have tripled since 1960.
Large portion sizes have a
psychological effect on appetite (the
want to eat). People are more likely to
overeat when more food is put in a
container.
Why have portion sizes increased so
drastically, and how can consumers
cope with this?
Hunger vs. Appetite
Appetite is psychological and physiological. It determines how
much you eat.
Psychological: Portion size, food plating
Physiological: Leptin, dopamine
Hunger is physiological - it makes you
feel like you need to eat something.
Low blood sugar
Metabolic set point hypothesis
Satiety is the sense of “fullness.”
Metabolic Set Point
One factor that affects both hunger and
appetite is how much fat a person already has!
Fat cells (adipocytes) make hormones that
regulate satiety. When you lose fat, your body
compensates to increase calories in and reduce
calories out by making you hungrier.
This is the metabolic set point hypothesis, and
helps explain why it’s so hard to sustainably
lose weight.
The brain has a built
in adipostat, which
is like a “thermostat”
for fat. Weight loss is
possible, but it
effectively requires
you to reprogram
your brain.
Fat Cell
Hyperplasia vs.
Hypertrophy
When we make more fat in our bodies, fat
cells need to either expand or divide to
store it.
Hypertrophy is when fat cells
individually get larger to store more fat.
This is the most common way fat grows in
adults.
The division of fat cells, which increases
the number of fat cells, is called
hyperplasia. This is especially common
in childhood and adolescent obesity. These
fat cells all secrete hormones and tend to
set the metabolic set point towards more
calorie storage. This is one reason why it’s
harder to reverse obesity that begins early
in life.
Caloric Input: Metabolism of Nutrients
Last lecture, we discussed how the digestive system breaks
starch, triglycerides, and proteins into sugars, fatty acids, and
amino acids.
But how does the body actually get usable energy (calories) from
these macronutrients?
There are three steps needed to harvest energy from sugar:
glycolysis, the Krebs cycle (Citric acid cycle), and oxidative
phosphorylation ( electron transport chain). These steps
produce the main “energy storage” molecule of cells: adenosine
triphosphate (ATP).
ATP
ATP is used by most cell
processes for energy. It
consists of adenosine (part of
DNA), and three phosphate
groups.
Breaking off one phosphate
and bonding it to oxygen
releases energy for use.
Releasing that phosphate turns
ATP into ADP (adenosine
diphosphate). We need to eat
and breathe to “recharge” ADP
back to ATP.
Glycolysis: First Step in Glucose Metabolism
Whoa! That’s a
lot of steps!
Can we simplify
this a little?
Glycolysis: First Step in Glucose Metabolism
Breaking down one
glucose (6-carbon
sugar) gives us two
pyruvate (3-carbon)
molecules, a little
ATP, and some
NADH.
NADH is a carrier of
high-energy electrons
we’ll use in a later
step!
Pyruvate gets turned into acetyl CoA
We lose one carbon
from each pyruvate.
This makes a little
more NADH and
releases carbon
dioxide waste.
Acetyl CoA can then
go into the
mitochondria, parts
of the cell specialized
for harvesting energy.
The Krebs Cycle (citric acid cycle)
harvests energy from acetyl CoA
The Krebs cycle gives us a little ATP,
makes more NADH and FADH2 (a
different electron carrier), and finishes
breaking down the sugar.
At this point, glucose has been broken
down into CO2. But what do we do with
the NADH and FADH2?
Organisms that don’t use oxygen
perform fermentation to get rid of the
extra NADH/FADH2. But we can use it
more efficiently.
The Electron Transport Chain (oxidative
phosphorylation) uses oxygen and NADH/FADH2
to make ATP
This step produces
the most ATP.
Oxygen is turned
into water, and
NADH is recycled
for later use.
We aren’t ready to
fully understand
what is happening
here… but we’ll
revisit it after we’ve
studied electrolytes.
Glucose
glycolysis
Fat
Pyruvate
Beta oxidation
Carbon chain processing is in red.
Electron carriers in blue
Acetyl CoA
A little ATP
NADH
Oxygen
Electron transport
chain
Mitochondrion
Some ATP
NADH
FADH2
Kreb’s
cycle
CO2
Lots of ATP
Water
Another Diagram (Same
Pathway, without Fat)
Oxygen and Human Life
While we can live for days without
water and weeks without food
(though it’s not recommended), we
can only survive for minutes
without oxygen.
Given everything we’ve seen here so
far…
Why is a constant supply of oxygen
so critical for survival? Why do we
die so quickly if we don’t breathe?
Brain Oxygen Demand
The brain uses 20% of the body’s
oxygen, roughly, because it needs a
huge amount of ATP.
The cells that make up the brain,
called neurons, need ATP to send
messages back and forth. Without
these messages, a person is braindead. When part of the brain loses
oxygen because of lack of blood
flow from a clot, a stroke results.
Calorie Output
There are three major ways we spend and release energy, burning
calories:
1. Basal metabolism (Resting Energy Expenditure): The energy we
burn just to maintain our body temperature and keep us alive.
2. Physical activity: Exercise and daily movement. In addition,
physical activity increases our Resting Energy Expenditure!
3. Digestion + Metabolism of Food: It takes energy to break down
food and harvest energy from it. Some foods take more energy to break
down than others. We call this the Thermic Effect of Food.
Resting Energy Expenditure
Resting Energy Expenditure consists of the calories you burn even when
you are not physically active or digesting food. Basal Metabolic Rate
specifically refers to the REE after waking up in the morning.
About 2/3rds of the calories you burn in a day are REE! Keeping your
body warm takes a lot of energy.
Having more muscle can increase REE. On the other hand, crash diets
and heavy food restriction lower REE, making it harder to lose weight
and maintaining the metabolic set point.
Aging also decreases REE.
Physical Activity
Physical
activity
Regular physical activity, which raises REE, is
better than occasional fits of exercise.
As the diagram shows, it takes a long time for
exercise alone to burn off calories!
Digestion/Thermic Effect of Food
About ten percent of the
calories you eat are burned in
the process of digestion.
This is the Thermic Effect of
Food. While it may be slightly
higher for protein, research
remains ambivalent on
whether this effect is
sufficient to explain weight
loss on high-protein diets.
A Maintainable Diet
A good diet is not one that a person uses for six months, then
drops. Many dieters regain the lost weight! (The often-quoted
figure of “95% of dieters regain lost weight” may be too
pessimistic, but the regain rate is high.)
To avoid this, long-term lifestyle change is needed. If you eat
fewer calories, you may feel less energetic. If you exercise more,
you may feel hungrier.
The only way to sustainably, safely lose weight is to eat
healthily and exercise regularly in a way you can maintain.