DIETS
Energy budget
Energy budget
The way in which our body processes energy may
be understood in terms of an energy budget. How
much energy we put in—through our food—and
how much we expend—our levels of activity—
determine what we have left over—our fat stores.
Metabolism
Metabolism refers to our body’s vital
chemical processes, including extracting
energy from food and using it to build
molecules and repair our cells. Around
40–70 percent of the energy we burn each
day goes into these basic metabolic
functions—the proportion depends on
how physically active we are.
GLUCOSE IN A
BLOOD VESSEL
GLUCOSE
PROTEIN
FOOD
Food broken down
The body digests the food and drink
that we consume, breaking it down into
basic biochemical components or
building blocks: amino acids, fatty
acids, and sugars.
1
Build and repair
Amino acids circulate
through the bloodstream and
are used by cells throughout
the body to build proteins used
for growth and maintenance.
2
Excess converted
to glucose
Any excess amino acids that
have not been used to make
proteins are converted by the
liver into glucose.
3
Main energy source
Glucose travels in the blood
and provides body cells with a
ready source of energy. Any
that is not used is stored
as glycogen.
4
FATTY ACIDS
FOOD
SUGARS
AMINO ACIDS
AMINO
ACIDS
AMINO
ACIDS
YOUR BODY MASS INDEX
(BMI) IS SIMPLY YOUR
WEIGHT IN KG DIVIDED
BY THE SQUARE OF YOUR
HEIGHT IN METERS
I
N
T
E
S
T
I
N
E
I
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R
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190
DIETS
Energy budget
191
Losing weight
When we deprive it of food, our body
exploits its stores of energy. First, it
uses all available glucose in the
blood. Glucose is replenished as the
liver breaks down its glycogen store.
When glycogen runs out, the body
turns to its fat stores. So the only
way to lose weight is to remain in an
energy deficit—using more calories
than you take in—for a prolonged
period. Do this too strictly for long,
however, and muscles waste, as the
body breaks them down to liberate
amino acids for energy.
Putting on weight
When we consume more calories
than we expend through
metabolism and exercise, we store
the extra energy, first as glycogen,
then as fat. Fat is stored under the
skin (subcutaneous) and around our
organs in our abdominal cavity
(visceral). It is visceral fat that leads
to obesity-related diseases. White
fat cells also secrete hormones and
hormonelike molecules, which
affect food intake (see pp.14–15)
and insulin secretion and
sensitivity (see pp. 216–17).
Glucose burning
If the body has a good
supply of glucose, it will use
this as its primary source of
energy until it runs out.
Storing carbohydrates
If we have a surplus of glucose that our
body has not used for energy, this is
taken up by our liver cells and stored as a
complex carbohydrate called glycogen.
Fat burning
If the body does not have
enough glucose to burn, it
will next turn to its fat
stores for energy.
Storing fat
When the liver reaches its full capacity for
storing glycogen, any extra calories that are
consumed are converted to fat and placed in
stores throughout the body.
Protein burning
When starved, the body
will take the extreme
measure of using amino
acids for energy.
Recently, scientists have discovered that
some adults have stores of brown fat that
burns to keep us warm. Previously, they
thought only babies had brown fat. They
have also discovered beige fat, which
can change to an energy-burning state
when the environment changes, such
as when the temperature drops.
Finding ways to maintain this burnable
fat in the long term could lead to
treatments for obesity.
BURNING FAT TO KEEP WARM
GLUCOSE AND
GLYCOGEN
TO ENERGY
FATTY ACIDS
TO ENERGY
AMINO ACIDS
TO ENERGY
Brown fat
used up in
reactions
that create
heat
Glycogen
store half full
Glycogen
store full
Regular
white fat
stores energy
GLUCOSE
TO GLYCOGEN
GLUCOSE
TO STORED FAT
DOES HAVING A
SLOW METABOLISM
MAKE YOU PUT ON WEIGHT?
No difference has been found
between the metabolisms of
overweight and slim people.
If anything, metabolic rate
increases as your body
size increases.
US_190-191_Energy_budget.indd 191 18/01/2017 09:47
190
DIETS
Energy budget
191
Losing weight
When we deprive it of food, our body
exploits its stores of energy. First, it
uses all available glucose in the
blood. Glucose is replenished as the
liver breaks down its glycogen store.
When glycogen runs out, the body
turns to its fat stores. So the only
way to lose weight is to remain in an
energy deficit—using more calories
than you take in—for a prolonged
period. Do this too strictly for long,
however, and muscles waste, as the
body breaks them down to liberate
amino acids for energy.
Putting on weight
When we consume more calories
than we expend through
metabolism and exercise, we store
the extra energy, first as glycogen,
then as fat. Fat is stored under the
skin (subcutaneous) and around our
organs in our abdominal cavity
(visceral). It is visceral fat that leads
to obesity-related diseases. White
fat cells also secrete hormones and
hormonelike molecules, which
affect food intake (see pp.14–15)
and insulin secretion and
sensitivity (see pp. 216–17).
Glucose burning
If the body has a good
supply of glucose, it will use
this as its primary source of
energy until it runs out.
Storing carbohydrates
If we have a surplus of glucose that our
body has not used for energy, this is
taken up by our liver cells and stored as a
complex carbohydrate called glycogen.
Fat burning
If the body does not have
enough glucose to burn, it
will next turn to its fat
stores for energy.
Storing fat
When the liver reaches its full capacity for
storing glycogen, any extra calories that are
consumed are converted to fat and placed in
stores throughout the body.
Protein burning
When starved, the body
will take the extreme
measure of using amino
acids for energy.
Recently, scientists have discovered that
some adults have stores of brown fat that
burns to keep us warm. Previously, they
thought only babies had brown fat. They
have also discovered beige fat, which
can change to an energy-burning state
when the environment changes, such
as when the temperature drops.
Finding ways to maintain this burnable
fat in the long term could lead to
treatments for obesity.
BURNING FAT TO KEEP WARM
GLUCOSE AND
GLYCOGEN
TO ENERGY
FATTY ACIDS
TO ENERGY
AMINO ACIDS
TO ENERGY
Brown fat
used up in
reactions
that create
heat
Glycogen
store half full
Glycogen
store full
Regular
white fat
stores energy
GLUCOSE
TO GLYCOGEN
GLUCOSE
TO STORED FAT
DOES HAVING A
SLOW METABOLISM
MAKE YOU PUT ON WEIGHT?
No difference has been found
between the metabolisms of
overweight and slim people.
If anything, metabolic rate
increases as your body
size increases.
US_190-191_Energy_budget.indd 191 18/01/2017 09:47
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