FOOD FUNDAMENTALS
Carbohydrates
Carbohydrates
Most of the food we eat contains carbohydrates.
They include sugar and starches, which provide
our body with energy, and fiber, which is vital
for a healthy digestive system.
What are carbohydrates?
Carbohydrate molecules are made
up of carbon, hydrogen, and oxygen
atoms, often in the form of
hexagonal or pentagonal rings.
If the rings are in ones or twos,
they are sugars, but if the rings
combine into unbranched or
branched chains, they become
starches and other complex
carbohydrates. Very long,
indigestible chains make up
dietary fiber (see pp.24–25). In the
body, sugars and starches are
converted into the sugar glucose—
our body’s primary source of energy.
Smelly breath
caused by ketones
Ketone
released
in breath
SUGARS
STARCHES
FIBER
DO CARBS
MAKE YOU FAT?
Carbohydrates can cause you
to gain weight if you eat too
many of them, but complex,
high-fiber carbohydrates
are a key part of a
healthy diet.
If you don’t eat enough carbs, your
liver converts fats into ketones and
protein into glucose, which are
used to generate energy. Ketogenic
diets can help weight loss, but not
much is known about their long-
term health effects. They can also
give you smelly breath!
NOT ENOUGH CARBS?
Milk and natural sugars
Natural sugars are found in milk
products, fruit, and some vegetables.
The fiber in some of these foods
ensures that the sugar is absorbed
at a gradual rate.
BROCCOLI
APPLE
MILK
Free sugars
These can be added to food as refined
table sugar, but are naturally present in
honey, syrups, and fruit juices. These
provide lots of “empty calories” and it is
easy to eat too much of them.
HONEY
FRUIT
JUICE
SYRUP
Refined starches
Only the simpler, more easily
digested starches are found in
refined carbohydrates such as
white flour and white rice. They break
down easily in the body, giving a quick
energy rush, but don’t keep you full for long.
Unrefined starches
These are found in foods
including whole-grain breads,
cereals, and beans. They are broken
down slowly, releasing energy over a
long period of time. They are also a good
source of fiber, vitamins, and minerals.
WHOLE GRAINS
BEANS AND
LEGUMES
WHITE
RICE
CAKE
WHITE
BREAD
A LOW-CARB DIET MAY LEAD TO
MOOD SWINGS AS CARBS HELP
THE BRAIN MAKE A CHEMICAL
THAT STABILIZES MOODS
US_022-023_Carbohydrates.indd 22 18/01/2017 09:41
22
FOOD FUNDAMENTALS
Carbohydrates
23
How the body uses carbohydrates
When we eat carbohydrates, our digestive tract breaks them down
into sugars, which are absorbed into the blood. Glucose is used
directly by our various organs and muscles as a source of energy.
Fructose—a simple fruit sugar that bonds with glucose to make
table sugar—can only be processed by the liver. People with
high fructose diets are at higher risk of type 2 diabetes, possibly
because fructose is more likely to be converted into fat.
Some glucose is stored
as glycogen, a complex
carbohydrate like starch
Glucose is used or
stored by the liver
L
I
V
E
R
Fructose
molecules
travel in the
blood
Glucose
molecules
travel in the
blood
Glucose travels
around the body
The heart uses
energy to pump
nutrients around
the body
Fructose is either
converted to glucose
or stored as fat
Using energy
Glucose is the
easiest and most
efficient fuel for the
body. Chemical
reactions in our cells
convert glucose (or
other molecules if
glucose isn’t available)
into molecules that
release energy.
Fat stores
Once the liver’s
glycogen stores are full, excess
glucose is converted into fat
and stored around the body,
to be used as fuel later if food
becomes scarce.
Absorption and
distribution
Long-chain, starchy
carbohydrates need to be
broken down into sugars
to be absorbed. Digestion
begins in the mouth and
continues into the small
intestine, where the sugars
pass into the bloodstream.
The liver’s role
If we eat more carbohydrates than we
need to use immediately, the liver stores the
excess as glycogen. When blood sugar levels
drop, the stored glycogen is converted back
into glucose to be used by the body.
3
4
1
2
B
L
O
O
D
V
E
S
S
E
L
S
M
A
L
L
I
N
T
E
S
T
I
N
E
Muscle cells
convert glucose
into energy
BRAIN
HEART
FAT
M
U
S
C
L
E
The brain is the
body’s most energy-
demanding organ
US_022-023_Carbohydrates.indd 23 23/02/2017 11:26
22
FOOD FUNDAMENTALS
Carbohydrates
23
How the body uses carbohydrates
When we eat carbohydrates, our digestive tract breaks them down
into sugars, which are absorbed into the blood. Glucose is used
directly by our various organs and muscles as a source of energy.
Fructose—a simple fruit sugar that bonds with glucose to make
table sugar—can only be processed by the liver. People with
high fructose diets are at higher risk of type 2 diabetes, possibly
because fructose is more likely to be converted into fat.
Some glucose is stored
as glycogen, a complex
carbohydrate like starch
Glucose is used or
stored by the liver
L
I
V
E
R
Fructose
molecules
travel in the
blood
Glucose
molecules
travel in the
blood
Glucose travels
around the body
The heart uses
energy to pump
nutrients around
the body
Fructose is either
converted to glucose
or stored as fat
Using energy
Glucose is the
easiest and most
efficient fuel for the
body. Chemical
reactions in our cells
convert glucose (or
other molecules if
glucose isn’t available)
into molecules that
release energy.
Fat stores
Once the liver’s
glycogen stores are full, excess
glucose is converted into fat
and stored around the body,
to be used as fuel later if food
becomes scarce.
Absorption and
distribution
Long-chain, starchy
carbohydrates need to be
broken down into sugars
to be absorbed. Digestion
begins in the mouth and
continues into the small
intestine, where the sugars
pass into the bloodstream.
The liver’s role
If we eat more carbohydrates than we
need to use immediately, the liver stores the
excess as glycogen. When blood sugar levels
drop, the stored glycogen is converted back
into glucose to be used by the body.
3
4
1
2
B
L
O
O
D
V
E
S
S
E
L
S
M
A
L
L
I
N
T
E
S
T
I
N
E
Muscle cells
convert glucose
into energy
BRAIN
HEART
FAT
M
U
S
C
L
E
The brain is the
body’s most energy-
demanding organ
US_022-023_Carbohydrates.indd 23 23/02/2017 11:26
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