they are made of. Glucose is one of the three nutritionally relevant monosaccharides found

in food. The prefix mono, for one, means that these molecules are made of a single sugar

unit. The two other monosaccharides are fructose and galactose. As you can see, they contain

the same six atoms of carbons and the same atoms of oxygen and hydrogen, but they are

arranged in a slightly different way, and this is enough to give them different properties.

Fructose is abundant in fruit and is sweeter than glucose. Galactose by itself is not very

common, but some is found in milk and dairy products.

Glucose, fructose and galactose are the only carbohydrates that our intestine can absorb.

Every other carbohydrate, before it can enter our body, must first be broken down into glucose,

fructose or galactose during digestion. Indeed, glucose, fructose and galactose are the building

blocks of every other existing dietary carbohydrate.

Carbohydrates made of two sugar units are called disaccharides, and they are also important

in food. When a molecule of glucose combines with a molecule of fructose, we get sucrose.

Sucrose is the common table sugar. It is abundant in sugar cane and sugar beet, from which it

is extracted to make table sugar, and then in honey, maple syrup and molasses.

When a molecule of glucose combines with a molecule of galactose, we get lactose. Lactose

is the main sugar present in milk and dairy products.

When a molecule of glucose combines with another molecule of glucose, we get maltose, some

of which is found in beer and liquors as a result of fermentations operated by yeasts

on starch.

Monosaccharides and disaccharides are referred to as sugars or simple sugars, to distinguish

them from longer chain carbohydrates which are called polysaccharides or complex carbohydrates,

and are made of many sugar units. The most abundant polysaccharide in plants

is starch, which is the carbohydrate they build for energy storage. Starch is made of

thousands of molecules of glucose linked together in long chains, which can be linear or branched.

It is abundant in grains, legumes, and tubers such as potatoes and yams.

Another important polysaccharide is glycogen, which is the main energy storage carbohydrate

in animals and is also made of thousands of molecules of glucose, but arranged in a different

structure, with shorter but more frequent branches. However, contrary to plants, animals

do not store a lot of energy as carbohydrates, and instead prefer to store their energy in

the form of fat. For this reason, while plant foods can provide a lot of carbohydrates as

starch, animal foods do not provide significant amounts of carbohydrates because they have

just a little glycogen, and most of it breaks down after the animal dies.

We ourselves store glycogen in our liver to have some glucose available in between meals

to maintain blood glucose concentrations stable. To make glycogen, glucose molecules are combined

together. When glucose is needed, glucose molecules will be detached one by one from

glycogen. However, glycogen takes up a lot of space and holds a lot of water, so our

glycogen stores are limited to a few hundred grams. If we don’t eat again within about

18 hours, these glycogen stores get completely depleted. If we do intense physical activity,

our muscles use up a lot of glucose and our glycogen stores get depleted faster. Once

glycogen is over, our liver has to start breaking down proteins to make glucose, leading to

loss of muscle tissue in the long term. Some glycogen is also stored directly in our muscle

cells. This glucose cannot be sent back to the bloodstream to maintain blood glucose

stable, but it can be used directly in the muscle especially during high intensity and

endurance exercise. For this reason, muscle cells of trained endurance athletes increase

the amount of glycogen that they can store, especially if they follow a particular dietary

strategy called carb loading.

Some other carbohydrates present in food are still made of glucose, fructose and galactose,

but they are not digestible, because they are linked together with a different type

of bond that our digestive enzymes are unable to break, and like we said, if we cannot break

a carbohydrate all the way down to the monosaccharides we cannot absorb anything. Although they cannot

be absorbed, these non-digestible carbohydrates are still very important for our health and

we classify them in a separate category which we call dietary fiber.

We will discuss dietary fiber later. But please note that in this course, whenever we use

the word carbohydrates, we only refer to the digestible carbohydrates that can be digested,

absorbed and provide energy, and we do not refer to fiber.

Let’s now spend a few words on how carbohydrates are digested and absorbed.

As we already said before, the only carbohydrates that our intestine can absorb are the three

monosaccharides glucose, fructose and galactose. Every other carbohydrate, in order to be absorbed,

must be broken all the way down to these single sugars units. This is the goal of carbohydrate

digestion. Any carbohydrate which cannot be broken down into single units, will travel

intact through the small intestine and become dietary fiber.

Cooking facilitates carb digestion: it softens connective structures in fibrous parts of

plants, and it hydrates starches, making them more digestible, so much so that if we were

to eat some raw potato, chestnut, pasta or rice, their starches would mostly travel intact

through our small intestine without being absorbed.

In our mouth, the enzyme salivary amylase starts breaking down starch into smaller units,

but it is soon inactivated by the stomach acidity. You can notice the activity of salivary

amylase if you chew thoroughly a piece of bread: after a while, it will start becoming

sweeter. This is because some starch has been broken down to maltose.

Not much happens to carbohydrates in the stomach. In the small intestine, pancreatic amylase

from the pancreas completes the breakdown of starch to units of the disaccharide maltose.

Enzymes located on the brush border then work on disaccharides, breaking them down into

their single sugar units. Sucrase breaks down sucrose into glucose and fructose, maltase

breaks maltose into two units of glucose, and lactase breaks down lactose into glucose

and galactose. The individual monosaccharides can then be absorbed and enter the bloodstream

through the portal vein directed to the liver. In the liver, fructose and galactose are almost

completely converted to glucose. The liver uses some glucose itself for energy, some

is sent back to the bloodstream to maintain blood glucose stable and for other cells to

use, some is used to replete glycogen stores, and any excess is converted to fat and stored

in the adipose tissue.