One important classification of carbohydrates
is based on the number of sugar units that 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.