Carbohydrates and Nutrition
Simple carbohydrates: Monosaccharides and Disaccharides
• Simple carbohydrates (“sugars”)
– Monosaccharides
• Glucose
• Fructose
• Galactose
– Disaccharides
• Maltose
• Sucrose
• Lactose
The monosaccharides can be directly absorbed into our bloodstream, so they don't require any
digestion.
However, the disaccharides need to be broken down to the individual monosaccharides.This process
is part of normal digestion and is called hydrolysis. So we can break down the disaccharides into the
individual monosaccharides through hydrolysis.
Hydrolyse
Monosaccharides Disaccharides
And as an example, we can have sucrose (table sugar), and it can undergo hydrolysis to yield the
individual monosaccharides glucose and fructose.
Through that process we are able to utilize the energy that is available in the disaccharides by
forming the monosaccharides which can subsequently be absorbed into the bloodstream and used as
an energy source.
Invert sugar is produced by the enzymatic cleavage of sucrose into glucose and fructose.
Complex carbohydrates
• Polysaccharides
– Glycogen
– Starches
, • Polymer of glucose
– Fibres
Fibres
• Dietary carbohydrates that are not subject to digestion by endogenous enzymes but may be
digested by bacteria in the colon.
Soluble fibre
Soluble fibre is a chemically diverse group that includes the following:
Pectin is part of the cell wall of plants. Its key feature is the abundant presence of
galacturonic acid. Pectin is often added during food manufacturing to impart gelation,
viscosity, texture and protein stability. It is used in food products such as jams and jellies
because of its ability to form stable gels.
Hemicellulose describes a diverse set of polysaccharides composed of different
monosaccharides. Apart from glucose, these monosaccharides include xylose, mannose,
galactose, rhamnose, and arabinose. Hemicelluloses are part of the cell walls of plants. An
example of a fibre that is part of the hemicelluloses is psyllium, which is one of the most
widely used fibre supplements in the world. Other hemicelluloses are guar gum and xanthan
gums. They are used extensively in food manufacturing as thickening agent due to its high
viscosity.
Fructans are polymers of fructose. The best-known representative of the fructans are the
highly fermentable inulins. Inulin is naturally present in various foods, such as asparagus,
leek, onions, and banana. In addition, inulin is added to foods as a functional pre-biotic food
ingredient to confer a health benefit.
Insoluble fibre
Insoluble fibre includes the following:
Cellulose is a linear chain of glucose, similar to amylose, except that the glucose units are
linked in such a way that our digestive enzymes cannot break them. As a result, cellulose
leaves the human body completely undigested. Ruminants such as cows have special
bacteria in their stomach that produce cellulase, allowing them to break down cellulose and
thus utilize cellulose for fuel.
Unlike other dietary fibres, lignins do not belong to the carbohydrates. They are complex
polymers of aromatic alcohols. It is the second most abundant natural polymer in the world,
next to cellulose.
Hemicellulose describes a diverse set of polysaccharides composed of different
monosaccharides. They have been discussed under soluble fibres.
Resistant starches are starches that are not subject to regular digestion in the GI tract. Depending on
the type, they may be partially fermented in the colon by the gut microbiota. Resistant starch can be
divided into distinct groups.
- RS1: physically inaccessible starch (coarsely ground or whole kernel grains),
- RS2: granular starch (uncooked potato, unripe banana flour),
, - RS3: retrograded starch (cooked and cooled starchy foods),
- RS4: chemically modified starches.
The third category includes for instance French fries or boiled potatoes that are cooled down and
kept in the fridge for an extended period of time. Simply leaving the French fries to cool down on
your plate will not result in measurable formation of resistant starch. Even when French fries or
boiled potatoes are kept in the fridge, only a portion of the starch will be retrograded and thus
escape regular digestion.
Please note that hemicellulose is included in the soluble and insoluble fibres. That is because certain
fibres within this class are soluble in water, whereas others are insoluble.
Carbohydrates in our Diet
Many foods are high in carbohydrates. The predominant sources of carbohydrate in most people’s
diet are starchy foods such as wheat, corn, rice, cassava and potatoes. The raw forms of these foods
(e.g. whole wheat, brown rice etc) also contain substantial amounts of fibre, which is mostly lost
during processing. Many foods are rich in carbohydrates due to their high sugar content, which is
present naturally (as in fruits) or added during processing. Crystalline table sugar is 100%
carbohydrate in the form of sucrose.
Meat contains only very small amounts of carbohydrate in the form of glycogen.
Sources of Dietary Fibre
Plant foods in their natural form usually contain substantial amounts of fibre. Most high fibre foods
contain a mixture of dietary fibres, although the main type of fibre present can differ considerably
between various foods. Processing of food often leads to loss of fibre. Sometimes, the colour can be
deceiving. Many breads in the Netherlands are made to look like whole wheat bread, but its main
ingredient is white flour, not whole wheat flour. Other breads are made to look like white bread (to
make it more appealing to children) but have fibre added. Always check the list of ingredients.
Animal products, including milk and milk products, contain little to no fibre. Sometimes, fibre is
added to yogurt to create functional foods.
, Carbohydrate Digestion and Absorption
Carbohydrate absorption occurs mainly in the ileum, a part of the small intestine. The products of
carbohydrate digestion are the three monosaccharides: glucose, galactose and fructose.
Pancreatic amylase and maltase are the enzymes essential for the full digestion of starch to glucose.
Pancreatic amylase breaks down starch to the disaccharide maltose. Maltose is, then, digested by
the enzyme maltase to yield individual glucose units that can be absorbed by enterocytes.
Starch amylase Maltose Maltase Glucose
Salivary amylase starts the digestion of starch in the mouth, but is not essential for full digestion.
Sucrase and lactase break down the disaccharides sucrose and lactose. These two disaccharides are
not present in starch.
Dietary fibre is also referred to as non-digestible carbohydrate and thus evades digestion until
reaching the colon. A portion of the dietary fibre is used by the bacteria residing in the colon to
produce a variety of products including short chain fatty acids (butyrate, propionate, acetate),
methane, hydrogen sulphide, and trimethylamine N-oxide.
