What are carbohydrates?

Carbohydrates are one of the four classes of biomolecules. The other three are lipids, proteins and nucleic acids. Though they are almost always quite distinct from the other three classes, providing a precise definition is a bit tricky. Technically, carbohydrates are molecules whose basic structure the general formula (CH₂O)_n, where n is any number from 3 onwards. (For instance, for glucose n = 6, producing the overall formula C₆H₁₂O₆.) Although this is the fundamental form, many carbohydrates have added functional groups too.

Carbohydrates are made up of monosaccharides ('single sugars'). Examples of these include the fairly well known glucose, fructose and galactose. Here's what glucose 'looks' like:

If two monosaccharides are joined together they are known as disaccharides. These include sucrose (the table sugar that you buy in shops, made from a glucose and a frutose molecule) and lactose ('milk sugar', made from glucose and galactose).

If a few more monosaccharides are joined together, but not too many, the carbohydrate is known as a oligosaccharide ('a few sugars'). In order to quantify this more precisely, many authorities recommend limiting this term to a carbohydrate of from 3 to 10 monosaccharides.

Finally, there are the polysaccharides - sugars made from many (more than 10) monosaccharides. Polysaccharides can be used as a storage form for sugars - i.e. if there is excess glucose, the glucose can be packaged into one big molecule, which is more efficient. In animals, this molecule is called glycogen; the related molecule in plants is called starch. Polysaccharides can also be used to as structural components, such as the cellulose that makes up green plants' cell walls.

You may have heard of a dietician advising people to eat less 'simple' carbohydrates and more 'complex' carbohydrates instead. By 'simple' he or she simply means mono- or disaccharides, whereas 'complex' refers to oligo- or polysaccharides. Simple carbohydrates taste sweet, whereas complex carbohydrates don't.

Carbohydrates are absorbed in the human small intestine. Only monosaccharides can be absorbed, however, so the body must break larger carbohydrates down. The three main sources of carbohydrates in the human diet are starches, lactose and sucrose.

• Starches are converted to maltose (a disaccharide) and other oligosaccharides by an enzyme from the pancreas, and to a lesser extent, from saliva.
• The cells of the small intestine then break down the lactose, sucrose, maltose and the oligosaccharides into their constituent monosaccharides.
• These are water soluble and absorbed immediately into the intestinal blood stream (the portal veins).

People who are lactose intolerant lack the functional enzyme (lactase) that breaks down lactose into its monosaccharides - glucose and galactose. I've posted a little more on this topic here.