Carbohydrates defined and explained

CARBOHYDRATES: chemically speaking, they include such diverse groups as aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers_

Carbohydrates may be divided into three principal groups, namely sugars, oligosaccharides and polysaccharides.

Sugars --> Monosaccharides (Glucose, galactose, fructose) - Disaccharides (Sucrose, lactose, trehalose) - sugar alcohols (Sorbitol, mannitol)

Oligosaccharides --> Malto-oligosaccharides (Maltodextrins) - Other oligosaccharides (Raffinose, stachyose, fructo-oligosaccharides)

Polysaccharides --> Starch (Amylose, amylopectin, modified starches) - Non-starch polysaccharides (Cellulose, hemicellulose, pectins, hydrocolloids)

What Is Total Carbohydrate?

Calculating carbohydrates by "difference"

The protein, fat, ash and water grams of a food are measured, subtracted from the total weight of the food and the remainder, or "difference", is agreed upon to be carbohydrate.

However, the "by difference" grams include plenty of non-carbohydrate components such as lignin, organic acids, tannins, waxes, etc.

Finally, a single global figure for carbohydrates in food doesn't tell most interesting things about the food's nutritional value because it can't identify the of carbohydrate types and doesn't describe physiological properties of those carbohydrates.

Sugar or Sugars?

The chemical term "sugars" describes the mono and disaccharides. "Sugar" is used to identify refined sugar and added sugar.

Healthy and unhealthy sugars

The terms "extrinsic" and "intrinsic" sugars were developed in UK to help the consumer choose between what were considered to be healthy sugars and those which were not. Intrinsic sugars were defined as sugars naturally occurring within the plant cell, while extrinsic sugars were those which were usually added to foods. However, nutritionists don't plan to measure these sugars separately in the diet or to incorporate them into food tables.

Complex carbohydrates

The term was introduced in order to distinguish sugars from fruit, vegetables and whole grains. The term is used to describe either starch, or the combination of all polysaccharides as well as to encourage consumption of what were considered to be healthy foods such as whole grain cereals, etc. However, it is now realized that starches, as complex carbohydrates, are not created equal: some are being rapidly absorbed and having high glycemic index and some being resistant to digestion (low glycemic index). Available and unavailable carbohydrate

Not all carbohydrates could be utilized or "metabolized". Available carbohydrates are defined as "starch and soluble sugars" and unavailable as "hemicellulose and fiber (cellulose)".

However, it can be misleading to talk of carbohydrate as "unavailable" because some carbohydrates seen as "indigestible " are in fact able to provide the body with energy through fermentation in one part of the intestinal tract or another.

A more appropriate substitute for the terms "available" and "unavailable" today would be to describe carbohydrates either as glycemic (i.e. providing carbohydrate for metabolism) or non glycemic or those that do not provide carbs for metabolism.

Tanya Zilberter, PhD

Source: FAO/WHO expert consultation on carbohydrates in human nutrition