Retrospective #31: Carbohydrates and Sugars

Are all carbohydrates sugars? Are all sugars carbohydrates? What is a carbohydrate? And what is a sugar? This is not chemistry class, but I think we all need to know the answers to these basic questions if we are going to guard our health. So, I’ll try to keep it simple and interesting. After all, we all have to eat, and making wise choices requires us to be well informed. There’s a lot of misinformation going around too, so listen up.

All carbohydrates are saccharides. The word saccharide comes from the Greek word meaning sugar. Carbohydrates are divided into four types: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides are smaller compounds, composed of one or two molecules, respectively, and are commonly referred to as sugars. These compounds very often end in the suffix “ose.” Examples include glucose (as in blood sugar), sucrose (as in table sugar), and lactose (milk sugar).

Polysaccharides are long strings of glucose molecules. Think of them as stored energy (e.g. as glycogen in humans and starch in plants) and as structural components (cellulose in plants). The term carbohydrate includes any food that is composed of long-chain glucose molecules -- the so-called “complex carbohydrates,” such as cereals, bread, rice or pasta, or the mono and disaccharides (“sugars”), such as those found in candy, jams, jelly and ice cream.

Glucose, fructose and galactose are the three monosaccharides. They are the simplest carbohydrates in that they cannot be broken down further into smaller molecules. Glucose is also, along with fat, a source of fuel for metabolism, glucose being always being the first used. When not immediately needed for energy, glucose is converted into its storage form, glycogen, mainly deposited in the liver and muscle cells.

The disaccharides (two molecule compounds) include sucrose (one glucose and one fructose molecule), lactose (one glucose and one galactose molecule) and maltose (two glucose molecules bonded in a special way). Oligosaccharides and polysaccharides are just longer chains of monosaccharides bound together. Oligosaccharides contain between three and ten monosaccharides and polysaccharides have more than ten monosaccharide units.

The human diet contains many foods high in carbohydrates: fruit, sweets, soft drinks, breads, pastas, beans, potatoes, rice and cereals. Carbohydrates are a common source of energy in living organisms; however, no carbohydrate is an essential nutrient in humans. Carbohydrates are not necessary building blocks of other molecules, and the body can obtain all its energy and other nutritional requirements from protein and fats.

The brain and neurons generally cannot burn fat for energy, but use glucose or ketones. Humans can synthesize some glucose (in a process called gluconeogenesis) from specific amino acids, from the glycerol backbone in triglycerides, and in some cases from fatty acids. Glucose is, however, a nearly universal and accessible and preferred source of calories. It is used first, either directly or indirectly (from glycogen in storage). Polysaccharides are also a common source of energy. Human beings can easily and quickly break down starches into glucose.

A commonly held belief among the public, and even among nutritionists, is that complex carbohydrates (e.g. starches) are digested more slowly than simple carbohydrates (sugars) and thus healthier, especially for Type 2 diabetics. However, sugar (sucrose, a disaccharide), contains 50% fructose which is does not raise blood sugar, while some carbohydrates (e.g. breads), are 100% processed and refined glucose, and raise blood sugar rapidly.

It is not sufficient, therefore, to buy foods that trumpet their containing “whole grains.” The primary ingredients (those listed first) may be “bleached all purpose flour” (a processed food), water and some form of sugar: dextrose, molasses, sucrose or HFCS (all highly processed), before whole grains are added. If you see them sprinkled on the surface of a loaf of bread, that surface has been browned and the whole grains adhered with brushed-on HFCS.

 N.B.: For the record, for those who would doubt my authority to make some of the representations made herein, this Retrospective was largely cribbed in 2011, some of it verbatim, from the Wikipedia entry for “Carbohydrate.”