Why might your bad breath mean you need a plate of chips? Why add a splash of vinegar? And why do the wine-drinking French not get fat?

You thought you knew all there was to know about carbs, but this is where it gets interesting, particularly if you’re an endurance athlete who wants to be light and lean, yet full of energy. (And free of halitosis.)

Name: (Obvious, when you think about it). It’s called a carbohydrate because it usually contains carbon, with oxygen and hydrogen in the same proportion as in water.

Use: The primary function of carbohydrates is to provide energy for the body, especially the brain and the nervous system. Your liver breaks down carbohydrates into glucose (blood sugar).

Forms: Complex and Simple.

Simple carbohydrates are one, two, or at most three units of sugar linked together in single molecules. Simple carbohydrates include sugars such as fruit sugar (fructose), corn or grape sugar (dextrose or glucose), and table sugar (sucrose).

Complex carbohydrates are hundreds or thousands of sugar units linked together in single molecules. Complex carbohydrates include everything made of three or more linked sugars.

In the past, simple sugars were considered bad and complex carbohydrates good, but the picture is much more complicated than that. (See Glycemic Index.)

Simple or complex, the digestive system handles all carbohydrates in much the same way: it breaks them down (or tries to break them down) into single sugar molecules, since only these are small enough to cross into the bloodstream. It also converts most digestible carbohydrates into glucose, because cells are designed to use this as a universal energy source.

Identification: Simple sugars are easily identified by their taste: sweet. Complex carbs, such as potatoes, are pleasant to the taste buds, but not sweet.

Food Sources: The sugars and the starch-and-fibres.

Simple carbohydrates are found in processed and refined sugars including table sugar, honey and candy, and in fruits, vegetables, and milk products. Simple carbohydrates are more easily digested by the body than complex carbohydrates.

Complex carbohydrates are combinations of starch and fibre. Any food with starch contains complex carbohydrates, for example bread, cereals, pasta, rice, and starchy vegetables like potatoes.

Quantity: Carbohydrates are interchangeable with fats for energy requirements. Humans, like most mammals, convert amino acids of proteins and glycerol of fats to glucose, which is why you won’t find an RDA listing for carbs on the packaging. Of course, a person on a low-calorie diet requires a minimum of about 50-75 g a day of carbohydrate to prevent ketosis. (A condition in which your body goes into starvation, due to a lack of carbs and therefore a depletion of stored glycogen, and feeds itself from fatty acids rather than glucose. Ketones are produced by the breakdown of these fatty acids. Their smelly side effect: halitosis. As Atkins Dieters will testify.)

Vitamins: Vitamins of the B group take part in the metabolism of carbohydrates at various stages, which is why carbohydrates increase the demand for the B vitamins. Mother Nature plays along: she’s combined vitamin B with carbohydrates in most grains and plants. Of course, modern methods of food refining, such as polishing rice, can lead to vitamin B deficiency and impaired metabolism of carbohydrates.

Glycemia: This is the concentration of glucose in the blood. It is usually expressed in milligrams per deciliter (mg/dl).

Glycemic Index (GI): This newish system for classifying carbohydrates calls into question many of the old assumptions about how carbohydrates affect health. It measures how fast and how far blood sugar rises after you eat a food that contains carbohydrates. Think of it as a ranking system for carbs.

White bread, for example, is converted almost immediately to blood sugar, causing it to spike rapidly. It''s classified as having a high glycemic index. Brown rice, in contrast, is digested more slowly, causing a lower and gentler change in blood sugar. It has a low glycemic index.

Carbohydrates that break down rapidly during digestion have the highest glycemic indices. These carbs require less energy to be converted into glucose, which results in faster digestion and a quicker increase of blood glucose. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic response equates to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids.

Energy and endurance: A low GI food will release energy slowly and steadily and is generally appropriate for everyone, especially diabetics, dieters and endurance athletes. A high GI food will provide a rapid rise in blood sugar levels and is suitable for energy recovery after endurance exercise.

Bread: The glycemic effect of foods depends on a number of factors such as the type of starch, fat content of the food and increased acidity of the meal (adding vinegar for example, will lower the GI). The presence of fat or dietary fibre can inhibit carbohydrate absorption, in this way lowering the GI. Unrefined breads with higher amounts of fibre, generally have a lower GI value than white breads, but, while adding butter or oil will lower the GI of bread, the GI ranking does not change: there is still a higher blood glucose curve after white bread than after a low GI bread such as pumpernickel. But… Many brown breads are treated with enzymes to soften the crust. This makes the starch more accessible and raises the GI, with some brown breads even having GI values over 100.

Good news: Alcoholic beverages have been reported to have low GI values. But don’t be fooled: beer has a moderate GI. Another potential reason French women don’t get fat: recent studies have shown that the consumption of an alcoholic drink prior to a meal reduces the GI of the meal by approximately 15%.

Processed carbs: One of the most important factors that determine a food''s glycemic index is how highly processed its carbohydrates are. Why? Processing carbohydrates removes the fibre-rich outer bran and the vitamin- and mineral-rich inner germ, leaving mostly the starchy endosperm.

Glycemic Load: (This is where it gets complicated and counterintuitive at times.) Some foods that contain complex carbohydrates, such as potatoes, quickly raise blood sugar levels, while some foods that contain simple carbohydrates, such as whole fruit, raise blood sugar levels more slowly. The reason: that old quantity versus quality issue.

Watermelon is the classic example: this sweet fruit has a high GI, but it’s still mostly water, with only a bit of carb. So how much will it really impact our blood sugar levels, even though the GI is so high?

Because of this, researchers have developed a new way of classifying foods that takes into account both the amount of carbohydrate in the food and the impact of that carbohydrate on blood sugar levels. This measure is called the glycemic load. A food''s glycemic load is determined by multiplying its glycemic index by the amount of carbohydrate it contains.

The simple message: Replace highly processed grains, cereals, and sugars with less processed whole-grain products. And only eat potatoes - once on the list of preferred complex carbohydrates - occasionally because of their high glycemic index and glycemic load.

Search: One of the most comprehensive lists of the glycemic index of foods was published in the July 2002 issue of the ‘American Journal of Clinical Nutrition’. A searchable database maintained by the University of Sydney is available online at www.glycemicindex.com.

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