Understanding Carbohydrates - A Beginner's Guide

The New Basics About Carbohydrates

Sugar coated donuts
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What Are Carbohydrates?

Carbohydrates, along with fats and proteins, are one of the three main classes of food. Carbohydrates are organic compounds consisting mainly of sugars, starches, and fiber.

Plants make carbohydrates during photosynthesis and store them as any of the saccharides (sugars) described below. They are used primarily for energy in the body. If carbohydrate isn't used in short order, it is stored.

A certain amount can be stored in the liver and muscles as glycogen, and the rest is stored as fat. Unlike protein and essential fats, our bodies can get along without dietary carbohydrate if needed.

These are the types of carbohydrates:

Monosaccharides and Disaccharides (Sugars or Simple Carbohydrates)

Monosaccharides are the simplest types of sugars, and are building blocks for the larger carbohydrate molecules. The most common is glucose, but fructose, galactose, and others are also in this group. All the carbohydrate in our food must be broken down into monosaccharides if it is to be absorbed by the small intestine, and these sugars are very readily absorbed as soon as they hit the intestines.

Disaccharides are combinations of two monosaccharide molecules. For example, a molecule of sucrose (table sugar) is made up of one molecule of glucose and one of fructose. Lactose, the sugar in milk, is made up of glucose and galactose.

Disaccharide molecules must be split up to be absorbed. Lactose intolerance occurs when people lack the enzyme needed to break lactose into the monosaccharide molecules that make it up. Sucrose is quickly broken down and absorbed.


“Oligo” is a prefix from Greek meaning “few”, so oligosaccharides are a few monosaccharides together.

There is no hard cutoff, but usually carbohydrates with 3 to 10 monosaccharides fall into this category. Oligosaccharides are found in certain plant foods -- legumes, onions, asparagus, and others. Many oligosaccharides are poorly absorbed in the small intestine and have drawn recent attention as “prebiotics” – food for certain beneficial bacteria in the colon.

More about oligosaccharides in Part Two of this series.

Polysaccharides (Starches or Complex Carbohydrates)

Longer chains of glucose molecules, often with many branches, are called polysaccharides. The configuration of the molecules has a lot to do with how quickly they are digested. One recent categorization of starches is to label them according to how the human body digests them.

Rapidly Digested Starch (RDS): As the label implies, rapidly digested starch breaks down quickly into simple sugars. Carbohydrates with a lot of RDS tend to have a high glycemic index -- that is, they cause blood glucose to rise quickly and stay high longer.

Some starch digestion actually begins in the mouth, with an enzyme in the saliva. There are many starchy foods, such as potatoes, which have higher glycemic index numbers than table sugar. Beans, on the other hand, have very little rapidly digested starch, and have a low glycemic index. Most of the starch in beans is split between the next two categories.

Slowly Digested Starch (SDS): Slowly digested starch breaks down over a period of time in the small intestine -- up to two hours. Often this is because it is physically difficult to break it into small bits. For example, the starch in whole barley, which is difficult to chew and otherwise mechanically break down, is almost half of the SDS type. Therefore, barley has a low glycemic index. In some foods, the slow digestion has to do with enzymes needed to digest the food.

Resistant Starch: Resistant starch actually reaches the large intestine (colon) without being broken down into sugars at all. In the colon, it is fermented by the bacteria there. The products of this fermentation, primarily short-chain fatty acids, contribute to colon health and may have other positive effects, including lowering insulin resistance and a lessened glycemic response to carbohydrates eaten later (the so-called “second meal effect”).

More about starch in Parts Three and Four of this series.


Recent research, including that into resistant starch, has brought the definition of fiber up for a certain amount of debate. Traditionally, fiber has been thought of as carbohydrate that reaches the colon undigested. Now, however, we know that along with resistant starch certain kinds of fiber are fermented in the colon -- in other words, digested further by the bacteria there, producing products useful for the body. Should it still be called fiber? What about undigested oligosaccharides which also feed bacteria in the colon? Fiber is turning out to be much more complex than we realized, and new interesting questions are being investigated about it.

More about this in Part Five of this series.

Return to Introduction
Part Two: Oligosaccharides and Prebiotics
Part Three: What You Need To Know About Starches
Part Four: Don't Resist This Starch!
Part Five: Surprising Facts About Fiber
Part Six: Seven Facts to Remember About Carbs

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