Brain health - food: Glycemic index and the digestible carbohydrates

The rate of carbohydrate digestion, and why it matters

(under development - see what's next)

Carbohydrate basics

Carbohydrates encompass a number of sugars, ranging from simple to complex, and a group of starches. Glucose, the simplest sugar, is special - it is simply chemically stored solar energy, made from red and blue light (the green is reflected away, making many plants look green!), carbon dioxide, and water. Glucose is the basic and preferred energy source for many plant and animal cells.

When we eat carbohydrates, their digestion results in production of the simplest carbohydrate, glucose. In our bodies, a steady and large amount of glucose is needed to keep organs working. Our brain requires an unusually large amount of the total available glucose in the blood. Our big brains are not free - they must be fed!

Because sugars attract water, they are not good storage forms for carbohydrates, so many plants convert sugars into starches, which are more stable, and are often stored in plant roots or in the stored food supply in seeds. These starches are primarily in two forms: amylose and amylopectin. This is important to us because these starches, commonly encountered in our plant foods, do not digest at the same rate. (The same is true of the various sugars, as well.)

Amylose is digested significantly more slowly, and this fact becomes a major factor in determining the glycemic index of a food. High-amylose starchy foods (like Basmati rice) have a lower glycemic index then other such foods (like the sticky rice more commonly eaten in Japan).

Animals have their own form of converted sugar (starch) - glycogen, which can be converted to glucose quickly (giving it a high glycemic index). We generally store enough glycogen in our liver for about 48 hours of sedentary living. If we're active during this time, we consume this stored energy proportionately faster. A marathon runner, for example, will use up his or her stored glycogen at some point during this long run, at which point running will become significantly more difficult - runners refer to this as "hitting the wall".

When excess sugar is available - sugar not needed for immediate metabolic consumption, it may be converted into nature's most efficient form of stored energy - fat. A gram of fat has about twice the calories of a gram of carbohydrates [1]. (For more information about fats we eat, see the notes on Oils.

The Glycemic Index (GI)

The GI of a food or meal is a number from 0 to somewhat over 100 which is used to indicate how a given food affects blood glucose. The higher the glycemic index (GI) of a food, the fast its ingestion results in an increase in blood glucose. It's a very practical measure of the effect a food has, with some important implications.

As blood glucose rises, our body's introduction of the hormone insulin into the blood also rises. Insulin facilitates transport of glucose into body tissues, but it also helps the conversion of glucose not immediately needed into fat, as well as having damaging effects on arteries and helping to promote the growth of tumors. It is appearing increasingly likely that one effect of having to produce frequent surges of insulin may well be that body tissues become increasingly insensitive to insulin [2]. Thus, this well may be a pathway to adult-onset diabetes (type II) - there is strong epidemiological evidence for this [3].

How the GI of a food is determined

A measured quantity of a test food is fed to a group of research volunteers. Each has previously had their response to glucose determined (it varies a bit from person to person). The effect of the test food on blood glucose levels is determined in the hours following ingestion. This effect is determined on several days (because an individual's reaction to various foods also varies from day to day). The effect of the test food is compared to that of glucose (which has a rated GI of 100) averaged across 8-10 people, resulting in a food's GI.

Factors affecting a food's GI

To apply GI ratings for foods to everyday life, it is critical to understand the several factors which determine the rate at which a food may cause an increase in blood sugar:

• The chemical nature of the carbohydrate in a food. Simple sugars act more quickly; so do starches with more branches in their molecule (amylopectin as opposed to amylose (see carbohydrates).
• The physical nature of the carbohydrate food. Bread, for example, has a generally high GI because it is made from finely ground grain (flour), and is usually acted upon by yeast during its preparation. Both of these factors act to create thin-walled bubbles of dough, and this makes digestion very much more easy, and thus faster. Pasta, also made from flour, has a much lower GI because it is ingested in rice-sized particles (after chewing); it can be made even slower to digest if it is not over-cooked. [4]
• The foods with which a given carbohydrate food is combined when eaten. Certain food elements act to slow carbohydrate digestion: both fiber and fat have this effect (this is not a reason to add fat to your diet, however!). So does acid in a food. All three of these slow the rate of transport of food through the digestive tract, which lowers the GI of intestinal contents [5].

Glycemic load

As it turns out, it's more useful to pay attention to the GI of a meal than to that of an individual food. This quality is known as the glycemic load of a meal. Mixed in your stomach, the foods of a meal become in essence a single food - one you made yourself. How quickly that meal changes your blood sugar is its glycemic load. It was mentioned above that pasta (especially if cooked right) has a lower GI than bread. Beans, which contain substantial amounts of carbohydrate, are far lower, because of all the fiber they contain.

Animal proteins have low GI values, but don't make an adequate diet in our culture because of what they do not contain - all the valuable phytonutrients contained in certain plant foods. [6]

What's next on this page...

• More information about the GI of individual foods - in a format you can print and use in your kitchen.
• More information about how to use the Glycemic load concept to manage your food consumption.
• ...and more!

Notes

[1] Weil (2000), chapter two, provides us with an excellent summary of the basics of human nutrition. He only occasionally gets a bit too detailed for ordinary readers, and he always has a good point he's trying to make. This reference is highly recommended.

[2] Weil, A. (2000), pp. 51-53.

[3] Weil, A. (2000), pp. 62-63.

[4] Weil, A. (2000), p. 53.

[5] Weil, A. (2000), p. 54.

[6] Hyman (2006), pp. 45-48.

References

Hyman, M. (2006). Ultra-metabolism - the simple plan for automatic weight loss. New York: Scribner.

Weil, A. (2000). Eating well for optimum health: The essential guide to food, diet, and nutrition. New York: Knopf.