How the Low-Glycemic-Index Diet Works

Summarized by Robert W. Griffith, MD
May 3, 2007

Summary

It's become recognized that low-glycemic-index diets are likely to have important health benefits, due to their avoidance of the high-low blood glucose swings induced by high-glycemic-index foods.

Introduction

Diet fads have come and gone for the last 100 years. But with the burgeoning obesity epidemic and the increasingly rapid spread of advertising the life-cycles have become shorter. For those trying to lose weight, the only really important mantra is "calories in - calories out = calories (fat) retained". However, better understanding of the metabolic processes has allowed us to give more attention to what is called the glycemic index of food. The present state of our knowledge is summarized in a Clinical Update appearing in the medical journal Lancet; here's a summary of the summary.

What Is the Glycemic Index?

The glycemic index (GI) is a ranking of carbohydrates on a scale according to the extent to which they raise blood sugar levels during the 2 hours after they are eaten. Sugar is given a score of 100. (In some methods white bread is used as the reference in which case glucose would be scored at 140.) Most breads, rice, breakfast cereals and potato products have a high GI, because processing methods allow the starch to become fully hydrated and then hydrolyzed into glucose in the digestive tract. Some experts refer to the glucose load (GL), which is the average GI multiplied by the amount of carbohydrate, and which takes into account the differing amounts of carbs in various foods.

The Effects of a High-GI Meal

A high-GI meal rapidly leads to high blood glucose and high blood insulin levels (insulin being secreted in response to the high glucose). At the same time, such a meal inhibits glucagon secretion (glucagon is a hormone that raises the level of glucose in the blood that's secreted by the alpha cells of the pancreas). The higher insulin and lower glucagon secretions are an attempt by the body to maintain normal physiology (this is known as homeostasis). The changes promote the uptake of nutrients in the liver, muscle, and fat, and suppress glucose output by the liver. Within 60 minutes the blood glucose begins to fall, and the release of fatty acids from the body fat is suppressed; these effects stimulate hunger and, if the opportunity is given, cause overeating. It's also thought that the glucose 'high' (and insulin 'high') followed by the glucose 'low' can increase the risk for diabetes, cardiovascular disease, and cancer. There's no evidence from clinical studies that a high-GI diet has any health benefits - quite the reverse.

A Little History

Homeostasis - the body's attempts to maintain the physiological status quo - evolved hundreds of years before the modern era when high-GI foods became available. Grain products and concentrated sugars only became popular with the agricultural revolution in the late 18^th and 19^th centuries. Increased processing of carbohydrates has led to breakfast cereals, sugared sodas, desserts, and the like, which are responsible for many of the nutritional problems of today. Man's sweet tooth can only be satisfied to a limited extent by sugar substitutes, and such surrogates are not enough to balance the other high-GI foods consumed nowadays - bread, cereals, and potatoes.

The Low-GI Diet

Low-GI foods, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin levels, and lessened glucagon secretion. The excessive swings in postprandial glycemia (i.e. the glucose level after a meal) are avoided. There's now good evidence that a low-GI diet has proven benefits for health, and no ill-effects have been described.

There are some uncertainties about trying to adhere to a low-GI diet if you're not a professional dietician. Food labels don't usually carry the GI (or the GL) of the food. Then again, the glycemic response may change when several foods are eaten in combination, as in a typical meal. And the concept is a little complicated for the general public to grasp immediately. In spite of these difficulties, it should be possible for the essentials of the low-GI diet to be explained in a simple fashion.

The USDA Food Pyramid is now well known to many consumers. A Low-GI Pyramid places greater emphasis on fruits, vegetables, lean protein, nuts and legumes. Also, healthy oils (e.g. olive oil) are welcomed, while grains (both unrefined and refined) are more restricted. Foods to avoid (or eat sparingly) are the high glycemic index foods. The pyramid looks like this:

Source

• Clinical Update: the low-glycemic-index diet DS. Ludwig, Lancet, 2007, vol. 369, pp. 890--892

Related Links
The Glycemic Index Diet - Does It Help in Diabetes?
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