The American Heart Association, the American Dietetic Association and other prominent groups endorse a unified set of guidelines for the optimum diet. These organizations exhort that the majority of calories in the daily diet should come from carbohydrates (55% to 60%); fat should provide no more than 30% of total calories; and protein should be kept to 10% to 15%.

In contrast, many popular diet books turn the standard diet on its head. As described in the entry on low-carbohydrate diets, the Atkins diet, the Zone diet, Protein Power, and other alternative dietary approaches turn thumbs down on carbohydrates. Instead, they advocate increased consumption of fat and/or protein. According to theory, the low-carb approach aids in weight loss (and provides a variety of other health benefits) by reducing the body's production of insulin.

The low glycemic index (low-GI) diet splits the difference between the low-carb and low-fat approaches. It maintains the low-carb diet's focus on insulin, but it suggests differentiation of certain carbohydrates over others rather than restricting carbohydrate intake.

All carbohydrates are not created equal when the evidence is examined. Some, such as pure glucose, are absorbed quickly and create a rapid, strong rise in both blood sugar and insulin. Others (such as brown rice) are absorbed much more slowly and produce only a modest blood sugar and insulin response. The proponents of the low-GI diet claim that eating foods in the latter category will enhance weight loss and improve health. However, as we shall see, there is as yet no solid evidence that low-GI diets enhance weight loss.

The precise measurement of the glucose-stimulating effect of a food is called its glycemic index. A food that has less effect in increasing blood sugar level (and therefore insulin) is given a lower glycemic index.

The glycemic index of glucose is arbitrarily set at 100. The ratings of other foods are determined as follows. First, researchers calculate a portion size for the food to supply 50 g. of carbohydrates. Next, they give that amount of the food to at least eight to ten people and measure the blood sugar response. (a group is used rather than an individual to ensure that the idiosyncrasies of one individual don't skew the results.) On another occasion, researchers also give each participant an equivalent amount of glucose and perform the same measurements. The glycemic index of a food is then determined by comparing the two outcomes. For example, if a food causes half of the blood sugar rise of glucose, it is assigned a GI of 50; if it causes one-quarter of the rise, it is assigned a GI of 25. The lower the Glycemic index, the better.

The determination of the Glycemic index has resulted in some surprises. Baked potatoes came back with a reading of 93. This rating is higher than that of almost all other foods, including ice cream (61), sweet potatoes (54), and white bread (70). Based on this finding, low-GI diets recommend that you stay largely away from potatoes. However, the concern regarding potatoes is probably unnecessary. See the discussion of glycemic load below.

A food must be tested to determine its actual glycemic index, but there are some general guidelines that are recognized. Fiber content tends to reduce the glycemic index of a food, presumably by slowing down digestion. Therefore, whole grains usually have a lower GI score than refined, processed grains. Fat content also reduces GI score. Simple carbohydrates (such as sugar) often have a higher GI score than complex carbohydrates (such as brown rice).

But, numerous exceptions to these rules exist. Factors such as the acid content of food; the size of the food particles; and the precise mixture of fats, proteins, and carbohydrates can substantially change the GI measurement. For a measurement like the glycemic index to be meaningful, it has to be generally reproducible between subjects. In other words, if a potato has a glycemic index of 80 in one person, it should have closely the same glycemic index when given to another person. The GI passes this test. The glycemic index of individual foods is fairly constant between people.

Thus, the GI of a food really does indicate its propensity to raise insulin levels. Whether a diet based on the index will aid in weight loss, however, is another story.

A low glycemic index diet is relatively easy to follow. Basically, you follow the typical diet endorsed by authorities such as the American Dietetic Association, but you choose carbohydrates that fall toward the lower end of the glycemic index scale. Books such as The Glucose Revolution give a great deal of information on how to make these choices, but do low glycemic Index diets aid in weight loss? There may be problems with this theory.

There are two primary theoretical reasons given why low-GI diets should help reduce weight. The most prominent reason involves insulin levels. Basically, these books show that low-GI diets reduce insulin release, and then take almost for granted the idea that reduced insulin levels should aid in weight loss. Unfortunately, there is little justification for the second part of this contention. Excess weight is known to lead to elevated insulin levels, but there is almost no meaningful evidence for the opposite: that reducing insulin levels will help remove excess weight.

Books espousing the low-GI diet give another reason for their approach. They claim that low-GI foods fill you up more quickly than high-GI foods, and also keep you feeling full for longer. As we shall see later, there is more evidence against this belief than for it.

A measurement called the Satiety Index assigns a numerical quantity to the filling quality of a food. These numbers are determined by feeding people fixed caloric amounts of those foods, and then determining how soon they get hungry again and how much they eat at subsequent meals. The process is similar to the methods used to establish the GI index.

The results of these measurements do not corroborate the expectations of the proponents of low-GI diets. It turns out, foods with the worst (highest) GI index are often the most satiating, exactly the reverse of what low-GI theory proponents would say.

A case in point, the Satiety Index tells us that potatoes are among the most satiating of foods. However, as noted above, the GI analysis gave potatoes a bad rating. According to the low-GI theory, you should feel hunger pangs shortly after eating a big baked potato. In real life, that doesn't happen.

Other contradictions between research findings and the low-GI/high-satiety theory include one study that found no difference in satiety between fructose (fruit sugar) and glucose when taken as part of a mixed meal, even though fructose has a GI more than four times lower than glucose.

Thus, the satiety argument for low-GI diets doesn't appear to hold up to inspection of the facts. So, is the glycemic index even the right measurement? Is it the wrong way to assess the insulin-related effects of food? The evidence would suggest this to be the case. Rather than making judgements from the GI, a more reasonable approach would be to use the measurement of glycemic load which measures the real effect of the food in question.




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