The glycemic index measures blood sugar response per gram of carbohydrate contained in a food, not per gram of the food, and leads to some odd numbers. For example, a parsnip has a glycemic index of 98, almost as high as pure sugar. If taken at face value, this figure suggests that dieters should avoid parsnips like the plague. In fact, parsnips are mostly indigestible fiber, and you would have to eat a few bushels to trigger a major glucose and insulin response.



This high number results because the glycemic index rates the effects per gram of carbohydrate rather than rating the effects per gram of total parsnip. The sugar present in minute amounts in a parsnip itself is highly absorbable. The high glycemic index rating of parsnips is thus extremely misleading. Books such as The Glucose Revolution take care of issues like this on a case-by-case basis by saying, for example, that you can consider most vegetables free foods regardless of their glycemic index. But in fact the same considerations apply to all foods and distort the meaningfulness of the scale as a whole.

This is taken into account by a different measurement, the glycemic load (GL). The GL is derived by multiplying the glycemic index by the percent carbohydrate content of a food. Therefore, it measures the glucose/insulin response per gram of food rather than per gram of carbohydrate in that food. The glycemic load of a parsnip is 10, while glucose has a relative load of 100. And remember our potato problem, that terrible goblin of GI diets? The glycemic load of a typical serving of potato is only 27, not such a goblin at all. These kind of numbers make a lot more sense.

So is there direct evidence suggesting low GI diets support weight loss? It is certainly possible that focusing on low glycemic-index or low glycemic-load foods will help you lose weight, even if the theoretical justification for the idea is weak. However, there is only preliminary evidence to support this possibility. The studies commonly cited, although promising, are too preliminary to prove much.

In one of these studies, 107 overweight adolescents were divided into two groups: a low-GI group and a low-fat group. The low-GI group was counseled to follow a diet consisting of 45% to 50% carbohydrates (preferably low-GI carbohydrates), 20% to 25% protein, and 30% to 35% fat. Calorie restriction was not emphasized. The low-fat group received instructions for a standard low-fat, low-calorie diet divided up into 55% to 60% carbohydrates, 15% to 20% protein, and 25% to 30% fat. Over a period of about 4 months, participants on the low-GI diet lost about 4.5 pounds, while those on the standard diet lost just less than 3 pounds.

However, this study does not say as much about the low-GI approach as one might think. The most obvious problem is that the low-GI diet used here was also a high-protein diet. It could be that high-protein diets help weight loss regardless of the glycemic index of the foods consumed as claimed by the proponents of high-protein diets.

The study was also flawed in that participants were not assigned to the two groups randomly. Instead, researchers consciously picked which group each participant would join. This major flaw introduces the possibility of intentional or unintentional bias. It is quite possible the researchers placed adolescents with greater self-motivation into the low-GI group, based on an unconscious desire to see results from the study. Modern medical studies always use randomization to prevent this kind of bias.

Finally, researchers made no effort to determine if participants followed their diets. It might be that those in the low-fat diet group simply didn't stick to the rules as well as those in the low-GI diet group because they found the rules were more challenging.

In another study, 30 overweight women with excessively high insulin levels were put on either a normal low-calorie diet or a diet that supplied the same amount of calories but used low-GI foods. The results over 12 weeks showed that women following the low-GI diet lost several pounds more than those following the normal diet.

Another small study involved overweight adolescents in which a conventional reduced calorie diet was compared against a low-glycemic load diet that with no calorie restrictions. The results showed that simply by sticking to low GI foods, without regard for calories, the participants on the low GI diet were able to lose as much or more weight as those on the low calorie diet.

However, conclusions based on observational studies are notoriously unreliable due to the possible presence of unidentified confusing factors. For example, because there is an approximate correlation between fiber in the diet and glycemic load, it is possible that benefits, when seen, are really due to fiber intake instead. Factors such as this one may easily obscure the effects of the factor under study, leading to contradictory or misleading results.

Intervention trials (studies in which researchers actually intervene in participants' lives) are more reliable, and some have been conducted to evaluate the low-GI diet. One such study followed 30 people with high lipid levels for three months. Low-GI foods were substituted for higher-GI foods during the 2nd month, while other nutrients were kept similar. Improvements were seen in total cholesterol, LDL cholesterol, and triglycerides, but not in HDL. A close analysis of the results showed that only patients who had high triglycerides at the beginning of the study showed benefit. Another controlled trial found that a high carbohydrate, low glycemic load diet optimized lipid profile as compared to several other diets.

Another approach to the issue involves analysis of effects on insulin resistance. Evidence suggests that increased resistance of the body to its own insulin raises the risk of heart disease. One study found that use of a low-GI diet versus a high-GI diet improved the body's sensitivity to insulin in women at risk for heart disease. Similar results were seen in a group of people with severe heart disease and a group of healthy people.

The evidence that a low-GI diet will help you lose weight is not yet very impressive. Its theoretical foundation is weak, and it appears to be using the wrong method of ranking foods regarding their effects on insulin. However, there's no evidence showing a low-GI diet causes harm. If you find that you lose weight with a low-GI diet, stick with it.

However, while the most popular low-GI diet books (The Glucose Revolution, Sugar Busters) recommend a diet that is generally reasonable and should be safe, it is easy to design some fairly extreme low-GI diets. For example, a diet consisting of nothing but lard would be a very, very low-GI diet, since the glycemic index of lard is 0. While it no longer seems that saturated fat is as harmful as once thought, a pure lard diet is probably not a good idea. If you run across a diet book that recommends achieving a low glycemic index by consuming an extreme diet, approach it with caution.