The GI index does not take into account the number, amount or type of fruits or alcoholic beverages included in this diet.

The GI value of a meal featuring every major food group is difficult to predict. Consumers should note that the GI diet is not approved by the U.S. Department of Agriculture (USDA) and that following the USDA's Food Pyramid may be the safest way to eat healthfully, although some might disagree.

Some junk foods, including chocolate, potato chips, and many candy bars, have low GI indexes; however, this may be because of their serving sizes. The safety of this diet is not yet well-researched, but consumers should avoid excessive consumption of these generally unhealthy products if they choose to abide by the GI diet.

Most of the information about the GI diet available on the Internet is also promoting a product related to this weight loss plan. Consumers should critically evaluate the bias of information about the GI diet available on the World Wide Web.

Foods that cause a large and rapid glycemic response or that have a high glycemic index will elevate energy and mood at initial phase, but will also release an excess amount of insulin, and drive blood sugar back down too low. Therefore, these foods may lead to a cycle of increased fat storage, lethargy and even more hunger.

The theory behind the glycemic index is to minimize insulin-related problems by identifying and avoiding foods that have the greatest effect on blood sugar, especially for those with diabetes.

Low GI diets have been shown to improve both glucose and lipid levels in people with diabetes (type 1 and type 2) and reduce insulin levels and insulin resistance. Low GI diets may also have benefits for weight control because they help control appetite and delay hunger.

There are several limitations of this diet. There is a scarcity of GI data, with only 5% of the foods having GI values. There is a wide variation in GI measurements; for example, baked Russet potatoes have been tested with a GI as low as 56 and as high as 111. GI values can be affected by preparation method, and by combination with other foods. There may be individual differences in glycemic response.

Reliance on GI may lead to overconsumption. GI should be used only for a rating of a food's carbohydrate content. If it is used as the sole factor for determining diet, individuals may easily end up overconsuming fat and total calories.

A major criticism of the GI diet is that the reference food used to determine the glycemic index, white bread, is popular only within a particular culture. Even proponents of this diet acknowledge that the glycemic index tests were developed for people who eat at least 50 grams of carbohydrates per day. Results for people whose foods do not feature large amounts of carbohydrates have not been investigated.