What is a low-glycemic diet?

Overview

Mainstream organizations, such as the American Heart Association and the American Dietetic Association, endorse a unified set of guidelines for the optimum diet. According to these organizations, the majority of calories in the daily diet should come from carbohydrates (55 to 60 percent), fat should provide no more than 30 percent of total calories, and protein should be kept to 10 to 15 percent.

However, many popular diet books turn the standard diet upside down. The Atkins
diet, the Zone
diet, Protein Power, and other alternative dietary approaches
reject carbohydrates and advocate increased consumption of fat or
protein, or both. According to theory, the low-carbohydrate (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 choosing certain carbohydrates over others rather than restricting
carbohydrate intake.

Evidence suggests that carbohydrates are not created equal. Some carbohydrates,
such as pure glucose, are absorbed quickly and create a rapid, strong
rise in both blood sugar and insulin. However, other carbohydrates (such as brown
rice) are absorbed much more slowly and produce only a modest blood sugar and
insulin response. According to proponents of the low-GI diet, eating foods in the
latter category will enhance weight loss and improve health. However, despite some
promising theory, there is no solid evidence that low-GI diets enhance weight
loss.

Besides weight loss, preliminary evidence suggests that the low-GI approach
(or, even better, a related method called low-glycemic load, which is discussed
later in this article) may help prevent heart disease. The low-GI approach has
also shown promise for treating and possibly preventing diabetes.

What Is the Glycemic Index?

The precise measurement of the glucose-stimulating effect of a food is called its glycemic index. The lower a food’s GI, the less potent its effects on blood sugar (and, therefore, on insulin).

The GI 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 grams (g) of carbohydrates. Next, they give that amount of the food to a minimum of eight to ten people and measure the blood sugar response. (By using a group of people rather than one person, researchers can ensure that the idiosyncrasies of one person do not skew the results.) On another occasion, researchers also give each participant an equivalent amount of glucose and perform the same measurements. The GI of a food is then determined by comparing the two outcomes. For example, if a food causes one-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 GI, the better.

When scientists first began to determine the GI of foods, some of the results drew skepticism. It did not surprise anyone when jellybeans turned out to have a high GI of 80 (after all, jellybeans are mostly sugar). Also, it was not unexpected that kidney beans have a low GI of 27 because they are notoriously difficult to digest. However, when baked potatoes showed an index of 93, researchers were stunned. 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 people avoid potatoes.

There are other surprises hidden in the GI tables. For example, fructose (the sweetener in honey) has an extraordinarily low GI of 23, lower than brown rice and almost three times lower than white sugar. Candy bars also tend to have a relatively good (low) GI, presumably because their fat content makes them digest slowly.

It is difficult to predict the GI of a food without specifically testing it, but there are some general factors that can be recognized. Fiber content tends to reduce the GI of a food, presumably by slowing down digestion. For this reason, 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).

However, there are numerous exceptions to these rules. 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 GI to be meaningful, it has to be generally reproducible among people. In other words, if a potato has a GI of 93 in one person, it should have nearly the same GI when given to another person. Science suggests that the GI passes this test. The GI of individual foods is fairly constant among people, and even mixed meals have a fairly predictable effect according to most studies.

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 a different issue.

Following a Low-Glycemic-Index Diet

Following a low-GI diet is fairly easy. Basically, one should follow the typical diet endorsed by authorities such as the American Dietetic Association, but in doing so, one should choose carbohydrates that fall toward the lower end of the GI scale. Popular books such as The Glucose Revolution (1999) give a great deal of information on how to make these choices.

Do Low-Glycemic-Index Diets Aid in Weight Loss?

There are two primary theoretical reasons given why low-GI diets should help reduce weight. The most prominent reason given in books on the low-GI approach 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. However, there is little justification for the second part of this argument. Excess weight is known to lead to elevated insulin levels, but there is little meaningful evidence for the reverse: that reducing insulin levels will help remove excess weight.

Books on the low-GI diet give another reason for using their approach. They state that low-GI foods fill a person up more quickly than do high-GI foods and that they also keep one feeling full for longer. However, there is more evidence against this belief than for it.

The satiety index. 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 low-GI diet proponents. As it happens, foods with the worst (highest) GI index are often the most satiating, exactly the reverse of what low-GI-theory proponents would say. For example, the satiety index claims that potatoes are among the most satiating of foods. However, the GI analysis gave potatoes a bad rating. According to the low-GI theory, one should feel hunger pangs shortly after eating a big baked potato. In real life, this does not happen.

There are numerous other contradictions between research findings and the low-GI/high-satiety theory. For example, one study 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.

Some studies do seem to suggest that certain low-GI foods are more filling than high-GI foods. However, in these studies the bulkiness and lack of palatability of the low-GI foods chosen may have played a more important role than the foods’ GI. Thus, the satiety argument for low-GI diets does not appear to hold up to scrutiny.

Is the Glycemic Index the Right Measurement?

There is another problem with the low-GI approach: It is probably the wrong way to assess the insulin-related effects of food. The GI measures blood sugar response per gram of carbohydrate contained in a food, not per gram of the food. This leads to some odd numbers. For example, a parsnip has a GI of 98, almost as high as pure sugar. If taken at face value, this figure suggests that dieters should avoid parsnips. However, parsnips are mostly indigestible fiber, and a person would have to eat a few bushels to trigger a major glucose and insulin response.

The reason for the high number is that the GI rates the effects per gram of carbohydrate rather than per gram of total parsnip, and the sugar present in minute amounts in a parsnip itself is highly absorbable. The high GI rating of parsnips is thus extremely misleading. Books such as The Glucose Revolution address issues like this on a case-by-case basis by arguing, for example, that one can consider most vegetables “free foods” regardless of their GI. In fact, the same considerations apply to all foods and distort the meaningfulness of the scale as a whole.

A different measurement, the glycemic load (GL), takes this into account. The GL is derived by multiplying the GI by the percent carbohydrate content of a food. In other words, it measures the glucose/insulin response per gram of food rather than per gram of carbohydrate in that food. Using this system, the GL of a parsnip is 10, while glucose has a relative load of 100. Also, the GL of a typical serving of potato is only 27.

Scientific Evidence

Theory is one thing and practice is another. It is certainly possible that making sure to focus on low-GI or low-GL foods will help a person lose weight, even if the theoretical justification for the idea is weak. However, there is only preliminary positive evidence to support this possibility, and the largest and longest-term trial failed to find benefit.

In one of the positive 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 percent carbohydrates (preferably low-GI carbohydrates), 20 to 25 percent protein, and 30 to 35 percent fat. Calorie restriction was not emphasized. The low-fat group received instructions for a standard low-fat, low-calorie diet divided into 55 to 60 percent carbohydrates, 15 to 20 percent protein, and 25 to 30 percent fat. In about four months, participants on the low-GI diet lost about 4.5 pounds, while those on the standard diet lost just under 3 pounds.

This study does not say as much about the low-GI approach as it might seem. Perhaps the most obvious problem is that the low-GI diet used here was also a high-protein diet. It is possible that high-protein diets might help weight loss regardless of the GI of the foods consumed. (In fact, this is precisely what proponents of high-protein diets claim.)

Another problem is that participants were not assigned to the two groups randomly. Rather, researchers consciously picked what group each participant should join. This is a major flaw because it introduces the possibility of intentional or unintentional bias. It is quite possible, for example, that researchers placed adolescents with greater self-motivation into the low-GI group, based on an unconscious desire to see results from the study. This is not an academic problem, and modern medical studies always use randomization to circumvent it.

Finally, researchers made no effort to determine how well participants followed their diets. It might be that those in the low-fat diet group simply did not follow the rules as well as those in the low-GI diet group because the rules were more challenging. Despite these many flaws, the study results are still promising. Losing weight without deliberately cutting calories is potentially a great thing.

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

In yet another small study, this one involving overweight adolescents, a conventional reduced-calorie diet was compared with a low-GL diet that did not have any calorie restrictions. The results showed that simply by consuming 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, in a large and long-term study, an eighteen-month trial of 203 Brazilian women, the use of a low-GI diet failed to prove more effective than a high-GI diet. Additionally, a smaller study failed to find a low-GI diet more effective for weight loss than a low-fat diet except in people with high levels of circulating insulin.

Possible Health Benefits

There is some evidence that a low-GI diet (or, even better, a low-GL diet) might help prevent cancer and heart disease. The low-GI approach has also shown promise for preventing or treating diabetes.

Heart disease prevention. One large observational study evaluated
the diets of more than 75,000 women and found that those women whose diets
provided a lower GL had a lower incidence of heart
disease. In this study, 75,521 women age thirty-eight to
sixty-three years were followed for ten years. Each filled out detailed
questionnaires regarding her diet. Using this data, researchers calculated the
average GL of each participant. The results showed that women who consumed a diet
with a high GL were more likely to experience heart disease than those who
consumed a diet of low GL.

Other observational studies suggest that the consumption of foods with lower GL may improve cholesterol profile: specifically, reduced triglyceride levels and higher HDL (good cholesterol) levels. These effects, in turn, might lead to decreased risk of heart disease. However, other observational studies have found little or no relationship between heart disease and GI or GL.

These contradictory results are not surprising, but even if the observational study results were entirely consistent, it would not prove the case for a low-GI approach. Conclusions based on observational studies are notoriously unreliable because of the possible presence of unidentified confounding factors. For example, because there is an approximate correlation between fiber in the diet and GL, it is possible that benefits, when seen, are from 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. For example, in the foregoing large weight-loss trial, the low-GI diet failed to prove more effective than a high-GI diet in terms of weight loss. The results did suggest, though, that a low-GI diet can improve cholesterol profile. However, this study was not primarily designed to look at effects on cholesterol.

A study that primarily focused on this outcome followed thirty people with high lipid levels for three months. During the second month, low-GI foods were substituted for high-GI foods, while other nutrients were kept similar. Improvements were seen in total cholesterol, LDL (bad) cholesterol, and triglycerides, but not in HDL. A close analysis of the results showed that only participants who had high triglycerides at the beginning of the study showed benefit. Another controlled trial found that a high carb, low-GL diet optimized lipid profile compared with several other diets. However, another study found that low-fat and low-GI diets were about equally effective in terms of profile.

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
the 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 in healthy people. While these results are
preliminary, taken together they do suggest that consumption of low-GI foods might
have a beneficial effect on heart disease risk.

Low-GL diet and diabetes. Two large observational studies, one
involving men and the other involving women, found that diets with lower GLs were
associated with a lower rate of diabetes. For example, one trial followed 65,173
women for six years. Women whose diets had a high GL had a 47 percent increased
risk of developing diabetes compared with those whose diets had the lowest GL.
Fiber content of diet also makes a difference. People who
consumed a diet that was both low in fiber and high in GL had a 250 percent
increased incidence of diabetes.

However, as always, the results of these observational studies have to be taken with caution. It is quite possible that unrecognized factors are responsible for the results seen. For example, magnesium deficiency is widespread and may contribute to the development of diabetes; whole grains contain magnesium and are also low-GI foods. Therefore, it could be that the benefits seen in these studies are actually caused by increased magnesium intake in the low-GI group, rather than by effects on blood sugar and insulin.

Furthermore, one observational study found no connection between the glycemic values of foods and the incidence of diabetes. Another observational study did find a correlation between carbohydrate intake (especially pastries) and the onset of diabetes, but no consistent relationship with GI. Other studies have found no relationship between sugar consumption (a high-GI food) and diabetes onset.

Thus, reducing dietary GL may help prevent diabetes, but this is not known for sure. Whether or not low-GI diets can prevent diabetes, going on a low-GI diet might improve blood sugar control for people who already have diabetes. However, the benefits seem to be small at most.

Other Uses and Applications

Weak evidence hints that a low-GI diet might help prevent macular
degeneration. Although there are theoretical reasons to
believe that the use of white sugar and other high-GI foods might promote colon
cancer, a large observational study failed to find any association between colon
cancer rates and diets high in sugar, carbohydrates, or GL.

It has been proposed that low-GI foods may enhance sports performance. One study involving a simulated sixty-four-kilometer bicycle race found no performance differences between the use of honey (low GI) and the use of dextrose (high GI) as a carbohydrate source. However, another study did find benefit with the consumption of a low-GI snack before endurance exercise. Finally, one study compared a low-GL diet with a high-carb diet in people with acne and found evidence that the low-GL diet reduced acne symptoms.

Conclusion

The evidence that a low-GI diet will help one lose weight is not impressive. Its theoretical foundation is weak, and it appears to be using the wrong method of ranking foods regarding their effects on insulin. Conversely, however, there is no reason to believe a low-GI diet causes harm.

While the most popular low-GI-diet books, such as The Glucose Revolution and Sugar Busters (1995), 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, because the GI of lard is 0. Although it no longer seems that saturated fat is as harmful as it was once thought to be, a pure lard diet is probably not a good idea. Any diet book or other source that recommends achieving a low GI by consuming an extreme diet should be approached with caution.

Bibliography

Chiu, C. J., et al. “Dietary Glycemic Index and Carbohydrate in Relation to Early Age-Related Macular Degeneration.” American Journal of Clinical Nutrition 83 (2006): 880-886.

Clapp, J. F., and B. Lopez. “Low- Versus High-Glycemic Index Diets in Women: Effects on Caloric Requirement, Substrate Utilization, and Insulin Sensitivity.” Metabolic Syndrome and Related Disorders 5 (2007): 231-242.

Ebbeling, C. B., et al. “Effects of a Low-Glycemic Load vs Low-Fat Diet in Obese Young Adults.” Journal of the American Medical Association 297 (2007): 2092-2102.

Noakes, M., et al. “The Effect of a Low Glycaemic Index (GI) Ingredient Substituted for a High GI Ingredient in Two Complete Meals on Blood Glucose and Insulin Levels, Satiety, and Energy Intake in Healthy Lean Women.” Asia Pacific Journal of Clinical Nutrition 14, suppl. (2005): S45.

Pittas, A. G., et al. “The Effects of the Dietary Glycemic Load on Type 2 Diabetes Risk Factors During Weight Loss.” Obesity 14 (2006): 2200-2209.

Smith, R. N., et al. “A Low-Glycemic-Load Diet Improves Symptoms in Acne Vulgaris Patients.” American Journal of Clinical Nutrition 86 (2007): 107-115.

Tavani, A., et al. “Carbohydrates, Dietary Glycaemic Load, and Glycaemic Index, and Risk of Acute Myocardial Infarction.” Heart 89 (2003): 722-726.

Wu, C. L., and C. Williams. “A Low Glycemic Index Meal Before Exercise Improves Endurance Running Capacity in Men.” International Journal of Sport Nutrition and Exercise Metabolism 16 (2006): 510-527.