Effects of a Low-Glycemic Load Diet on Resting Energy Expenditure and Heart Disease Risk Factors During Weight Loss

Abstract & Commentary

Synopsis: The effect of caloric restriction upon metabolism was gated by whether sugars or fats were preferentially restricted. A low-glycemic load diet (glucose restriction) yielded more favorable biochemical profiles than restricting fats.

Source: Pereira MA, et al. JAMA. 2004;292: 2482-2490.

Glycemic load refers to the effect of food intake upon subsequent blood glucose levels. All else being similar, a lower glycemic load diet results in a lower postprandial blood glucose and better satiety. In contrast, a high-glycemic load diet appears to elicit hormonal changes that stimulate postprandial hunger and, therefore, increases postprandial food intake. High-glycemic load diets therefore may predispose to obesity.

Resting energy expenditure is essentially the same as what used to be termed basal metabolic rate. The higher the basal metabolic rate, the more calories the body burns independent of physical or mental exertion. It has been argued that individuals with a genetically endowed lower basal metabolic rate are prone to obesity and that they have difficulty losing weight because their BMR falls even lower during calorie restriction.

The present study was undertaken to determine if the kinds of calories that are restricted during weight loss matter in terms of basal metabolic rate and lipoprotein profile. Two types of energy restriction (ie, diets) were compared. One diet was low in fat but had a high glycemic index and was similar to what is currently recommended. The other diet had a low glycemic index but was higher in fat content. In the short term, both diets led to equivalent amounts of weight loss in overweight individuals with a body mass index > 27.

The study found that a lower fat, higher glycemic diet (as compared to higher fat, lower glycemic diet) led to higher insulin and glucose levels (greater insulin resistance) and lower basal metabolic rate (lower resting energy expenditure) despite equivalent weight loss and similar changes in body composition. The lipoprotein profile was better in those who consumed a higher fat but low-glycemic load diet. Pereira and colleagues speculate that the small changes associated with a higher fat and low-glycemic load diet, when summed across a longer time frame, would make a considerable difference in health outcomes and weight change.

Comment by Sarah L. Berga, MD

A subtitle for this article might be: Metabolism Made Simple. Pereira et al have done an artful job of making an exceedingly complicated topic understandable. This is a topic of considerable interest to patients. Patients seem to know that how they diet matters, but they would also like to be reassured that their intuitions are on target. Indeed, with the flood of recommendations available, it is extremely confusing for physicians and patients alike to make sense of all the hype. Enter the present clinical investigation. Not only is this study expertly and cleverly designed, the results are also simply conveyed. If you have any interest in this topic, read this article. It will be worth your investment in time and cognitive energy. I have heard many patients tell me that when they are dieting they are very "sensitive" to food types. Many report a sense of malaise when they eat the "wrong" foods. The present study found that subjects who ate a lower glycemic, higher fat diet has less malaise and less hunger. It appears that they were able to sustain their dieting for longer as well.

It has been hypothesized that the reason patients do not continue to lose weight during dieting is that the body "defends against" ongoing weight loss by decreasing resting energy expenditure (basal metabolic rate). A variety of physiological mechanisms may exist to limit weight loss in the face of ongoing caloric restriction, but the present study was not designed to address the topic of metabolic mechanisms. Instead, Pereira et al sought to answer the question as to which diet is best for forestalling this otherwise important adaptive response in resting energy expenditure to self-imposed famine. Their results suggest that this compensatory physiological response to fuel restriction, ie, failure to continue to lose weight due to slowing of basal metabolic rate, is gated in part by dietary composition. Paradoxically, higher fat diets yield higher resting energy expenditure and therefore, across longer time intervals, should afford greater weight loss. Conventional wisdom dictates just the opposite, ie, lower fat diets are supposed to result in both better lipoprotein profiles as well as better metabolic outcomes. The present study challenges this dictum by showing that diets with low-glycemic load but higher fat content improve lipoprotein profiles and preserve

basal metabolic rate, thereby allowing ongoing weight loss. Further, Pereira et al point out that individuals experiencing a larger decline in resting energy expenditure during weight loss may feel more fatigued, colder, and hungrier and that these symptoms make adherence with dieting more difficult. In summary, the higher fat, lower glycemic load diet is better on all counts. Dieting may also compromise mood and cognitive powers, but the present study did not examine these end points. It remains to be seen, then, which diets are best for key brain outputs, but my limited clinical and personal experience would suggest that low-glycemic, higher fat diets are better for the brain, too!

Sarah L. Berga, MD, James Robert McCord Professor and Chair, Department of Gynecology and Obstetrics, Emory University School of Medicine, is Associate Editor for OB/GYN Clinical Alert.