Weight Loss Requires Drop in Calories, Not Low GI

Question: Does a low glycemic index diet contribute to weight loss?

Answer: The quick answer to this question is “No.” Successful weight loss requires a reduced energy diet and an increase in physical activity. Nevertheless, diet books continue to suggest that high glycemic index (GI) foods trigger high insulin levels that in turn cause low blood glucose that spurs new cravings for food.¹

To investigate the diet books' claims, the definition of GI first must be clarified. The GI refers to the area under the 2-hour glucose curve following consumption of 50 g of digestible carbohydrate. It does not measure how rapidly blood glucose levels increase; when figures are available, you see the peak glucose response from foods or meals occurring at approximately the same times, although the peak responses may show a modest difference. Insulin responses from low versus high GI meals, when reported, are parallel.² Researchers don't know if, long term, the GI of a diet can be changed. It appears that most people already eat a diet in the moderate GI range.^3–6 In a 12-month study that implemented a low GI diet versus a usual GI diet, there were no differences in mean GI at the study's end.⁷

In the first published debate on the use of GI diets for weight loss, in 2002, one researcher advised against counseling obese patients to follow a low GI diet based on a review of 20 studies (all <6 months in duration).⁸ The review found weight loss on a low GI diet in four studies—and on a high GI diet in two studies—with no differences in 14 studies. The average weight loss was 1.5 kg (3.3 lb) on the low GI diet and 1.6 kg (3.5 lb) on the high GI diet. Another research team differed with those findings, concluding from epidemiological evidence and a weight-loss study in obese adolescents that GI does play a role in weight loss and satiety.⁹ Interestingly, in two subsequent weight-loss trials in obese young adults, no difference in body weight decreases were found when implementing a low GI diet compared with other weight-loss diets.^10,11

Low GI and high GI diets for weight loss in adults have been compared in randomized clinical trials. In one 10-week study, no significant difference in weight loss occurred between the high and low GI groups.¹² Another study compared three diets (high GI, low GI, and high fat), all with calorie levels 500 kcal less than each subject's estimated energy needs.¹³ At 12 weeks, changes in weight loss and improved insulin sensitivity were significant in all groups, but no differences between and among the groups were found. All groups maintained their weight loss and improved insulin sensitivity independent of diet composition.

More recently, an eloquently designed 1-year trial compared the effects of two energy-restricted diets differing in GI.¹⁴ All food for the two diets (40% carbohydrate from low GI foods compared with 60% energy intake from high GI foods) was provided to the subjects for the first 6 months. During the next 24 weeks subjects took responsibility for food preparation and continued their assigned diet. Both groups attended weekly behavioral support sessions. No significant differences between groups were found in weight or body fat loss, mean energy intake, hunger, satiety, and metabolic rate for ≤12 months. The investigators concluded that their findings provide more rigorous support that wide variability in the balance of dietary macronutrients and glycemic GI has little effect on long-term weight loss during calorie restriction. They noted a tendency for weight and body fat regain in the low GI group, which suggests “that reduced energy intake may be somewhat harder to sustain with low GI regimens long term.”¹⁴

Finally, it should be noted that the GI is not necessarily the best indicator of healthy food choices. Soft drinks, candy bars, and premium ice creams have low to moderate GIs. The GI of foods can be lowered by adding fat or adding or substituting sugars, especially fructose and sugar alcohols. Even University of Toronto nutrition professor Thomas Wolever, MD, PhD, one of the originators of the GI concept, has noted, “Whole-wheat bread, brown rice, and brown spaghetti all have the same GI values as their `refined' white versions.”¹⁵

The bottom line is that calories count! Energy restriction and an increase in energy expenditure should continue to be the focus of weight loss and maintenance intervention efforts. Health professionals and the public know this, but they still want an easier answer. ▪