Is the low-glycemic diet a red herring?

The glycemic index, which indicates how different types of carbohydrates affect blood sugar levels, was originally developed as a tool to help diabetics manage their diets.  But the wider nutrition community quickly jumped on the bandwagon, arguing that a low-glycemic diet (one that avoided sharp spikes in blood sugar) would be healthier for everyone.  It wasn’t long before the low-glycemic diet (a variation on the low-carb diet) became a hot weight-loss trend.

The hypothesis behind the buzz is that a high-glycemic diet stimulates the conversion of dietary carbohydrates into stored fat.  By consuming a low-glycemic diet, you would theoretically reverse (or at least avoid) this effect. High-glycemic foods also tend to be calorie-dense—you can consume a lot of calories before you get full—and because they are so quickly digested, you get hungry again sooner than you would with low-glycemic foods.  Or so the theory goes.

As much logical sense as all the arguments make, the research doesn’t seem to be backing up the notion that a low-glycemic diet will lead to quicker, easier, or longer-lasting weight loss. In a carefully controlled study at the Human Nutrition Research Center at Tuft's University, dieters followed one of two calorie-restricted diets: The low-glycemic diet was a Zone-style diet (40% carbs, 30% protein, 30% fat), featuring low-glycemic foods such as bean and barley stew, low-fat cottage cheese, and pumpernickel bread. The other diet, which included things like sweet potatoes and bagels, contained 60% carbs, 20% protein, and 20% fat—more in line with the government’s dietary guidelines. Both diets cut total calories by about a third.

The two groups followed their diets for a total of six months, at which point the low-glycemic dieters were slightly ahead in terms of weight loss, with an average loss of 10.4% of their starting weight, compared with 9% in the high-glycemic group.  After a full year, however, both groups were precisely the same: Each group had maintained an 8% weight loss.

Of course, mere weight loss is not the only thing that matters. Were the dieters on the low-glycemic plan any less hungry?  Did they find it easier to stick to the diet? Interestingly, the researchers found that there were no differences in the dieters' reported hunger levels or satiety.  On the other hand, during the second six months of the study, the low-glycemic dieters did gain back more of the weight they’d lost, which suggests that the higher-carb diet may be easier to maintain long-term.

One last important point: The results I summarized above are for the whole group. However, when the researchers broke the subjects into subgroups they noted an important difference.  People who had signs of insulin resistance (a prediabetic condition in which insulin secretion increases) lost more weight on the low-glycemic diet; those with normal insulin response, on the other hand, did better on the high-glycemic diet.  Which takes us full circle:  A low-glycemic diet appears to be very beneficial for those with diabetes, borderline diabetes, or insulin resistance—a group estimated to include up to 25% of all Americans.  Dieters without insulin problems may actually have more weight-loss success on a higher-carbohydrate/lower-fat diet.

You can read more about this study here.

I should also note (because if I don’t, I know you will!) that apart from weight loss, which was the subject of this particular study, a low-glycemic diet may still have important benefits for health such as lowering the risk of diabetes, heart disease, and other inflammation-related conditions. For more information about the glycemic index, see ND’s glycemic index topic page.  For those interested in managing this aspect of their diet, ND includes the estimated glycemic index (eGL) as part of its complete nutritional analysis of every food and recipe.