Just what’s behind that ‘low-glycemic’ label?
Add one more label to the list consumers are increasingly being asked to parse: This one declares food items as “low glycemic,” and refers to a food’s effects on blood sugar levels. Low-glycemic diets have become popular in England and Australia, based on studies that suggest they could help manage diabetes and prevent heart disease and obesity, and they’re now making headway here in the U.S.
The idea that a low-glycemic diet could improve health -- namely by stabilizing blood sugar and thus preventing overeating -- originated with researchers at the University of Toronto in the 1980s and has been most extensively developed (and promoted) by scientists at the University of Sydney in Australia.
The thinking behind the diet is easily summarized: different carbohydrates are metabolized differently in the human body. Some are digested slowly, causing relatively modest, gradual changes in blood sugar levels. Others, meanwhile, are quickly digested and cause blood sugar to peak and plummet rapidly and dramatically after a meal -- and hunger to set in again quickly.
About a decade ago, several small studies suggested that a diet of mostly low-glycemic foods might help with weight loss. In one study, pregnant women on high-glycemic diets gained more weight by the time they gave birth than women who ate low-glycemic diets. In another, obese women on a low-glycemic diet lost more weight than those on a high-glycemic diet; a separate study of obese children produced similar findings. In a larger study, of men, a high-glycemic diet was associated with being bigger around the middle.
Such findings helped inspire dozens of low-glycemic diet books and cookbooks as well as a certification process that has put “low glycemic” labels on a handful of foods in the U.S., including Uncle Sam Cereal, Ezekiel Sprouted 100% Whole Grain Bread and Silk Organic Soymilk.
The low-glycemic label indicates that a food ranks between 1 and 55 on the glycemic scale. The scale assigns foods a ranking of 0 to 100 based on how much, and how quickly, they cause blood sugar to rise and fall: zero for meat, eggs and most vegetables; around 10 for whole milk and chickpeas and other legumes; and as high as 100 for highly processed baked goods such as doughnuts, croissants and pancakes. A food’s glycemic-index ranking reflects, among other things, the type of starch it contains, how much fat and fiber is in it, and how processed it is. A low-glycemic food is typically high in fiber, low in fat, and not processed.
In 2006, researchers at the University of Sydney published results of the first randomized clinical trial to evaluate the low-glycemic diet’s effects on health. The study, published in the Archives of Internal Medicine, assigned 129 young adults to one of four diets, two of which were low on the glycemic index. All of those on the low-glycemic-index diets (and none of those on the high-glycemic diets) saw a drop in their LDL (or “bad”) cholesterol levels.
That study suggested that low-glycemic diets held promise for those wishing to avoid heart disease and maybe lose weight in the process. Two reviews of low-glycemic diet studies, published earlier this year in the American Journal of Clinical Nutrition, confirmed other claims about low-glycemic diets.
One review examined 37 studies and concluded that a low-glycemic-index diet was as effective in preventing diabetes and heart disease as diets high in whole grains and fiber. The reviewed studies also suggested that eating foods low on the glycemic scale could reduce the risk of gallbladder disease and breast cancer.
The other review, of 45 relevant clinical trials, concluded that eating a low-glycemic diet appeared to help people with diabetes, namely by improving blood sugar control. But it also concluded that there was minimal evidence that a diet low on the glycemic scale could help people lose weight.
Researchers haven’t settled on what, exactly, might make low-glycemic diets healthful. Some have speculated that such diets simply increase consumption of fiber and other so-called unavailable carbohydrates, which may reduce the risk of heart disease and some types of cancer on their own.
And though a low-glycemic food may be healthful in isolation, what matters most in weight loss and chronic disease prevention is the overall diet. Unlike study subjects, individual consumers aren’t likely to choose every item in their diet based on its glycemic index ranking.
Nor should they, said Judith Stern, professor of nutritional sciences at UC Davis. Not every low-glycemic food is healthful, she pointed out; fructose-sweetened beverages, for instance, rank low on the glycemic index because the sugar has minimal effect on blood sugar levels, while many healthful whole fruits rank near the middle, around 50.
Research suggests that the glycemic index is an effective way for people with diabetes to manage their blood sugar. But its benefits for the population at large may be minimal. In the end, Stern said, the glycemic-index diet, like countless diets that have come before it, is just another gimmick.
“But for some people,” she added, “gimmicks work.”
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