By Rachel Johnson, Ph.D, M.P.H., R.D.
There are times when I feel like I need to duck when the subject of the glycemic index (GI) comes up. This system of ranking foods according to how much they raise blood sugar (glucose) was first developed as a tool to help people with diabetes control their blood sugar. Now, it’s squarely in the public mainstream: low-GI diet books crowd bookstore shelves, many diet plans have “low-glycemic” variations and Australian supermarkets have foods labeled with their GI ratings. But a huge debate about the value of the GI is raging in the nutrition community. Some of my colleagues are staking their careers on the GI’s importance, while others disparage its value; I’ve seen their arguments come close to blows. What makes the index so controversial? And until the dust settles, is there anything we can apply to our own eating? First, a little background.
The glycemic index measures how much a fixed quantity of different foods raises your blood-sugar levels compared with a standard, pure glucose (GI=100). Foods with a high GI value (greater than 70) tend to cause a higher spike in blood sugar—and in insulin, the hormone that helps glucose get into cells. The spikes are especially problematic for people with diabetes, who lack an effective insulin system to clear the sugar from their blood. And, because high-GI foods are so quickly metabolized, they tend to make you hungry again sooner, says David Ludwig, M.D., Ph.D., a Harvard endocrinologist and author of Ending the Food Fight (Houghton Mifflin, 2007). Ludwig’s research found that obese teenage boys were hungrier after they’d eaten a high-GI breakfast of instant oatmeal, and ate 600 to 700 calories more at lunchtime than when they’d breakfasted on moderate- or low-GI meals like steel-cut oats or omelets.
By contrast, lower-GI foods (under 55) are metabolized more slowly, and are believed to keep your appetite on a more even keel. Some experts think that by tempering blood-sugar surges, eating low-GI foods may even help prevent the damage to cells that’s caused by high blood-glucose concentrations.
But others poke holes in the GI concept for lots of reasons. “Despite what the diet books say, the glycemic index does not measure how rapidly blood-glucose levels increase,” argues Marion Franz, R.D., C.D.E., a nationally recognized diabetes expert who wrote the introduction to The EatingWell Diabetes Cookbook. “Blood glucose peaks at about the same time with most foods—and the differences between the highest and lowest glycemic responses are small.”
Those in the anti-GI camp also point out another major weakness in using the system: the glucose response to foods can vary widely from person to person, and even in the same person from day to day, so the numbers don’t tell the whole story. A low GI score is no guarantee of healthy fare, either: cola (63), potato chips (54) and even some candies (a Snickers bar is 55) qualify as low or moderate GI. “But the biggest problem is that the GI looks at single foods, and the real issue is what happens with meals,” says Franz. A high-GI potato becomes a low-GI meal if you add a pat of butter, because the added fat helps slow the absorption of the potato’s carbohydrates. These complexities, she says, are too confusing to make the GI useful for most people.
Ludwig, who regularly tests GI principles in his research studies, responds: “Tell that to the thousands of people who come to our clinics!” (I told you the arguments can get heated.)
Despite these limitations, some studies suggest the GI concept holds promise. Following low-GI eating principles can help people with diabetes fine-tune their blood-sugar responses and may even help people with prediabetes lower their risk of progressing to full-blown disease. New research connects low-GI diets with lower risk of age-related macular degeneration, a major cause of blindness, and other work suggests a possible link with reducing risk for heart disease and even colorectal cancer.
And of course, there’s the tantalizing possibility that by its moderating effects on blood sugar and thus appetite, eating a low-GI diet may help people lose weight. Unfortunately, research results in this area have been mixed. Ludwig has found that low-GI diets seem to be most effective in people whose bodies secrete more insulin: more often “apple-shaped” people, who accumulate extra fat around their waists, compared to people with lower-body fat (“pear shapes”). “Apple-shaped people who have done poorly on traditional low-fat diets may do especially well on a low-glycemic-load diet,” he says. And, regardless of body shape, those who followed low-glycemic diets improved their triglyceride and HDL cholesterol levels, he added; both are important risk factors for heart disease.
Will the great GI debate end anytime soon? Knowing nutrition scientists as well as I do, I don’t think so. But I believe the concept makes some sense as long as we don’t get too hung up on the numbers. In the end, choosing low-GI foods is common sense: for the most part, they’re more natural, whole, unpolished and unprocessed. Getting more of these types of foods is smart eating, no matter which side you’re on.