The glycemic index -- the rate at which a food raises blood sugar -- is a buzzword in many diet books, and the centerpiece of the highly touted South Beach Diet. It's also an invitation to reconsider the healthy side of carbohydrates.
Proponents say eating foods with low index numbers, such as yogurt and peanuts, help keep glucose and insulin levels in check, reducing disease risk later in life, in addition to helping some people lose weight. "There is evidence that lower GI diets help with many of the diseases of the Western world -- diabetes, heart disease, and cancer," said David Jenkins, a scientist at the University of Toronto.
The concept could provide a happy medium between the low-fat and low-carb extremes that have trumped so many dieters. "We think a low-GI diet is the perfect compromise," said David Ludwig, a director of the Optimal Weight for Life program at Children's Hospital Boston. "There are healthy fats and healthy carbs. You don't have to restrict any category."
Other nutritionists argue that people should focus on calories rather than the glycemic index. Most of the benefits of lower GI foods like yogurt are unrelated to their effect on blood glucose, said Patricia Vasconcellos, a nutritionist and spokeswoman for the American Dietetic Association. "These foods are higher in fiber and lower in saturated fat, so they tend to be better nutritionally and also lower in calories."
The glycemic index can steer people to healthy foods, but it can also be extremely misleading.
Mashed bananas, for instance, have a higher glycemic index than whole ones, ostensibly because mashing them makes their calories faster to digest. Carrots end up looking like one of the world's worst foods, while ice cream and candy, which virtually all nutritionists agree are empty calories, come off well. Wheat bread is ranked slightly worse than white, though most experts believe wheat is a more nutritious choice.
Some of these anomolies can be explained by looking at the way the glycemic index is calculated: By averaging the blood sugar levels of eight to 10 volunteers after they have eaten 50 grams of carbohydrates of a particular food. Carrots look bad according to this measure, because you'd have to eat a pound and a half of them to add up to 50 grams of carbohydrates. For this reason, many scientists talk about the glycemic load of a particular food, rather than its index, which puts a serving of carrots back in the health-food category, and makes candy look worse.
Quirks aside, low glycemic index foods tend to be healthy choices.
According to several human studies, benefits of eating a low GI diet range from controlling hunger pangs to keeping insulin levels in check. "When carbs are digested slowly, that can help us from getting hungry before the next meal," said Walter Willet, an epidemiologist at the Harvard School of Public Health and an advocate of the index. "There is also some evidence that eating low GI foods helps people stay on a healthy diet."
Willet and others also have shown that people who eat a low GI diet have lower risk for heart disease and diabetes, regardless of weight.
Scientists don't know the exact mechanism behind the low-GI benefit, but they say fluctuating insulin levels are the key. When you eat refined carbs, like those in white bread, the sugars are quickly broken down into glucose. Blood sugar shoots up, and the body responds with insulin, which lowers blood sugar levels by signaling cells to take in glucose.
"Over time, fluctuating blood sugar levels put stress on the cells that produce insulin and tire them out," Ludwig said. These long-term changes -- known as insulin resistance -- are the likely culprit behind the increased risk of diabetes and heart disease, he said. Problems with insulin also may slow a person's ability to lose weight, because cells convert glucose into storage, rather than burning it for fuel.
To try to parse out which benefits were due to low GI and which to high fiber and protein, Ludwig fed rats a diet that was identical in every way -- including nutrients and fiber -- and varied only in the type of starch. "After two months," he said, "the animals that ate a higher GI diet had almost twice the fat and less muscle mass. That's the opposite of what you want to happen."
The high GI rats also had higher levels of insulin and sugar in their blood, and three times the level of triglycerides, all risk factors for heart disease and diabetes.
Over time, the animals on the high GI diet began gaining so much weight that scientists cut back their food intake. Even on less food, they gained more weight than their lower GI counterparts. "It looked like their metabolism slowed down," Ludwig said. "In mice that ate a higher GI diet, insulin directed nutrients into storage as fat and away from muscle."
While other countries have jumped on the GI bandwagon -- Australia is considering adding GI measures to food labels -- most US researchers aren't quite ready for a change in the federal nutrition guidelines. "We need more long-term human intervention studies," said P. K. Newby, a nutrition scientist at the Human Nutrition Research Center on Aging at Tufts University. She said it's not yet clear whether eating by the index helps people lose and keep off extra pounds.
Other researchers argue that only certain groups need to worry about the glycemic index. David Jenkins, the scientist who originally developed the measure in 1981 as a way for diabetics and athletes to keep their blood sugar under control, said eating low glycemic foods is important but not fundamental. He said the concept is probably most significant for people who are overweight or inactive, and may already have problems with glucose. If you're thin, Jenkins said, "it doesn't matter what the GI is of the foods you eat."
And all nutritionists seem to agree that people don't have to slavishly follow a glycemic index chart to get some benefit. Ludwig said: "Easy guidelines, such as cutting down on refined starch and concentrated sugar, and eating lots of fruits, vegetables, nuts, legumes, healthy fats and adequate protein, would encompass 90 percent of the GI idea."
Ludwig is currently recruiting participants for a long-term, large-scale clinical trial of glycemic index. Participants must be between 18 and 35, moderately overweight to obese, and willing to commit to 18 months of individual and group nutritional counseling. Call 617-355-2500 for more information.