First it was fat, then it was carbs and, in recent years, the buzzword for the diet-conscious has become glycemic index. That's a measure of how quickly a food is broken down and absorbed by the body, and it's the driving principle behind such weight-loss plans as the Atkins and South Beach diets. But while scientific studies have documented the impact of too much dietary fat and carbohydrate on the body — making us heavier and increasing our risk of diabetes and heart disease — the evidence has not been as clear for high- or low-glycemic index foods. So, a new study published today in the Journal of the American Medical Association adds welcome evidence in favor of keeping your glycemic load in check — particularly if you have diabetes.
Dr. David Jenkins at the University of Toronto and St. Michael's Hospital and his colleagues report that a low-glycemic-index diet — including foods such as fruits, vegetables, beans, nuts, flaxseed and quinoa — is better at lowering blood glucose levels in patients with diabetes than a high-fiber diet. Patients in the study who were assigned a low-glycemic diet reduced their blood glucose levels, as measured by the amount of hemoglobin A1C in their blood, by 0.50%, compared to an 0.18% drop in similar patients eating a diet high in cereal fiber. (See the top 10 food trends of 2008.)
Although the overall blood-sugar reductions were small, "this is a very important trial," says Dr. David Ludwig, director of the Optimal Weight for Life Program at Children's Hospital in Boston. Many previous studies in the field have been underpowered, involving small numbers of subjects, Ludwig says, but this trial was large and looked at important endpoints. "Its results carry additional weight compared to many of the past trials," he says.
The Canadian team analyzed data from 210 diabetes patients, all of whom were taking medication to control their blood sugar. Because the medical management of diabetes still leaves patients with two to four times the average risk of heart disease, the study's authors wanted to explore whether a stricter diet could reduce that risk. The theory is that foods that break down quickly in the gut and flood the blood with glucose — high-glycemic foods — put an exceedingly heavy burden on the body to churn out enough insulin to process the sugar, leading to diabetes. Low-glycemic foods, which take longer to break down and result in a more even tide of glucose in the blood after a meal, should therefore be easier for the body to handle and may stave off the progression of diabetes.
Half of the patients in the study were placed on a low-glycemic index diet, and kept a journal of what they ate for six months. The other half consumed a "brown," or high-fiber, diet rich in cereal fibers including wheat, whole-grain breads, brown rice and potatoes with their skins, and also kept a journal of their food choices. All participants were told to avoid high-glycemic foods (the glycemic index of a food is typically measured as the amount by which a 50 g portion raises blood sugar compared with white bread or pure sugar), such as pancakes, muffins, bagels, French fries, potato chips and cookies.
After the six months, not only did the low-glycemic diet group show lower levels of blood glucose, but they also enjoyed a 1.7 mg/dL boost in HDL, or good cholesterol, while the high-fiber group experienced a .2 mg/dL drop in their HDL. Studies have shown that raising HDL levels is one way to prevent heart attacks, but it's not clear yet whether the current findings translate to any practical protection against heart disease. (See 9 kid foods to avoid.)
Jenkins is the first to admit that the glycemic index of particular foods is only one of many dietary factors that may impact heart disease risk in diabetes patients. Critics of the glycemic-index concept note that the way the body processes food — and the time that it takes to break it down — is affected by myriad factors, including how food is cooked, in what combinations it's eaten, and even what time of day a person eats. Al dente pasta, for example, has a lower glycemic index than fully cooked pasta, and for some people, eating later in the day raises the glycemic index of foods that would be lower if eaten earlier. Dr. David Katz, director of the Yale University Prevention Research Center argues, therefore, that it may be more instructive to focus on the "glycemic load" of an entire meal — a combined measure of the glycemic indices of individual foods — rather than looking at one food at a time. "Measuring the glycemic property of a diet at large is in fact quite useful," he says.
So, what does that mean for people who are deciding which diet to try in the new year? Researchers say it's too soon to eschew a high-fiber (or low-fat or low-carb) plan to go low-glycemic load. "There are certain quirks that make [understanding] the glycemic index more complicated than understanding carbohydrates and fats," says Dr. John Buse, president of medicine and science at the American Diabetes Association. "The science in the low-glycemic index field is certainly less robust than in other nutrition management fields." Buse notes that the study's findings do not discount the value of a high-fiber diet in diabetes prevention — in this particular trial, the low-glycemic index diet was in fact higher in fiber than the high-fiber plan.
"There is this sense that we have a beauty contest of diets," says Katz. "But I don't think we are really obligated to chose between a low-glycemic diet and a high-fiber diet. There are benefits to combining them, for everyone." In other words, the bottom line is the same as it's always been: Eating a well-balanced diet — with enough fruits, veggies and low-fat protein, and restricting excess fat, sugar and processed foods — is probably the smartest choice for anyone, whether or not you have diabetes.