When weight loss is the goal, most diets restrict calories. It is a relatively simple concept--a person can lose weight by taking in fewer calories than he or she expends. But does it matter where the calories come from? It might, according to findings from a small study published in the December 2005 issue of the medical journal Diabetes Care. Researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University discovered that a diet's overall "glycemic load" may be an important determinant of weight loss, but only for some people.
Senior author Susan Roberts, PhD, director of the Energy Metabolism Laboratory at the Center says, "Our results suggest that in the future there may be a way to predict who will do best on a low glycemic load diet." The key, they have found, may be in knowing a person's level of insulin secretion.
"Insulin is a hormone that is important in glucose (sugar) metabolism," explains senior author Andrew Greenberg, MD, director of the Obesity and Metabolism Laboratory at the Center. "The regulation of body weight is, at least in part, influenced by how much insulin a person secretes in response to a load of glucose, as well as by how sensitive that person is to insulin's glucose-lowering effects."
"In our study," says first author Anastassios Pittas, MD, assistant professor at Tufts University School of Medicine, "everyone lost some weight as a result of restricting calories, but people who had high levels of insulin secretion and ate a diet with a low glycemic load lost the most weight."
As part of the ongoing Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) trial at Tufts, the authors studied 32 healthy overweight adults on a reduced-calorie diet for six months. Half of the subjects were randomly assigned to a low glycemic load diet, and the other half followed a diet with a high glycemic load.
"A food's glycemic load is a relative measure of how much carbohydrate is in the diet and how quickly that food is converted in the body to blood sugar. Foods with lower numbers typically have a greater proportion of protein and fat, which usually result in a smaller rise in blood glucose following a meal. Examples of low glycemic load foods include salads with oil and vinegar dressing, high fat granola cereal, and most fresh fruits and vegetables. Glycemic load may not be the 'be-all, end-all' of weight-loss diets for everyone," says Roberts, who is also a professor at the Friedman School of Nutrition Science and Policy at Tufts, "but it significantly enhanced weight loss in our high-insulin-secreting subjects."
"Our findings may eventually have implications for individualizing weight-loss diets," says Roberts. "We need to confirm our results with further studies of larger groups of subjects first, but measuring insulin secretion might be a simple way to target dietary recommendations that help enhance successful weight loss." Greenberg, who is also an assistant professor at the Friedman School, notes that "only when we have completed these future studies can we determine whether these tests will be useful for making recommendations for the general public."
Pittas AG, Das SK, Hajduk, CL, Golden J, Saltzman E, Stark PC, Greenberg AS, Roberts SB. Diabetes Care, (December) 2005; 28: 2939-2941. "A Low-Glycemic Load Diet Facilitates Greater Weight Loss in Overweight Adults With High Insulin Secretion but Not in Overweight Adults With Low Insulin Secretion in the CALERIE Trial."
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy at Tufts University is the only independent school of nutrition in the United States. The school's eight centers, which focus on questions relating to famine, hunger, poverty, and communications, are renowned for the application of scientific research to national and international policy. For two decades, the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University has studied the relationship between good nutrition and good health in aging populations. Tufts research scientists work with federal agencies to establish the USDA Dietary Guidelines, the Dietary Reference Intakes, and other significant public policies.
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