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| David Ludwig, MD, PhD |
"A major question in the field of obesity is, why can some people do well on conventional weight-loss diets, while others on the very same diets do very poorly?" says David Ludwig, MD, PhD, of Children's Hospital Boston, the study's senior investigator. "The usual answer is motivation and compliance -- that people just don't stick to their diets. But our findings show that biology determines why some people do well on one weight-loss diet and not on another."
Low-glycemic-load diets limit rapidly-digested carbohydrates that sharply raise blood sugar and insulin levels (such as white bread, refined breakfast cereals and concentrated sugars) in favor of carbohydrates that raise blood sugar more slowly (such as whole grains, most fruits, vegetables, nuts and legumes).
In a study published in The Lancet in 2004, Ludwig and colleagues found that rats whose insulin levels were highest 30 minutes after receiving a dose of oral glucose (sugar) gained the most weight when fed high-glycemic-load diets. They hypothesized that people who make a lot of insulin are similarly sensitive to the effects of glycemic load, and might be more responsive to a low-glycemic load diet.
Study participants were randomized to either a low-fat diet or a low-glycemic-load diet. All underwent initial testing to measure their insulin response. The diets involved no calorie restrictions or portion-weighing; instead, dieters received counseling encouraging them to eat until satisfied, changing only the kinds of foods they ate. Every six months, they received unannounced phone calls asking what they ate the day before; based on their responses, the two groups seemed to be adhering similarly to their diets. Protein and fiber consumption, physical activity and satisfaction with the diet were similar for the two groups.
During the first six months, high insulin secretors (those in the top half of insulin secretion) lost 2.2 lbs/month on the low-glycemic-load diet, versus 0.9 lbs/month on the low fat diet. At 18 months, total weight loss was 12.8 lbs in the low-glycemic-load group, but only 2.6 lbs in the low-fat group. Those on the low-glycemic-load diet also had a significantly greater decrease in body-fat percentage, and did not regain weight between six to 18 months -- a time when regain virtually always occurs.
Among dieters in the bottom half of insulin secretion, weight loss and changes in body fat did not differ significantly between the low-glycemic-load and low-fat diets.
"These findings can allow clinicians to individualize the treatment of obesity by first giving patients an oral glucose tolerance test," says Ludwig, director of the Optimal Weight for Life clinic at Children's Hospital Boston and author of the new book Ending the Food Fight: Guide Your Child to a Healthy Weight in a Fast Food/Fake Food World. "People who make a lot of insulin may do especially well on diets that reduce glycemic load. They tend to do very poorly on low-fat diets, which are generally high in carbohydrates and raise insulin levels even further, which in turn causes weight gain."
Regardless of insulin secretion, the low-glycemic-load diet had advantageous effects on components of the metabolic syndrome, a condition closely related to diabetes and heart disease: high-density lipoprotein (HDL) or "good" cholesterol increased and triglycerides decreased. Patients on the low-fat diet did not have these improvements, but they did have reductions in low-density lipoprotein (LDL) or "bad" cholesterol.
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