May 15, 2007 - Why do certain diets work for some people and not for others? Blame biology, a new study suggests.
The study shows that obese people using a weight loss strategy called the low-glycemic-load diet lost nearly 13 pounds over a year and a half -- but only if their bodies secreted high amounts of insulin in response to sugars.
Obese high-insulin secretors who tried a low-fat diet lost only 2.5 pounds. On the other hand, people who were low-insulin secretors lost the same amount of weight -- a little more than 4 pounds after a year and a half -- on low-glycemic-load and low-fat diets.
"The low-glycemic-load diet was effective for a lot of the individuals who were high-insulin secretors and who previously had challenges losing weight and keeping it off," study researcher Cara B. Ebbeling, PhD, co-director of obesity research at Children's Hospital Boston, tells WebMD.
Ebbeling, David S. Ludwig, MD, PhD, and colleagues report the findings in the May 16 issue of the Journal of the American Medical Association.
No ‘One Size Fits All’ Weight Loss Diet
Ebbeling, Ludwig, and colleagues randomly assigned 73 obese men and women aged 18 to 35 either to a low-glycemic-load diet or to a low-fat diet. All of the study participants got intensive counseling and motivational support for their diets.
Their main finding was that a person's success with a particular weight loss diet might have more to do with biology than with willpower.
"The key question is, 'Why do some people succeed on a low-fat diet while others fail?'" Ebbeling says. "The usual explanation is that people who succeed just have more willpower. This study indicates that differences in insulin secretion are at least part of the reason why people don't succeed at a low-fat diet. Such people may do better on a low-glycemic-load diet."
The most important thing about the Ebbeling/Ludwig study is that it challenges the "one size fits all" mentality that surrounds weight loss diets, says Madelyn Fernstrom, PhD, founder and director of the Weight Management Center at the University of Pittsburgh Medical Center.
"What is newsworthy about this study is it offers a biological explanation for the observation that different people respond differently to the same diet," Fernstrom tells WebMD. "There is a subgroup of people who squirt out a lot of insulin in an exaggerated response to sugary foods -- and maybe these are people who respond much better to a low-glycemic-load diet."
Low-glycemic-load diets are often called "slow-carb" diets. The idea of these diets is to avoid starchy and/or sugary carbohydrates -- such as white potatoes or white rice -- and to eat lots of fruits, nonstarchy vegetables, legumes, and whole grains.
It's an offshoot of high-fiber diets that cut cholesterol and heart disease risk, says David J.A. Jenkins, MD, PhD, DSc, director of the clinical nutrition and risk factor modification center at St. Michael's Hospital, Toronto, and professor of nutritional sciences at the University of Toronto.
"Many of the things we use in our high-fiber diet -- such as barley, oats, and psyllium -- have significantly lower glycemic indices than their low-fiber counterparts," Jenkins tells WebMD. "The sticky parts of fiber keep your blood sugar down and your cholesterol down. Two birds are hit with one stone."
Studies of the low-glycemic-load diet get mixed results. The new study offers an explanation, says Stephen Cook, MD, assistant professor of pediatrics at the University of Rochester, N.Y. And the findings may explain why a diet that works at one stage of life does not work later in life.
"We know insulin sensitivity changes over time," Cook tells WebMD. "People in their 20s and 30s are much more sensitive than older people. So in looking at this data, I think it is important to keep in mind the fact that adults go through different developmental stages, just as children do."
Cook says there's still no good clinical test for insulin secretion. But Fernstrom suggests that the current three-hour glucose challenge test could be adapted to a 30-minute version acceptable to most patients.
"If the prediction made in this study holds out with further study, this could be a very useful tool," Fernstrom says. "This study shows that everybody who is fat does not have problems with glucose and insulin, but some do. So maybe we can identify a subgroup of people where we can say, 'This kind of diet could work really well for you.'"