Clinical trials of weight loss diets have produced inconsistent results, and Harvard researchers at Children’s Hospital of Boston think they know why:
Insulin is a hormone that prompts our fat and liver cells to store energy from the foods we eat, and some people naturally produce higher levels of insulin. Which means their bodies can translate carbohydrates (sugar/glucose) to fat more efficiently.
The new Children’s Hospital study (reported in the Journal of the American Medical Association*) tested the natural insulin secretion/response levels of 73 overweight young adults at baseline. This involved a serum insulin concentration test at 30 minutes after administration of a 75-gram dose of oral glucose.
The researchers then assigned the 73 subjects to one of two popular diets for six months:
n A low-fat diet – 55% carbohydrate and 20% fat.
n A low-carb or “low glycemic load” (GL) diet – 40% carbohydrate and 35% fat.
The low-GL diet includes more foods that are digested slowly and raise blood sugar levels slowly (such as whole grains, fruits, vegetables, nuts, legumes) and fewer refined carbohydrates or “fake foods” (such as processed breakfast cereals, white bread, and concentrated sugars), which are digested quickly and tend to raise blood sugar levels quickly. Importantly, all participants on both diets were instructed to eat until they were satisfied, and not to count calories.
At follow-up 12 months later, the researchers found that the reductions in body weight and body fat percentage for the two diet groups as a whole were similar.
But for the 28 individuals in the two diet groups whose insulin response levels as measured at baseline were “high” (above the median), the low-GL diet produced greater reductions in both:
n Body weight (average 12.76 lb loss with low-GL diet, vs. 2.64 lb with low-fat diet), and
n Percent body fat (average reduction of 2.6% with low glycemic load diet, vs. 0.9% reduction with low-fat diet).
And in terms of follow-up measures affecting heart health:
n Plasma high-density lipoprotein (HDL or ‘good’) cholesterol and triglyceride concentrations improved more for subjects on the low-glycemic load diet.
n While low-density lipoprotein (LDL or ‘bad’) cholesterol improved more on the low-fat diet.
“These findings could allow clinicians to individualize the treatment of obesity by first giving patients an oral glucose tolerance test,” says David Ludwig, MD, PhD, the study’s senior investigator.
Next an Adult Study; Analysis of Underlying Physiology
This research team is currently recruiting obese adults in the Boston area, aged 18 to 40, for a follow-up weight-loss study that will provide all meals for a six-month period, according to a press release at the Children’s Hospital website.
And beyond that, they plan to investigate what causes some people to produce higher levels of insulin than others.
* “Effects of a low-glycemic load vs. low-fat diet in obese young adults: A randomized trial,” by CB Ebbeling, et al. Journal of the American Medical Association. 2007. May 16;297(19):2092-102. To see an abstract of the article, click here.