A diet low in simple carbohydrates may be more effective than a low-fat diet for people whose bodies naturally produce higher insulin levels, a trial suggests.

In Wednesday's issue of the Journal of the American Medical Association, researchers said those with higher insulin levels dropped nearly five times more weight (5.8 kilograms) over 18 months by following a diet lower in simple carbs than with a low-fat plan (1.2 kilograms). 

"The main finding of our study is that a simple measure of insulin secretion predicted weight and body fat loss on low-glycemic load and low-fat diets," the study's authors wrote.

"Reducing glycemic load may be especially important to achieve weight loss among individuals with high insulin secretion."

A low-glycemic load diet emphasizes eating most fruits and vegetables, beans, and low-fat dairy products — foods that are absorbed more slowly and keep insulin levels in the blood more stable than refined breads, cereals containing white flour and sugary drinks.

In the study, Cara Ebbeling, co-director of obesity research at Children's Hospital Boston, and her colleagues followed 73 obese people aged 18 to 35 for six months of the diet intervention and then 12 months of follow-up

Half of the participants ate either a low-glycemic diet made of 40 per cent carbohydrates and 35 per cent fat while the rest followed a low-fat diet that was 55 per cent carbohydrates and 20 per cent fat.

Before starting the diet, participants took a standard glucose tolerance test that shows whether they produce high or low amounts of insulin in response to glucose in food.

Insulin's main job is to take sugar out of the bloodstream and store it as fat. Processed foods that are high on the GI index cause insulin levels to spike instead of stabilizing, which is the goal.

Long-term success

The low-glycemic diet also improved high-density lipoprotein or HDL cholesterol, the so-called good cholesterol, and triglycerides, more than the lower-fat plan, the researchers found, confirming previous findings.

Differences in insulin secretion may help explain why conventional low-fat diets seem to work for some people but not others, Ebbeling said.

People following the low-glycemic diet also seemed to be more successful at keeping the weight off.

It's not clear why no differences in weight loss were found between the two diets for people who produced low amounts of insulin. Most people produce medium levels of insulin, which the study did not address.

Experts on glycemic response cautioned against relying on a test to find the best diet, since weight loss depends on eating healthy in the long term as well as exercising.

"I would say the first thing to do if you're wanting consistent weight loss is to look at the diet, any diet, and say to yourself, 'Is this the way I can eat the rest of my life?' " Rick Gallop, former president of the Heart and Stroke Foundation of Ontario and author of The G.I. Diet, told CBC Newsworld on Wednesday.

"If you can't, don't start it."

To be successful, a diet plan needs to address the three classic reasons why people have trouble sticking with a diet, Gallop said:

• They feel hungry or deprived.
• Counting calories and weighing and measuring foods is too complex for people strapped for time.
• The diet makes them feel unhealthy.