TUESDAY, May 15 (HealthDay News) -- A diet slightly higher in fat and lower in processed carbohydrates -- such as refined cereals, white bread and white sugar -- may help people who secrete insulin at higher levels lose more weight than a low-fat, higher-carb plan.

That's the conclusion of a new study that may help to explain why a person's rate of insulin production -- and not dedication to a diet -- can determine their weight loss success.

In the study, those with higher insulin levels lost nearly five times as much weight on the lower-carb plan than on the low-fat plan, dropping 12.8 pounds in 18 months compared to just 2.6 pounds.

The study findings are published in the May 16 issue of the Journal of the American Medical Association.

Cara Ebbeling, co-director of obesity research at Children's Hospital Boston, and her colleagues followed 73 obese young adults -- ages 18 to 35 -- during a six-month intervention and then over 12-months of follow-up. They wanted to look at individual differences in insulin secretion and see if different diet plans worked better, depending on a person's levels of insulin -- the hormone that converts blood sugar to energy for cells.

The key question, Ebbeling said, was, "Why do some people have success with low-fat diets and others don't? We hear that question all the time. The usual explanation is that some people are more motivated than others."

But there may be more to the story than simple motivation, her research suggests. "Biology offers another explanation for why some people succeed with a conventional low-fat diet and others do not," she said.

Differences in insulin secretion are at least part of the reason why some overweight people don't have success with low-fat diets, Ebbeling said.

For the study, half the participants followed a diet that was 55 percent carbohydrates and 20 percent fat. The other half ate a diet that was 40 percent carbohydrates and 35 percent fat. At the study's start, the researchers measured the participants' blood levels of insulin after they had been given a 75-gram oral dose of glucose.

Overall, no differences in weight loss were found between the lower carb group and the low-fat group.

But those with above-average insulin levels lost more weight on the lower carb diet -- 12.8 pounds at 18 months -- than those on the low-fat diet -- 2.6 pounds at 18 months.

The lower-carb plan also improved high-density lipoprotein or HDL cholesterol, the so-called good cholesterol, and triglycerides, more than the higher-carb, lower-fat plan. But the low-fat diet improved so-called bad (LDL) cholesterol more than the lower-carb plan.

Overweight people who can't lose weight or keep it off on a traditional low-fat diet might do better on a low-glycemic load diet, Ebbeling said.

Exactly why isn't known, she said.

A low-glycemic load diet emphasizes carbs with a low-glycemic index. The term low-glycemic index refers to the quality of carbs and how fast they are absorbed. Foods with a low-glycemic index are absorbed more slowly, thus keeping insulin levels more stable, she said.

To eat a low-glycemic load diet, avoid foods such as white bread, refined cereal, cookies and sugary drinks, Ebbeling advised. Emphasize fruits, vegetables, legumes and minimally processed grains, she said.

Lona Sandon, a spokeswoman for the American Dietetic Association and assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center at Dallas, said, "This study solidifies what was felt or anecdotally known by experts."

One caveat, she added, is that many people have no idea if they secrete more insulin than normal. "Just because you are overweight does not necessarily mean you are producing higher levels of insulin," she said.

Ebbeling said those who want to know their insulin-production levels can ask their doctor for an oral glucose tolerance test.

More information

To learn more about healthy eating, visit the American Dietetic Association.

SOURCES: Cara Ebbeling, Ph.D., director of obesity research, Childrens Hospital Boston; Lona Sandon, assistant professor of clinical nutrition, University of Texas Southwestern Medical Center at Dallas; May 16, 2007, Journal of the American Medical Association

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