Q. My neighbor and I went on a low-fat diet together. She lost weight and I didn't. Why?

A. Assuming you both ate the same amount of calories, differences in body composition (percent body fat versus muscle), frequency of past dieting attempts and amount of physical activity could influence your results. Differences could also be related to the amount of insulin your body secretes after meals.

Insulin is a hormone that converts blood sugar into energy for cells. A study published in the May 16 issue of the Journal of the American Medical Association showed a relationship between insulin levels and success with different diet plans; 73 obese young adults were assigned to either a low-glycemic load diet (40 percent carbohydrate, 35 percent fat) or to a low-fat diet (55 percent carbohydrate and 20 percent fat). The study lasted 18 months. Researchers wanted to learn why some people have success with low-fat diets and others don't. Although will power can play a role, sometimes there's more to the story.

Food and drinks that are high in processed carbohydrates such as sodas and white rice have a high glycemic load. This means, they cause a rapid rise in blood sugar after they are eaten. Low-glycemic-load foods are sometimes called "slow carbs" because they enter the blood gradually and have less effect on blood sugar. They include vegetables, fresh fruits, whole grains and starchy beans.

In this study, participants who secrete insulin slowly lost equally on both diets. In contrast, those with high insulin levels lost more weight on the low-glycemic-load diet (12.8 pounds) compared with the low-fat diet (2.6 pounds).

In people who pump out a lot of insulin in an exaggerated response to sugary foods and processed starches, reducing the glycemic load of the diet may keep insulin levels steady.

Past studies on low-glycemic-load diets produced mixed results. Sometimes they showed weight loss, sometimes not. This may be because nobody compared insulin levels in the participants!

Just because you're overweight does not mean your body produces excess insulin. The only way to know is by having your doctor do an oral glucose tolerance test.

When it comes to weight control, one size does not fit all. If you've had trouble losing weight on a low-fat diet, you may want to try decreasing the glycemic load in your diet.

Amy Tousman is a registered dietitian and certified diabetes educator with Nutrition Unlimited in Kailua. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.