A study suggests that a switch from quick-burning carbohydrates to breakfast foods that don't cause a rapid rise in blood sugar may help increase the fat burning effects of exercise.

The small British study tested whether glycemic index, a measure of how foods impact blood sugar, could affect how much fat is burned during a workout. 

Glycemic index is measured on a scale from 1 to 100. Low glycemic index foods like oats, barley, non-starchy vegetables and bran cause only a small bump in blood glucose. High glycemic index foods, like white bread and processed foods, cause blood sugar to spike.

Researchers followed eight overweight, sedentary women who each completed two trials. In the first trial, they ate a breakfast with a high glycemic index three hours before walking 60 minutes.

For the second trial, they ate a low glycemic index breakfast three hours before the same exercise.

The results, published in the May 2009 issue of The Journal of Nutrition, showed that fat oxidation was twice as great during exercise when women ate low glycemic index carbohydrates for breakfast. What’s more, the women continued to burn more fat for three hours after a low glycemic index breakfast and exercise. 

Doctors believe this is because consuming a breakfast with a low impact on blood sugar keeps the body from producing too much of the hormone insulin. Insulin tells the body to store, rather than burn, fat, making it tougher to lose weight.

“If you want to consume a meal before exercise but still burn as much fat as possible during exercise, chose low-glycemic index foods in the hours before exercise instead of high glycemic index foods,” says Emma Stevenson, PhD, associate director of the Brain, Performance and Nutrition Research Centre in the School of Psychology and Sports Sciences at Northumbria University, New Castle, U.K.

Stevenson says exercising in a fasted state would result in the highest fat burning rates but, "many individuals prefer to consume breakfast before a workout and so a low glycemic index meal provides a good solution."

Test breakfasts for each group totaled 265 calories. The low glycemic index breakfast consisted of muesli, skim milk, apple juice, an apple, canned peaches and yogurt. The high glycemic index breakfast included corn flakes, white bread, jam, skim milk, margarine and a carbonated glucose drink. The fiber content was higher in the low-glycemic index breakfast – 3.5 grams compared to 1.5 grams in the high glycemic index meal.

Researchers say the benefit of their study is that it looks at a population not studied before. “Previous research in this area has only focused on endurance trained individuals who are better at utilizing fat as an energy source anyway and on exercise at a higher intensity,” Dr. Stevenson explained.

“That is an important point,” says Eric Westman, MD, an expert on low glycemic diets at the Duke Lifestyle Medicine Clinic in Durham, N.C. “For a population that would tend to be older, if you are going for a mild activity, one way to enhance your muscle function and also fat burning is to lower the carbs,” he said.

This was a very small study, with only eight participants who only had one low-glycemic index meal and one high glycemic index one. Dr. Westman says while that is a downside to the research, he doesn’t think it invalidates it. “I don’t see any reason why it wouldn’t have the same effect over and over. If you keep carbohydrates down every day, it ought to have the same effect,” he said.

Going forward researchers would like to do longer term studies to see if the combination of low intensity exercise and low-glycemic index breakfasts over an extended period of time helps with loss of fat mass and to see if the same responses to the meals were evident in overweight or obese individuals. Dr. Stevenson says more research is also needed to look at different combinations of high-glycemic index and low-glycemic index foods at breakfast to see if they have a similar effect and to see if there are any longer term effects.

Dr. Westman says he’d also like to see further research that involves lowering carbohydrates even more to see if you get an even better effect.