There is much debate amongst doctors, nutritionists, and fitness experts, as to the optimal circumstances for exercise. Some believe that early morning, pre-food consumption exercise, is best for burning calories and enhancing metabolism, while others have a variety of ideas of what is best to consume before physical activity. A recent study suggests that consuming a meal low in glucose before exercising is most effective in burning fat and promoting general health.
The general idea of why low glycemic meals, as well as short-term fasting, before exercise, promotes weight loss, is that the body utilizes and burns body fat, instead of unavailable glucose. Burning fat and increasing metabolism are essential mechanisms in the prevention of obesity, diabetes, heart disease, and numerous related conditions.
The current study fed individuals either high glycemic meals, or low glycemic meals, before physical activity. Plasma free fatty acids (FFA), which when high indicate the burning of fat (specifically, fat oxidation), rose for both groups during exercise, but significantly more for those whom had consumed low glycemic meals beforehand. This observation suggests that while all forms of exercise and healthy nutrition are beneficial, for those looking to best utilize the time they have for physical activity, should look to excercise after consuming meals low in glucose.
Source: Defeat Diabetes Foundation: Price, Suzanne. The Journal of Nutrition news release. April 2009.