Eat to Turn Up the Heat
Jerry BrainumWhen to eat for maximum fat burning during exercise
Many studies show that eating foods that are rapidly used energy sources, such as carbohydrates, just before a workout blunts the use of fat as a fuel. Carbohydrate drinks do, however, offer a few notable advantages when drunk during the workout. Drinks that contain the proper level of carbs (8 percent or less) combined with some electrolytes (minerals) to speed fluid uptake provide hydration even more efficiently than plain water. Carb drinks maintain energy levels during workouts that exceed an hour and blunt the rise in cortisol that would normally occur after an hour of hard training. But it’s all at the expense of fat burning.
I’ve seen people eat full meals immediately before or even during workouts. Such meals will have a delayed digestion, as blood is shunted from the gastrointestinal area to the working muscles, so the food goes more or less undigested until the workout ends. In addition, weight training is powered mainly by stored muscle glycogen, which is a reflection of what you ate 24 to 48 hours earlier. The only benefit derived from eating just before or during a workout is psychological.
But what’s the time interval after a meal if you want to burn the most fat during a workout? A new study, involving obese women over age 50, examined the issue.1 That group may seem to have little relevance for a younger population, but the principles governing the study apply to anyone, regardless of age.
The women ate meals either one or three hours before exercise. Not only did exercising three hours after a meal result in greater fat oxidation, but the level of fat burning was also similar to what happens after fasting. The three-hour interval led to lower heart rates during exercise and lower glucose, lactate and insulin levels, all of which favor fat burning.
Why did eating a meal one hour before exercise lead to less fat burning? The authors explain that eating so close to a workout increases the carbohydrate content of the blood, resulting in a blunted fat-burning effect. The meal produces higher levels of lactate, which blunts fat burning during exercise. Elevated insulin levels after a meal also block fat release and oxidation during exercise.
The lowered heart rate occurred because of the heart’s role in supplying blood for digestion purposes the first hours after a meal. By the three-hour point the meal was digested, leading to less stress on the heart during exercise.
Even though the study featured obese older women, the relationship between fat use and exercise applies to anyone, regardless of sex or age. In effect, the longer you wait between your last meal and your workout, the greater the level of bodyfat you’ll burn during the workout.
1 Dumortier, M., et al. (2005). Substrate oxidation during exercise: impact of time interval from the last meal in obese women. Int J Obesity. In press.