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The wisdom about exercise is that muscles need to have nourishment after a strenuous workout. They need glucose and water to synthesize the glycogen that pumps them up, as amino acids to rebuild the protein fibers that give the muscle its power.

Conventional Wisdom Suggests Fasting at Exactly the Wrong Time

If these nutrients are not available in the two hours after the workout, experts tell us, the muscles miss out on a window of opportunity made possible by greatly increased sensitivity to insulin that enables them pull nutrients out of the bloodstream with ease. Since it takes time for nutrients to travel through the digestive tract to the bloodstream, many athletes plan pre-workout and post-workout mini-meals for maximum nutrition.

There are also athletes who take the point of view that it is more important to burn fat during exercise, so they do their exercise routines on an empty stomach. It impossible to fat cells to release fat at the same time, these athletes and their trainers believe, so the best course of action is to workout on a completely empty stomach.

Which group of experts is right?

It turns out that both assertions are correct, for different kinds of workout routines.

If your goal is to build muscle, nutrient timing is important but not essential. There is a limited window of opportunity during and after a workout when muscle sensitivity to insulin is increased approximately 50-fold. Muscles can take up nutrients during this time better than at any other, but there are limits to absorption.

Eating a 2-pound (kilogram) steak after 2 hours of weightlifting is not going to create 2 pounds of muscle. At most, the muscles will absorb about 40 grams (1-1/2 oz) of protein in the recovery period. They also need about the same amount of carbohydrate. They don't need fat specifically for recovery and rebuilding. If you do a high-intensity workout, you may need up to 20 grams more protein and carb.

What about fat? If the idea is to burn belly fat, first you have to get the fat out of fat cells. That requires  at the very least not putting more fat into them!

Resistance exercise and tongue-hanging-out high-intensity aerobic exercise burns glucose, not fat.

Aerobic exercise at a moderate pace over a period much longer than even trained athletes can maintain maximum intensity, about an hour, or more, burns fat. Optimal muscle building routines are high-intensity and take relatively little time. Fat-burning routines are low-intensity and take a lot of time.

If you do an hour of moderate exercise, chances are you will burn about 500 calories. But if you do an hour of moderate exercise, replenish your carbs and amino acids for muscle maintenance, and then fast, you will burn fat at a higher rate until you start replenishing your fat cells by eating again.

Fasting is great for burning fat, but the experts have it backwards. Fast after your workout to burn fat. Or, to burn fat, just fast. If you don't eat anything at all for at least 17-18 hours, two things happen. One  is that your body greatly increases the production of growth hormone to preserve muscle. You do have to eat within 48 hours, but for about a day, muscles are spared.

The other thing that happens is that fat cells start to release fatty acids into the bloodstream for use as fuel. The brain continues to use glucose as its primary fuel, but you have about 6 or 7 hours that your body can burn fat—whether you exercise or not. But you will burn more fat if you fast after a moderately paced aerobic exercise activity.

Instead of burning 500 calories of carb and fat, you may burn up to 2,000 calories of mostly fat. That's very quick, real fat loss without losing any muscle.

It's important not to take any advice to extremes. If you take insulin, for instance, don't start fasting to burn fat. Don't fast for days on end to lose weight. About 24 hours is a maximum, and about 17 hours is a minimum for healthy fat burning. Simply working out in the afternoon, taking your recovery drink, and then not eating again until you have a late breakfast the next day is an optimal method of fasting for fat loss.

  • Brad Pilon, personal communication, 25 June 2011.
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