Why You Can’t Substitute Fat and Protein for Carbohydrate in Athletic Endeavour

by Paul Rogers on May 31, 2010

Low-carb dieting still has many followers after many years. Some people find it works for weight loss — if they can stick to it for any length of time.

Yet the proponents and supporters of low-carbohydrate dieting are always looking for one more angle to boost the somewhat low credibility of low-carb eating among most professionals in the nutrition and exercise disciplines. And they have made inroads into groups like bodybuilders and weight trainers. Paleo dieting, although not strictly low-carb, is essentially low-carb for athletes because of the restrictions placed on what you can eat in the carbohydrate realm. When you have a lot of energy to consume — in excess of 4000 calories (17,000 kjoules) per day for example — restricting grains and sugars is a sure way to ensure that your athlete under-eats. The Paleo Diet for Athletes is doomed to failure. And so are all forms of low-carb dieting for athletes. It just won’t work — and here’s why.

High-fat diets for athletes

Carbohydrate is somewhat limited as an energy fuel. We store about 100 grams in the liver and more or less around 500 grams in the muscles, both as glycogen. Blood glucose provides a little more. Fat is much more abundant; in fact, we carry so much, that theoretically we could run marathons, triathlons and longer events and still have fat energy to spare. No problem.

Except there is a problem. Glucose supplies ‘fast’ energy to the muscles and fat supplies ‘slow’ energy. Of course, there is no such thing as fast and slow energy, only the speed at which an energy source can provide that energy to working muscle. Fat just doesn’t cut it. And what’s worse, you can’t cheat either. When researchers tried to use a fat-loading protocol in training to enhance the way the athlete’s body uses fats in endurance events while still maintaining full glycogen supplies, all that happened was that it down-regulated the rate at which glucose could be supplied to muscle. That’s right: too much fat loading slowed down glycogenolysis, which is the retrieval of glucose from muscle glycogen. What this means in practical terms is that when you want to follow that acceleration up a big hill in the Tour de France or the Boston Marathon, you might not be able to kick it down a gear. That’s an easy way to get dropped. (Stellingwerff et al.)

More protein, less carbohydrate in weight training could lose you muscle and strength

This interesting piece of research comes from the Journal of Physiology Online and the Energy Metabolism Laboratory, Department of Kinesiology/Exercise Science Program, University of Rhode Island (Benjamin et al.) They studied two groups of novice weight trainers. Subjects were randomly assigned to a low carbohydrate (3.4 g/kg), higher protein diet (1.5 g/kg); or a high carbohydrate (5.0 g/kg), lower protein diet (1.2 g/kg). Both diets exceeded the protein recommended dietary allowance.

Here’s what they found. The low-carb group had a greater strength loss after exercise when compared with high-carb group. The low-carb exercisers also had a reduced protein turnover and synthesis during recovery. The authors concluded that dietary carbohydrate, compared to protein, may be a more important nutrient to the novice weight lifter when recovering from eccentric exercise-induced muscle damage.

You can juggle numbers here, talk about cycling carbs and many other approaches, but after a heavy workout you need to protect those amino acids from breakdown for glucose so that they can do what they do best — and that’s building muscle and other tissue. Having sufficient carbohydrate in the post-workout meal or drink will do just that.

  • Stellingwerff T, Spriet LL, Watt MJ, Kimber NE, Hargreaves M, Hawley JA, Burke LM. Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab. 2006 Feb;290(2):E380-8.
  • Benjamin L, Blanpied P, Lamont LS. Dietary Carbohydrate and Protein Manipulation and Exercise Recovery in Novice Weight-Lifters. JEPonline 2009;12 (6):33-39.

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