The New Rules of Marathon Nutrition: Sports Drinks

Part 3: Don't Let Your Sports Drink Become a Crutch

In Part 2 of this series, Active Expert Matt Fitzgerald explored how the low-carb approach affects running performance. In this article, Matt delves into the sports drink versus water debate to reveal what works best for race-day preparation. 

Ten years ago I used to advise runners to use a sports drink during all of their training runs lasting longer than an hour. This advice was based on research demonstrating that sports drinks enhance performance in races and time trials lasting more than 60 minutes. Of course, there's a difference between a long training run of an hour or more and a race of equal duration. Long training runs are not as challenging because they are done at lower intensities. Even so, I reasoned, with the advantage of a sports drink, runners would run two or three percent faster in their long training runs than they would otherwise, and would thereby derive a greater training effect from them.

More: Which Fluid Hydrates Best: Sports Drinks or Water?

Today I no longer advise runners to use a sports drink during all of their training runs lasting an hour or longer. Why not? Because I have learned that the benefits of using a sports drink come at a cost. Yes, sports drinks boost performance in longer races and workouts. But scientists have discovered that they also function as a sort of metabolic crutch. If you rely on sports drinks too heavily in training for longer races such as marathons, you may not become as fit or perform as well on race day as you otherwise would.

One of the most important ways in which running increases aerobic fitness is by stimulating the muscles to synthesize new mitochondria, which are the microscopic "factories" within muscles cells that carry out aerobic metabolism. One of the main fuels that the mitochondria use in aerobic metabolism is glycogen, which is the stored form of glucose. During long races and workouts, the supply of glycogen in the muscles becomes largely depleted. It appears that glycogen depletion is an important trigger of mitochondrial synthesis. In other words, the depletion of glycogen fuel that occurs in long runs stimulates the muscles to make new mitochondria, an adaptive response to physiological stress that boosts aerobic fitness for future runs.

More: An Overview of Energy Sources

When you use a sports drink during a run, however, your muscles are given an alternative source of glucose that allows them to conserve their glycogen supplies. As a result, you finish long runs with more leftover muscle glycogen when you use a sports drink than when you drink only water or nothing at all. And the ultimate consequence of this difference is that your muscles create fewer new mitochondria after a long run that is fueled with a sports drink.

More: Sports Drink Myths Debunked

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