Switching from a low-carb diet to carb-loading right before a race may help you lose weight and run faster

By Rachael Schultz
June 17, 2015
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There's a nutrition war dividing the endurance athlete community: For decades, runners ascribed to the dogma of carb-loading. After all, science has long supported running on a pool of carbohydrates. But there's a growing trend of endurance athletes switching to a low-carb, high-fat diet to avoid the energy crash pretty much every runner experiences if they don't refuel mid-race.

The argument is over whether we should choose quality or quantity when it comes to racing fuel. Recently, we covered The Truth About the Low-Carb High-Fat Diet, but the gist is this: Carbs are the most efficient form of fuel for our bodies. They can be digested and converted into energy much faster than other nutrients, explains Lisa Moskovitz, R.D., CEO and founding dietitian of the New York Nutrition Group. However, our muscles can only store around 2,500 calories of carbs at once, compared to the 50,000 calories of fat. You can teach your body to burn fat by restricting your carb intake, which can help you avoid that sudden wall of exhaustion on a long run caused by a depleted store of carbs.

The problem? Some studies and athletes have found that burning fat makes it harder for athletes to go fast. And if you're going for endurance, your body may not adapt fully to fat burning, so you can still hit a wall come race day.

The good news is that there's a compromise between all carbs all the time and a high-fat, low-carb diet-it's called training low and racing high. It works like this: You train on a low-carb, high-fat (LCHF) diet for five to 10 days to teach your body to convert fat into fuel, explains Georgie Fear, R.D., author of Lean Habits For Lifelong Weight Loss. Your daily diet comes from roughly 50 percent healthy fats, 25 percent carbs, and 25 percent protein. Then, one to three days before a race, you start carb-loading with 80 percent of your diet coming from (healthy!) carbohydrates, 10 percent from fat, and 10 from protein. Your body has adapted to burn fat, but it also remembers how to process carbs. The theory is that you gain the advantages of training on a low supply of carbs (the ability to burn more fat than usual), but you avoid any effect on speed from the missing carbs. You even get an extra energy boost since carbs turn to fuel so quickly, explains Fear.

Training low and racing high can be especially helpful for people who like the idea of a LCHF diet year round, but are still hitting a wall on their long runs, says Pam Bede, R.D., sports dietitian for Abbott's EAS Sports Nutrition. "Sports nutrition products go into circulation pretty quickly since they're so heavily carb-based, so the muscles can run off that much easier than a high-fat snack like nuts, which will take much longer to digest," she explains.

One of the biggest parts of becoming a better, faster runner, though, is figuring out which training and racing diet is right for you. While there are huge weight loss and endurance training benefits to this eating plan, it can seem like a massive undertaking, especially for those of us who get more fired up about fitness than nutrition. The good news? There's another way to reap most of the benefits without adhering to low carbs 24/7. Unfortunately, it involves dreaded two-a-days. You hit the gym in the morning after a light snack, which will allow your body to burn through your carb stores. Refuel with a high-fat lunch, and then, when you head out for another, lighter afternoon sweat session, your body doesn't have any carbs to pull from. This forces your muscles to mobilize fat stores, Bede explains.

But while the logic adds up, there's still little research around training low and racing high. What nutritionists do have are positive anecdotes from their clients. "Several recreational marathoners I work with trained on low carbs and raced on high carbs in preparation for the Boston Marathon and were pleased with the results," Fear says. So if you want, see how it works for you. If, after two weeks of limiting your carbs, you still feel terrible and lethargic on your runs, suck down a gel, Bede advises. Still feel exhausted? A LCHF diet may not be right for your body-and there's nothing wrong with that.

Since the fitness-focused approach doesn't require much of a diet change, you could also see how your body responds to that. And if you just want to dip your toes in the pool? Forgo breakfast before a morning workout every now and then (assuming you're not going too hard!). If you don't refuel before your a.m. run, you haven't eaten in at least 10 hours, meaning your body has no carbs to burn off and will likely tap fat stores to keep your legs moving. In fact, a 2013 study in the British Journal of Nutrition found that exercising in a fasted state can burn almost 20 percent more fat than sweating well fueled.