A low carb diet can help you lose weight, but research also shows that a low carb diet may help endurance athletes maximize their performance. Now that it’s the season for all sorts of endurance events (marathons, triathlons, Ironmans and more), I asked Jeff Volek, Ph.D., R.D., an expert on low carb diets for endurance athletes, to tell us why going low carb might improve your performance.

Q: Traditionally endurance athletes rely on high carb diets. You recommend a low carb diet for endurance athletes. Why?

A: A high carb diet locks an athlete into a dependence on glucose as the dominant fuel for exercise. It’s a fact that when we digest and absorb carbs as sugar, the body is forced to prioritize burning that sugar while simultaneously impairing access to and use of fat.  Storage of sugar in the body as glycogen is limited to about enough to last one day, or just a couple hours of hard exercise. To prevent this small carb fuel tank from running dry, frequent ingestion of carbs is necessary to prevent a fuel crisis.  The strategy of repeatedly shot-gunning sugar-laden gels and drinks to keep the carb furnace stoked is an approach used by many athletes, but it’s worth asking whether this is optimal, healthy, and sustainable. For many athletes, the answer is clearly no.

So how can you break the carb habit and “train” your body to use fat more efficiently? Just performing regular aerobic exercise allows greater amounts of fat to be burned, but this can only go so far.  Even the highest caliber endurance athletes will experience dramatically increased fat burning by cutting back on carbs in their diet. We’ve named this process of switching to fat as fuel “keto-adaptation”, and the evidence showing that fat is a premium fuel continues to mount because it is a more efficient source of calories that provides more energy.

Q: How long would it take an endurance athlete to transition from a high carb diet to a low carb diet?

A: Keto-adaptation changes the way your cells use fuel, and this takes time. As your body decreases its dependence on carbs, performance actually decreases during the first couple weeks. But by about four weeks, performance capabilities are back to normal and often exceed previous levels as time passes and adaptation becomes more complete.  

Q: How effective is a low carb diet in preventing an endurance athlete from “hitting the wall” during a race?

A: Hitting the wall is an energy crisis in the brain that happens when glucose supply is not able to keep up with demand. As a protective mechanism, the brain signals the body to shut down to preserve any available glucose. It is a catastrophic type fatigue that leaves a profound impact on athletes. You might be thinking that when glucose is limiting, the body can smoothly switch over to tapping its more abundant fat fuel tank. Unfortunately, this doesn’t work like flipping a switch (unless you were previously fat-adapted).  Most endurance athletes know this, and they purposefully carry quick energy in the form of sugar to avoid hitting the wall, which is essentially the brain responding to a real-time lack of fuel.   It’s ironic that when this fuel crisis hits, the body still has tens of thousands of kilocalories tucked away in fat cells. It’s a bit like being stranded in a boat in mid-ocean and dying of dehydration – just inches away from lots of water, but the wrong kind.

Q: Can you describe a typical day of eating for a high carb endurance athlete vs. a low carb endurance athlete?

A: Most people will find they get the best results eating 50 grams of carbs a day or less.  It’s important not to overdo protein on a low carb diet because too much protein can also interfere with ketosis.  The best approach is to experiment with finding the right amount of carbs and protein, coupled with regular monitoring of blood (not urine) ketones, which is now available with simple finger-stick instrument.  Given that this diet is low in carbs and moderate in protein, the majority of calories need to come from fat, but limit those rich in polyunsaturated fat (e.g., corn, soybean, safflower, cottonseed and peanut oils). And saturated fat is ok. We have repeatedly shown that on a low carb diet, blood levels of saturated fat decrease because a fat-adapted body prefers to burn them as fuel. And finally, a quick comment on salt is warranted. In the fat-adapted body, the kidneys tend to discard more water and salt, which can result in a general “washed-out” feeling.  An easy “solution” is to take an extra 1 to 2 grams of sodium per day as broth, bouillon or soup. And, in particular, on days you exercise, be sure to take 1 gram of sodium to prime your circulation 30 minutes before your workout.

Q: Typically endurance athletes “carb load” the night before a race. What does a pre-race meal look like for a low carb athlete?

A: The pre-exercise meal is not nearly as important when you are fat-adapted. Unless the exercise is well beyond a couple hours in duration, most fat-adapted athletes don’t bother with pre-exercise meals.

Q: During races, many endurance athletes supplement with gels and other supplements containing fast-acting carbs to keep their energy levels up during a race. What should endurance athletes on a low carb diet eat during a race?

A: In races, calories are not nearly as important because a fat-adapted athlete is able to use their fat stores for fuel more efficiently. Having said that, many ultra-endurance athletes still trickle in some carbs intermittently (up to 25 grams an hour) during long races and training bouts, but much less than they normally would before being fat-adapted. Many athletes are also using slow-release forms of carbs (oats, chia seeds, non-starchy vegetables, nuts and some fresh fruit, etc.) to minimize the insulin response and not impair fat burning.