Featured Expert

Lou Schuler

Lou Schuler is an award-winning journalist and the author, with Alan Aragon, of The Lean Muscle Diet.

The more we know about nutrition, the more confused we get. It’s natural to assume that if a diet is good for one thing, it must be good for everything. Thus, a healthy diet—however you define “healthy”—should also work for sports performance. And if a diet works for weight loss, we figure it must be a healthy diet, which means it’s also the best diet for whatever our fitness goals happen to be. 

These are all safe assumptions for most of us, most of the time. “It’s really about your goals for training,” says Susan Kleiner, Ph.D., R.D., author of Power Eating and a nutritionist with decades of experience working with athletes at every level. As she points out, you don’t need a high-performance diet if your workouts aren’t long enough or hard enough to drain your muscles of the fuel they use for long runs, or high-effort tasks like sprints or hill climbs, or the final minutes of a soccer game, when whoever is least gassed could very well decide the outcome.  

It’s when your training turns ambitious that you need your diet to support those ambitions. The popular diet of the moment probably won’t work. (Whatever your goal—packing on muscle, going the distance, or losing that gut—here are 26 Ways to Feed Your Body for Results.) 

“Recreational runners and triathletes are typically more motivated than the average person to change their diet in pursuit of health and fitness goals,” says Matt Fitzgerald, author of Racing Weight and many other books about endurance training and nutrition. “But when they do, they often choose the same diets that nonathletes choose. Fad diets are all they really know, so that’s what they fall back on.”

Fitzgerald believes the problem comes down to branding. “There’s no name for, and very little awareness of, the diet that most professional endurance athletes follow,” he says.

With few exceptions, they use a diet he describes as “agnostic healthy eating.” It’s obviously nutritious, with lots of whole foods that are natural to wherever they happen to live, or typical of their culture. Thus the diet of a Kenyan marathoner will look different from that of an American triathlete or a European Tour de France hopeful.

But while the foods differ, the diets have a lot of common characteristics, which are relatively simple and surprisingly intuitive. We’ll start with the most important one.

1. You must eat enough food to support your training

More often than not, popular diets become popular because they help people lose weight. And all weight-loss diets do the same basic thing: they give you a way to eat less. But a lower-calorie diet works against the goal of athletic performance in two ways.

First, Kleiner says, it reduces your energy output—the number of calories you burn while training. So if your primary goal is weight loss, you give yourself a roadblock if you cut too many calories.

Second, it limits how hard you can work. “Anyone who underfuels for even a few weeks will notice their inability to train at high intensity for very long, or at all,” Kleiner says. (If you don’t have a routine for your race yet, try this Advanced Sprint Triathlon Training Plan.)

The takeaway: Some diet plans suggest cutting 500 or more calories a day, with the goal of losing a pound of fat a week. But Kleiner thinks that’s too much to cut when you’re also training hard. “With my clients, we never say, ‘I only exercise to lose weight,’” Kleiner notes. “Don’t you also want to get better?”

2. You must eat carbs

Your body has two main sources of energy: fat and carbohydrate. At rest you burn slightly more fat, but as soon as you crank up your heart rate, you also crank up the percentage of carbohydrate you use for energy. This isn’t news; we’ve known it since a study was published in 1920. The carbohydrate comes primarily from glycogen stored in your muscles. The longer your workout lasts, the less glycogen you have to work with.

Because your muscles will never give up all their glycogen—they’ll always keep a little in reserve—there’s no advantage to a pure low-carb diet for an endurance athlete. Fitzgerald says that when runners reduce their carb intake, their performance level drops, and notes that this has been shown in the majority of studies comparing high- and low-carb diets for endurance athletes.