American Fitness Magazine

A Low-Carb Diet for Athletes?

Nancy Clark
Nancy Clark
| Stay Updated with NASM!


Originally appeared in the winter 2015 issue of the American Fitness Magazine.

Have you been hearing stories that fats are better than carbs as fuel for endurance athletes? 

 Maybe you have wondered if scientific research supports those stories. To find the latest science, I attended the American College of Sports american fitness magazine winter 2015 coverMedicine (ACSM) 62nd Annual Meeting.

 At this year’s meeting in San Diego (May 2015), I was able to verify that carbohydrates are undeniably the preferred fuel for athletes.

The following information highlights some of the research on carbohydrates, and how as a nutrition professional, you should educate your clients on their importance. You can also inform them on how many carbs to eat for weight loss

Highlights on Carbs

Louise Burke, PhD, RD, head of sports nutrition at the Australian Institute of Sport, confirmed that carbs are indeed an essential fuel for athletes who train hard and at high intensity. That is, if you want to go faster, harder and longer, you’ll do better to periodise your eating around these hard training sessions with carb-based meals (pasta, rice) rather than with meat and a salad doused in dressing—a high protein and fat meal.

Carbohydrates (grains, vegetables, fruits, sugars, starches) get stored as glycogen in muscles and are essential fuel for high-intensity exercise. Athletes with depleted muscle glycogen experience needless fatigue, sluggishness, poor workouts and reduced athletic performance. (These complaints are common among my clients who mistakenly limit carbs, believing they are fattening. Not the case. Excess calories of any type are fattening!)

Clearly, the amount of carbohydrate needed by an athlete varies according to length and intensity of exercise. Fitness exercisers who train at low or moderate intensities need fewer carbs to replace muscle glycogen stores than do elite athletes who perform killer workouts. Ultramarathoners who do long, slow, “fat-burning” runs can get away with a lower carb intake—unless they want to be able to surge up a hill or sprint to the finish. 

 A study with CrossFit athletes showed that those who reduced their carb intake (think Paleo Diet™) simultaneously reduced their ability to perform as well during their high-intensity workouts. Those who ate less than 40% of their calories from carbs (≤ 3g carbs/lb body weight/day or < 6g carb/kg) were outperformed by the higher carb group. Eat more sweet potatoes and bananas!

Some avid carb-avoiders endure a very low (< 20–50g/day) carb ketogenic diet. They “fat-adapt,” burn more fat, and hope to perform better. Yet, most studies with athletes in ketosis do not show performance benefits. Plus, is the diet sustainable? Who really wants to live in ketosis? No pasta, no potatoes, no birthday cake, no fun…

Additional Findings on Carbohydrates

British exercise physiologist Ron Maughan, PhD, asked, “Why would you even want to burn more fat than carbs during exercise?” He explained that fat, as compared to carbohydrate, requires more oxygen to produce energy. Burning more fat means that you have to work at a higher fraction of your maximum oxygen uptake. “Isn’t that the opposite of what you want to happen?”

Some athletes claim a key benefit of being fat-adapted is a reduced need to consume food during endurance exercise—and thereby reduce the threat of intestinal distress. Hence, fat-adapting seems like a logical plan for numerous endurance athletes who fear sour stomachs and “fecal urgency.” The problem is, if they want to make a surge, sprint to the finish, or dig deeper to go harder or longer, they will lack the glycogen required for that higher intensity burst.

Hence, their best bet would be to train their bodies to accept food during exercise. By experimenting during training and seeking help from a sports dietitian, an athlete can figure out which fluids and foods will settle well. Perhaps a different brand of sports drink or gel, or a swig of maple syrup, could offer the needed fuel without creating distress.

Concerns about carbs causing inflammation have prompted some athletes to avoid wheat and other grains. Yes, if you have celiac disease (an inflammatory condition with telltale signs of constipation, diarrhea, bloating, stomach discomfort and/or persistent anemia), you certainly should avoid gluten. But only 1% of the population has celiac disease and up to 10% may have other wheat-related issues. Research by Canadian sports nutritionist

Dana Lis, RD, suggests that gluten does NOT cause inflammation in athletes who do not have celiac disease or a medical reason to avoid gluten. Those who claim to feel better when eating a Paleo-type or gluten-free diet may have become more nutrition-conscious. They feel better because they are eating better on their whole-foods diet (as opposed to their previous “junk food” diet).

Carbohydrates from colorful vegetables and fruits are particularly important for athletes. They help keep your body healthy. Case in point, Montmorency cherries. The deep red color of these tart cherries comes from a plant compound (anthocyanin) that reduces inflammation and muscle soreness.

Athletes who consume concentrated tart cherry juice “shots” (or drink 8 oz of tart cherry juice twice a day) recover better after hard exercise than their peers without tart cherry juice. For master’s athletes, tart cherry juice can help calm the inflammation/pain associated with osteoarthritis. 

The bottom line:

1)  Enjoy a foundation of quality carbs at each meal to fuel muscles. 

2)  Include a portion of protein-rich foods in each meal to build and repair muscles (for example, scrambled eggs + bagel; turkey in a sandwich; grilled chicken with brown rice and veggies). 

3)  Observe if you perform better. 

Each person is an experiment of one, and we are just beginning to understand the genetic differences that impact fuel use, weight and sports performance. Your job is to learn what works best for your body and not to blindly accept the latest trendy nutrition advice. Be smart, fuel wisely and have fun, And if you are looking to be a leader in nutrition and shine a light on misinformation, check out the fitness certification bundles that NASM has to offer. 

The Author

Nancy Clark

Nancy Clark

Nancy Clark, MS, RD, CSSD (Board Certified Specialist in Sports Dietetics), counsels active people at her private practice in Newton, Mass. (617.795.1875). For more food help, enjoy reading her Sports Nutrition Guidebook and Food Guide for Marathoners. They are available at


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