The Truth About Carbs: How Low Should You Go?

Spring is traditionally the time of year for endurance athletes to up their training intensity and do some serious calorie burning workouts. Ironically, it’s also the time of year many athletes want to lose some winter weight.

That’s when carbohydrate recommendations can really get confusing. High carb or high protein? Is it possible to lose weight and still get in shape for racing?

Carbohydrates: The Maximum Requirements

To answer this question, let’s start by looking at the maximum carbohydrate requirements that an individual could need. Our best information about the upper limit of carbohydrate requirements comes from cyclists who race in the Tour de France. Unlike runners, these athletes can eat an impressive amount of food while racing, and indeed, those that win have mastered the art of grazing without missing a pedal stroke.

Just in case you are not familiar with the Tour, here’s the low down on the event specifics. The race lasts 22 days; the course covers 4000 kilometers, has 30 mountain passes, and only 1 rest day. Typically, the average Tour participant eats 6000-7600 calories per day, with up to 80% of their calories coming from carbohydrates. Obviously, if you’re competing in a 2-hour triathlon, you won’t need to consume quite this much, but an Ironman may be a different story.

In a controlled study which simulated the Tour de France over 2 days of exhaustive exercise, researchers studied the effects of providing a high glucose polymer sports drink during exercise in relation to endurance and protein balance. The results showed that when cyclists had an unrestricted amount of sports drink available, they consumed 80% of their calories from carbohydate, with 30% of the carb calories coming from the liquid sports drink and the rest from food. The average carbohydrate intake was about 8g carbs/pound of body weight. At this level of carbohydrate intake, the athletes were able to maintain protein balance, which means they did not break down muscle for energy despite doing exhaustive exercise. Their protein intake was between 1.5 to 1.8 g/kg (.7 g/pound).

A comparison group of cyclists who were given a 50% fructose sports supplement, which is a sweeter tasting, higher osmolarity carb source, were not able to take in as much of the supplement due to tolerance problems. This comparison group was not able to maintain their weight, nor did they stay in protein balance, despite eating the same amount of protein as the high carb group. In fact, they continued to show signs of protein breakdown for 36 hours following exercise.

So what does this research tell us? First, if you’re doing back-to-back days of long duration, exhaustive exercise, carb intake must be at least 8 grams/pound of body weight in order to prevent muscle breakdown. The ingestion of high glucose polymer sports drinks during exercise is probably necessary in order to meet total caloric requirements and prevent protein from being used as a fuel source both during exercise and following exercise.

Protein requirements for extreme events like an Ironman or Tour are about double the recommended daily allowance (RDA) for the general population, falling in the range of 1.5 to 1.8 g/kg body weight. For a 150-pound male, this is roughly 102 – 122 g/day. Most athletes will be able to eat this amount of protein without the use of supplements. However, these guidelines are based on high biological value protein sources such as eggs, meat, or fish. These protein sources provide 7 grams of high biological value protein per ounce, meaning all essential amino acids are present. Vegetarians who depend on complementing a mix of amino acid sources from grains and nuts will need to eat more protein to insure adequate intake of essential amino acids.

Carbohydrate Minimum: How Low Can You Go To Lose Weight?

Now let’s look at the other extreme: high protein weight loss diets. I have had a lot of sheepish emails from citizen-class athletes who are going with the low carb/high pro approach to dump some body fat. Not surprisingly, many of them have been successful. But is this the best approach?

The bottom line is that in order to lose weight, you have to burn off more calories than you eat.  Calories count. And no matter what diet you choose, all weight loss will be a combination of some fat and some muscle. For an endurance athlete, timing when you try and lose weight is an important consideration. Based on what we’ve learned from the Tour cyclists, intense exercise will promote loss of muscle if energy balance (i.e. weight maintenance) isn’t achieved. Clearly, during intense exercise, adequate carb intake is necessary to prevent muscle from being used for fuel.

Studies of endurance athletes performing low to moderate exercise (45% – 50% VO2 max) tell us a different picture. When exercising at low intensity, given an equal level of calories, endurance trained athletes will burn greater amounts of fat if they are consuming a high protein (1.2 g pro/lb) diet than a moderate protein diet of .45 g/lb. (slightly above the RDA). This increased reliance on fat may possibly reduce the amount of fatigue an active person feels while dieting, because glycogen stores will not become as depleted. For an athlete willing to keep their exercise at a low intensity level, a reasonably high protein, moderate carb, low fat diet may be a good approach.

What happens if athletes severely limit their carbohydrate intake? Research tells us that it becomes difficult to exercise due to feelings of fatigue caused by glycogen depletion. Limiting carb to 100-300 g/day may result in loss of muscle strength particularly when exercise is performed at higher intensities.

The reality is that most of us will never compete in the Tour de France. We will, however, face weight problems from time to time. As tempting as quick weight-loss diets are, experience with training has taught most of us that there are no quick fixes but that patience and persistence will eventually pay off.

Carbohydrate Guidelines at a Glance

How much carbohydrate should you eat? As much as you need.

Training Period                         Intensity                                    Carbohydrate Recommendation

Off-season                                low-moderate                                  2.3 – 3.2 g/lb

Pre-race                                     moderate-high                                 3.2 – 5.5 g/lb

Pre-race/Race                        high intensity, sustained                  4.5 – 5.5 g/lb


Plan ahead, and adjust diet needs to match training. Carbohydrate needs may fluctuate from day-to-day depending on exercise duration and intensity.

For an in-depth review of protein and carb requirements, check these references:


Coyle, Edward F. “Highs and Lows of Carbohydrate Diets.” Sports Science Exchange. 17.2             (2004): 1-8. Gatorade Sports Institute <>.

“Eating, Drinking, and Cycling. A Controlled Tour de France Simulation Study, Part II. Effect of             Diet Manipulation.” International Journal of Sports Medicine. 10(1989):41-48.

Tarnopolsky, Mark. “Protein Requirements for Endurance Athletes.” Nutrition. 20 (July/Aug.             2004):662-668.

About the author

Donna Marlor, MA, BSN, RD is a free-lance writer for magazines on health related topics and maintains a private practice for sports nutrition and weight management (

Experience:  Donna has worked as a dietitian in corporate wellness facilities and hospital based cardiac rehab programs. She has been a psychology and nutrition instructor at a community college and has taught weight management classes for health care facilities. She is a former Northern Michigan University cross-country skier, and continues to compete at a master’s level in running and x-c skiing events.

Education:  Donna has a master’s in educational psychology and a bachelor’s degree in foods and nutrition. In addition, she is a registered nurse. She is a member of the American Dietetic Association Sports and Cardiovascular Nutrition Practice group.

From Donna Marlor: “Sports nutrition fuels every athlete from the novice to the elite. By eating a healthy diet, you will have more energy and perform better no matter what your age or ability.”