Aerobic Endurance Athletes’ Diet: Why High Carbohydrate?

By Arnie Baker, M.D.

Macronutrient Mix & Carbohydrate Key Points

• Healthy diets are 60% carbohydrate, 25% to 30% fat, and 10% to 15% protein.
• Carbohydrate is the preferred fuel source for high-intensity exercise.
• Aerobic endurance athletes who exercise more than 10 hours per week benefit from a diet slightly higher in carbohydrate—typically up to 65% carbohydrate, 20% fat, and 15% protein.
• Even higher percentage carbohydrate diets may sometimes be best for aerobic endurance athletes.
• Aerobic endurance athletes may need 7 to 10 grams of carbohydrate per kilogram (3 to 4.5 grams per pound) of body weight per day to replace or top up glycogen stores.

Macronutrient Mix

The traditional high-performance aerobic-endurance diet consists of 60% to 70% of calories as carbohydrate, 10% to 15% as protein and 15% to 25% as fat. Such a diet is typical of Tour-de-France riders. This is referred to as a high-carbohydrate diet.

Low-carbohydrate diets always have their followers. Current examples include the South-Beach, the Atkins, and the 40–30–30 diet, in which carbohydrate makes up only 40% of total calories, and protein and fat make up the divided remainder.

Some have referred to these diets as “high fat.” In terms of percentage fat content, such diets may be typical of the average US diet. However, they have a higher than average protein content.

Carbohydrate, protein, and fat can all be used to make energy. Approximately 4 calories are produced from each gram of carbohydrate or protein metabolized; about 9 calories are produced from each gram of fat.

Where Energy Goes

The body needs energy to keep the brain working, the heart pumping, the kidneys filtering blood. The amount of energy needed for basal metabolic activities depends on the size of the individual—but let us say the average is about 1,000 calories per day.

The body also needs energy for physical activities—everything from light activity including walking to the heavy activity of high-end endurance exercise. Heavy activity can use several thousand calories a day.

Food energy that the body does not need does not evaporate. The body does not excrete calories. All calories ingested are either used to produce energy or stored as fat. Excess carbohydrate and protein are converted to fat and stored along with the excess dietary fat in the body’s fat deposits. (Importantly, the reverse does not happen except to a very minor degree—only a small portion of fat can be converted back to carbohydrate.)

How the Body Makes Energy

The body uses fat, carbohydrate, and protein to make energy via partially different metabolic pathways. Protein is usually used in building muscle or other functions; its contribution to energy production is relatively small and will be ignored in this discussion.

At rest and at low levels of activity, relatively more fat is used for energy production. As activity levels become more intense, more carbohydrate is used to make energy.

Fat requires relatively more oxygen to burn than carbohydrate. Although fat contributes to energy production during exercise, carbohydrate is the key to high efforts. Once your heart rate climbs over 75% of your maximum, more than 50% of your energy is coming from carbohydrate.

Glycogen Is Crucial for High-End Energy

When not depleted, the body has about 2,000 carbohydrate calories stored in the form of glycogen. About 500 calories of glycogen are found in the liver and about 1,500 calories are found in muscle.

A one-hour time trial uses up almost all the glycogen stored in muscle. It is easy for a high-performance athlete to burn up almost all stored glycogen with a day’s workout. We need glycogen replacement for repeated day-after-day training. Glycogen exhaustion limits or eliminates the role of stored carbohydrate in long-distance endurance activities that take most of a day or longer to complete.

Performance Time Related to Glycogen

There is a direct relationship between the amount of time a fit individual can perform threshold level work and the amount of glycogen present. The more glycogen initially present, the longer an individual can maintain an anaerobic-threshold level of effort.

A well-rested, recovered athlete has 100% of normal muscle glycogen. Prior exercise or not replacing carbohydrate results in lower levels. It is possible for muscle to have more than a “normal” amount of glycogen through a process known as glycogen loading, described below.

An athlete with 100% of normal muscle glycogen might be able to exercise at 75% of VO2 max for 80 minutes. With 75% of normal muscle glycogen, exercise time might be reduced to 60 minutes. With glycogen loading to 125% of normal muscle glycogen, exercise time might increase to 100 minutes.

A daily program of two hours of activity leads to reduced glycogen levels. Glycogen levels are maintained in proportion to the amount of carbohydrate ingested. Glycogen exhaustion occurs quickly unless a high-carbohydrate diet is maintained. On the morning after three days of heavy endurance training, an athlete consuming a 70% carbohydrate diet still has about 75% of normal glycogen levels. An athlete consuming a 40% carbohydrate diet has less than 15% of normal levels.

Glycogen Loading

Increased glycogen stores can be created through what is called glycogen loading. This involves a period of (1) glycogen use or exhaustion with heavy exercise, followed by (2) reduced activity accompanied by a high-carbohydrate diet. (Athletes also used to consume a high-fat diet in the first period, but further studies have shown that there is no need to incorporate this strategy to successfully load glycogen.)

A Lot of Carbohydrate Is Needed

Do some very rough arithmetic. A 132-pound (60 kilogram) bicycle racer might use 1,800 calories in basal metabolism and 2,200 in training or racing, for a total of 4,000 calories.

Of those 2,200 calories, 400 might come from fat and 1,800 from glycogen for high-energy use. All that glycogen needs to be replaced in order for the bicycle racer to work as hard the next day.

Carbohydrate calories are needed elsewhere. For example, the brain works only on glucose—it cannot burn fat or protein. No metabolic processes are 100% efficient.

Let us assume that we need about 2,400 carbohydrate calories to replace the 1,800 lost glycogen calories, to fuel the brain, and to account for inefficiencies.

Therefore, we need 2,400 out of a total 4,000 calories to be carbohydrate, or 60%.

If only 40% of her calories consumed that day come from carbohydrate, full glycogen replacement cannot be achieved.

More Carbohydrate—To a Point

Keep in mind that although carbohydrate has been studied for decades, the role of intramuscular fat is relatively poorly studied. Normally, about 2,500 calories are stored as intramuscular fat.

Typical of many refueling studies, one recent study showed that increasing calories from 57% carbohydrate to 68% or 88% carbohydrate results in more muscle glycogen after repeated bouts of exercise, in proportion to the amount of carbohydrate ingested.

Although both the 68% and 88% carbohydrate diets increased intramuscular glycogen, the 88% carbohydrate diet led to decreased muscle triglyceride concentrations.

Absolute Carbohydrate Calories

It is commonly accepted that most aerobic endurance athletes should consume a diet relatively high in carbohydrate—65% to 70% of total caloric intake.

Many find this approach simplistic, and say it is more important to ingest enough carbohydrate calories to replace those lost through exercise. This often amounts to the same thing, but reflects an approach to the reasoning underlying the simplification. For example, it is not that an athlete consuming 4,000 calories per day needs 65% of calories from carbohydrate; it is that 7-10 grams per kilogram per day—up to 2,400 carbohydrate calories for a 132-pound (60 kilogram) athlete—are needed to replace those lost during exercise.

Studies show that although athletes are able to get the percentage right, based on 7-10 grams per kilogram per day, less than 20% of men and women athletes consumed enough carbohydrate.

What Do Scientific Studies Show?

If you look at scientific research, you have to look at research designed to answer the right questions.

Are you a RAAM rider looking to improve fat metabolism? Are you a weekend warrior? Are you a recreational rider, riding a few times a week at no more than 75% of your maximum heart rate? Are you a frequent high-end training and racing athlete?

The literature supporting high-carbohydrate diets for high-end aerobic endurance athletes is massive, international, and accepted.

The literature supporting higher-fat diets is small. The only study I was able to find concerned athletes who consumed a meal of high (45%) fat vs. low (20%) fat the night before a cycling ergometer test. The riders were rested, were not subject to previous glycogen depletion, and had no breakfast.

40–30–30 proponents often quote this study, saying that it shows the superiority of increasing fat in the athlete’s diet. One could just as easily say it supports the notion of eating breakfast!

Diet and Health

The current medical wisdom is that reducing fat in our diets is important for general health. It is believed that fat contributes to heart disease and cancer. Fortunately, the high-carbohydrate diet for athletes achieves these very aims.

Insulin and High-Carbohydrate Diets

Proponents of a higher fat diet (for example, the 40–30–30 diet) point to diabetes and “carbohydrate poisoning.” They claim that high loads of carbohydrate are associated with high insulin levels. They say that insulin contributes to the conversion of carbohydrate to fat, and that increased fat stores contribute to insulin resistance and diabetes. Therefore, so the argument goes, we should reduce our intake of carbohydrate.

I partially agree. However, insulin also increases the formation of glycogen. Moreover, not all carbohydrate causes a rapid rise in insulin levels. The glycemic index—the degree to which foodstuffs increase blood sugar—is variable for different carbohydrates. Complex carbohydrate is the mainstay of the American Diabetes Association’s dietary recommendations.

Moreover, the body’s insulin response to a sugar load during exercise is reduced by the body’s secretion of catecholamines (adrenaline and related compounds). Although carbohydrate consumed before exercise does increase insulin levels, it still results in improved performance.

Pre-, During- and Post-Exercise Feeding

Athletes can divide calorie intake into two areas: calories in and around the training or racing, and all the rest.

Bottom Line

Aerobic endurance athletes emphasize carbohydrate in their diets, consuming more than 60% of daily calories from carbohydrate.

Aerobic endurance athletes need 7 to 10 grams of carbohydrate per kilogram (3 to 4.5 grams per pound) of body weight per day to replace or top up glycogen stores.