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By Matt Fitzgerald
Endurance athletes are commonly advised to aim for a target of 60 percent of carbohydrate in their daily diet. This recommendation is based on scientific studies dating back more than four decades that have shown that average carbohydrate intake levels in western diets—which hover around 50 percent of total calories—are not sufficient to optimize performance in extreme endurance tests such as marathons.
The 60-percent rule is only loosely research-based, however. There are no studies demonstrating that the 60-percent carbohydrate target is ideal for any endurance athletes, let alone all endurance athletes. In the aggregate, the research in this area does not support any one-size-fits-all carbohydrate intake level but instead suggests that carbohydrate intake, in both absolute amounts and in relation to total caloric intake, should vary with the training load.
The less you train, the less carbohydrate you need to perform optimally. The more you train, the more carbohydrate you need. While the 60-percent rule does address these realities to some degree (60 percent of the 5,000 calories an elite triathlete might eat daily is more than 60 percent of the 3,000 calories a typical age-grouper might eat), it does not address them adequately.
Training does increase protein and fat needs too, but not to the same degree that it increases carbohydrate needs. So the 15-miles-a-week runner might perform optimally on a diet that’s significantly less than 60 percent carbohydrate, while the triathlete who training 15 hours a week might perform best on a diet that’s significantly more than 60 percent carbohydrate. Put another way, it’s not carbohydrate as a percentage of total calories that you need to worry about. Rather, it’s the absolute amount of carbohydrate (that is, the total number of carbohydrate grams you eat). Therefore each athlete needs to select a daily carbohydrate intake target that is appropriate to his training workload and shape his diet to meet or exceed this target without failing to get adequate amounts of fat and protein.
A good illustration of this principle is to be found in the diet of the legendary Greek ultrarunner Kouros during a five-day, 600-mile footrace. To get through that event, Korous had to eat and drink a tremendous number of calories—almost 56,000 over the course of the race—and nearly all of these calories—95.3 percent—came from carbohydrate. Korous was wise to fuel his body in this seemingly unbalanced manner, as the body of a trained endurance athlete cannot store more than about 800 grams of carbohydrate yet burns carbs at a rate of nearly one gram per minute even during moderate-intensity exercise such as ultrarunning. Thus if Korous had consumed “only” 60 percent of his 56,000 daily calories in the form of carbohydrate, his glycogen stores would have been depleted long before he reached the finish line (even despite the fact that the body can convert a certain amount of dietary fat into carbohydrate).
At the other extreme, endurance athletes who train at very low levels can get significantly less than 60 of their calories from carbohydrate without making low muscle glycogen levels a limiting factor in their training. For example, a 150-pound person who runs four hours per week needs to consume approximately 520 carbohydrate calories per day to keep his nervous system running properly and an additional 400 carbohydrate calories daily to replenish carbohydrate used during exercise. To maintain his body weight, this person needs to consume roughly 2,500 calories daily. Therefore this individual can do just fine on a 37-percent carbohydrate diet, give or take.
Few endurance athletes train at the same level year-round. Therefore one important implication of the idea that the optimal carbohydrate intake level is dependent on training volume is that your carbohydrate intake level should change as your training volume does. In determining an appropriate carbohydrate intake level for your training volume, it is much more convenient to focus on the absolute amount of carbohydrate (in grams per kilogram of body weight) you eat instead of carbohydrate as a percentage of total calories. The following table will help you consume the right amount of carbohydrate for your training volume (and body mass) at any given time.
|Training Volume||Recommended Carbohydrate Intake|
|≤4 hours/week||5-6 g/kg|
|5-6 hours/week||6-7 g/kg|
|7-10 hours/week||7-8 g/kg|
|11-14 hours/week||8-9 g/kg|
|15-19 hours/week||9-10 g/kg|
|20-24 hours/week||10-11 g/kg|
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