Rethinking the 60 percent rule, 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.  Read more in the full article about how many calories Kouros had to consume per day – and how many of them were from carbohydrate!