Carbohydrates for Marathoners

by Ben Connelly

Even many non-runners have heard about the importance of eating carbohydrates during marathon training. But with many conflicting theories of nutrition, even seasoned veterans can feel confused. I hope to clear up some misconceptions today. 

Glycogen and Tapering:

Marathon runners are commonly advised to increase their carbohydrate intake during tapering, in order to build up glycogen stores. Yet many runners misunderstand this to mean they should eat more overall. In fact, what you want to do during the taper is increase carbohydrate intake while decreasing consumption of protein and fat (and alcohol). You want carbs to take up a greater proportion of your caloric intake, but you do not want to eat more calories. 

Actually, you probably want to eat fewer calories as you drop your mileage. When you run less, you burn fewer calories. And if you eat more calories than you burn, you will gain weight. 

Some coaches recommend people try to gain a few pounds during tapering, mostly in the form of water weight and glycogen stored in the muscles. Yet elite marathoners, from Meb to Shalane Flanagan, are doing the opposite: cutting down to a minimum racing weight. And they already have lower bodyfat than 99.9% of people in the world. While recreational runners should not attempt to cut down to unsustainably low weights, for the majority of American runners, losing a few pounds before race day is more likely than gaining a few pounds to improve performance.

You do need to have enough glycogen and water stored in your muscles to avoid hitting the wall. But you can achieve that without overeating and also gaining bodyfat. Extra weight will slow you down. Above a certain threshold, extra glycogen and water will not help your performance. And there is more that goes into avoiding the wall than just glycogen.

What About Becoming Fat-Adapted?

On the other end of the spectrum, many people have heard for at least the last 20 years that carbs make them fat. This has been disproven in numerous studies.(1,2,3) Too much food makes you fat, and it matters less whether that means carbs, protein, or fat(4). Type of carbohydrate does play a role, which I will cover in more detail in my next post.

In fact, distance runners (and all athletes), need carbohydrates in order to perform optimally. Not as much as some believe, but definitely more than you can get on a low-carb diet. 

But some still advocate runners learn to become “fat-adapted,” meaning that they train their bodies to burn fat as fuel instead of carbohydrates, doing away with the need to consume carbs during a marathon. 

There is some utility in occasionally training in a carbohydrate depleted state. Increasing your resistance to hunger and fatigue and decreasing your dependency on frequent eating can improve your health and your athleticism.

But to truly become fat-adapted, you have to eat a low-carb diet (without cheat meals) for many months while still training. And during that time, your training will suffer. Let us say that an athlete did want to become Keto-adapted (truly fat-adapted). After 18 months in ketosis, or more, an athlete could theoretically return to the same level as before they entered ketosis.(5) But they would sacrifice 18 months of training that could have improved their performance. And they would struggle even to train at a reduced level during that time. 

Theoretically, fat-adapted athletes could run fast marathons. But I would argue they could run even faster if they ate carbohydrates. 

Stay Tuned:

Next week, I will turn to specifics. What types of carbs to eat. When to eat them. How to eat during tapering. Eating during a race (I may do a whole post on that). And more. Be sure to check back in a week.








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