Can Low-Carb Diets Be Good For Runners?

A lot of fitness enthusiasts support eating low-carb lifestyle diets adapted from the traditional Atkins diet… typically with labels like Keto and Paleo, as well as carb-limited variants like the Bulletproof, Carnivore or Primal diets.

The obvious problem for runners interested in these diets is that running is the one form of exercise that demands a LOT of quick-burning glycogen, which can only be properly supplied by a diet rich in carbohydrates. Running minds like Hal Higdon and Matt Fitzgerald outright recommend avoiding low-carb diets and to build your diet around 60+% carbohydrates. Fitzgerald in fact found in his research for his book The Endurance Diet that pretty much every elite coach and endurance athlete he consulted with subsisted on a diet rich in carbohydrates.

While a carb-rich diet may in the long term pose a problem for hormonal and muscular health, the flip side is that the converse low-carb, fat/protein-rich diet comes with two major concerns:

1. It doesn’t have a lot of long-term evidence behind it. While we have observed various positive health benefits in the short term, contradicting the traditional folklore that high-fat/protein diets cause heart trouble… we have yet to observe if this diet seriously damages health in old age.

2. Endurance athletes, unlike weightlifters and other athletes who perform their work in shorter bursts… typically require a lot of readily-available glycogen to sustain their training, and a low carb diet simply will not provide that.

The low-carb diet’s energy replacement, ketones, still is glacially accessible compared to glycogen. A runner could manage on such a diet but they could not aspire to run particularly fast or hard for long, since their fuel has to come from slower-burning fat.

Again, it’s not impossible to run longer distances if you decide to go keto or otherwise low-carb. Some out there do it, and in fact a lot of ultra and marathon runners have done it to some extent.

The general rule: Beyond high intensity intervals and other brief bursts… you just simply cannot run at too high of an intensity. Your easy run pace is pretty much the only pace you can sustain for more than a few moments at a time.

What ultra runners do, however, is live on a low-carb diet… and then before/during races they briefly reincorporate sugar and carbs, which after the desensitizing low-carb diet provides such a drastic insulin boost when ingested by someone who doesn’t consume it at all that it provides substantially more energy for races than it would otherwise. Many, after the race, cease ingesting carbs once again.


So… could going low-carb work for runners? I think it could on one condition: Pretty much all of your running and training has to be at low intensity. No matter what your pace is, the intensity must be such that most of your energy burn comes from fat… or else you will simply burn out.

You will not get enough carbohydrate to fully replenish glycogen otherwise. You’ve got to move easily enough, at a suitably low intensity, for ketones and fat to fully provide the energy needed.

Most who swear by low-carb eating get all their exercise from weightlifting and brief, high-intensity aerobic training. Even if they deplete glycogen stores, those are nowhere close to zero, and they have plenty of time for what little carbs they ingest to re-supply enough to keep going over time.

Over time, those who do this gradually lose glycogen capacity… unless they occasionally re-feed carbohydrate. Ultra runners don’t lose much capacity over time because of this, and given they focus on extremely long easy runs they’re getting most of their energy from fat and ketones anyway.

Otherwise, some weight loss from long term low-carb eating (much like water weight) can be the product of displaced, diminished glycogen. Much of the weight loss runners experience after a marathon is the total depletion of glycogen, which can add up to a few pounds after factoring in the water required for its storage. Lighter bodyweight obviously makes moving your body easier and slightly reduces the caloric energy requirements (and needed fat/ketones) for exercise.

But overall, low-carb dieting should be avoided IF high performance in sub-ultra endurance races is important to you. You’ll never see the likes of Mo Farah or Michael Phelps go low-carb in preparation for a race. When being fast is important, glycogen and the routine ingestion of carbs becomes very important.

Whether experimenting with low-carb eating as a runner is important to you should depend on your goals. It may be a possible fit. It may be a horrible fit.

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