Think a Low-Carb, High-Fat Diet Might Help Your Running? Think Again!

One of the hottest areas of research in exercise science lately concerns the effects of low-carbohydrate, high-fat (LCHF) diets on endurance athletes. The latest addition to the literature on this subject comes from a study led by A.J. Heatherly at Middle Tennessee State University and published in Medicine & Science in Sports & Exercise.

Eight middle-aged, recreationally competitive male runners served as subjects. After being weighed and completing an initial performance test consisting of a 5 km road time trial, all eight volunteers were placed on a LCHF diet in which 70 percent of daily calories came from fat and carbohydrate intake was limited to 50 grams per day or less. At the end of the three-week intervention, the runners were weighed again and repeated the performance test.

On average, the men lost 2.5 kg (5.5 pounds) on the LCHF diet. Their 5 km performance did not change significantly, but the average time in the second time trial was 28 seconds faster.

At first blush, these results would appear to indicate that it is a good idea for runners to adopt a LCHF diet. But in fact it is not, for two reasons. The first and smaller reason is that the study was not controlled. All eight subjects went on the diet under scrutiny and none stayed on their normal diet for comparison’s sake. For this reason, the study design failed to account for the well-known familiarization effect. Any time a runner does the same time trial twice in three weeks, he can be expected to run a little faster the second time even if he hasn’t changed his diet or training because he knows the course and has a preexisting mark to aim for.

The bigger reason that the results of this study should not be interpreted as a win for the LCHF diet is that the runners failed to improve very much despite losing a substantial amount of body fat. A runner who loses 5.5 pounds of blubber should run much faster over 5 km regardless of any effect that the specific diet used to stimulate the weight loss might have on metabolism. The fact that the subjects in this study did not get much faster suggests that the benefit of losing weight was counteracted by something else. And from prior research we know what that something else was: High-fat diets are proven to impair carbohydrate-burning capacity and increase the energy cost of running at any given speed. In other words, they make runners less efficient and strip them of their highest gears.

If a LCHF diet were the only way to lose weight, then it might still be worth considering based on these findings. But it’s not. I can guarantee that if the same eight runners had simply improved the overall quality of their diet without changing the balance of macronutrients in it, they would have lost a comparable amount of weight without impairing their carbohydrate-burning capacity or their running economy in the process. Indeed, I myself recently lost 9 lbs in precisely this manner and it was a major contributor to my setting a marathon PR of 2:39:30 at age 46.

Lastly, let me just say this: Thank goodness the LCHF diet is not the optimal diet for endurance performance, because it is a tedious, ultra-restrictive, and no-fun way to eat!