Researchers found that when the participants followed a keto diet and exercised at less than 60 percent of their VO2 max—the amount of oxygen you can efficiently consume and use—their exercise efficiency remained the same.
But in exercise over 70 percent of VO2 max, their speed and endurance declined. In fact, running speed at VO2 max declined by 5 percent after keto-adaptation—when your body shifts from using carbs to using fat as the primary energy source.
This suggests that the keto diet would be just fine for moderate running efforts, but if you want to get faster, or work out at more intense levels, like powering up a hill, it may not be the best diet plan for your goals.
One reason? Fat is a less-efficient fuel source when you are exercising at high aerobic capacity, study author and registered dietitian David Shaw, Ph.D. (c), of the Auckland University of Technology, told Runner’s World. It requires more oxygen to create energy than carbohydrates do. Plus, the fatty acids were not as helpful in producing energy needed for muscle contraction, which may have contributed to the loss of speed.
So, should all runners skip keto? That depends on what your goals are.
“It ultimately depends on how fast you want to go,” Shaw said. For someone wanting to break two hours in a marathon, then a keto diet probably won’t help; rather, it will probably slow you down.
On the other hand, if you simply want to go for a few hours at a low-to-moderate intensity, then you may find that performance can be maintained following a keto diet.
