Impact of the Keto Diet on Exercise

The ketogenic diet, popularly referred to as simply “keto,” has recently gained significant attention—both positive and negative. Many people praise the diet and credit it as an effective weight loss tool, while others have heavily criticized it.


What is the Keto Diet?

While specific breakdowns may depend on the source, a 2018 Harvard Health Letter defined the keto diet as a strategy that “aims to force your body into using a different type of fuel. Instead of relying on sugar (glucose) that comes from carbohydrates (such as grains, legumes, vegetables, and fruits), the keto diet relies on ketone bodies, a type of fuel that the liver produces from stored fat.”

Several recent studies have questioned whether athletes could benefit from “going keto”—with mixed outcomes.

Mixed Results of Keto Diet Effectiveness

One of the latest studies, published in Medicine & Science in Sports & Exercise, examined the effects of a short-term keto diet in male endurance athletes, defined as maximal oxygen uptake (VO2max), 59.4 ± 5.2 ml⋅kg-1⋅min-1. For 31 days, participants followed either their habitual diet (HD) (43 ± 8% carbohydrate and 38 ± 7% fat) or an iso-energetic ketogenic diet (4 ± 1% carbohydrate and 78 ± 4% fat). Participants underwent a fasted graded metabolic test (~ 25 minutes). Participants also ingested either a high-carbohydrate meal or an iso-energetic low-carbohydrate, high-fat meal and completed a run-to-exhaustion trial at 70% of their VO2max, during which they were given either a carbohydrate or iso-energetic fat supplementation. The ketogenic diet participants maintained efficiency at < 60% VO2max, but efficiency suffered at > 70% VO2max, “as evident by oxygen uptake that could not be explained by shifts in respiratory exchange ratio and increased energy expenditure,” noted the authors. Diet had no impact on time to exhaustion.

Another study found that performance could suffer on the ketogenic diet compared to one that provides the body with a higher amount of carbohydrates. Reporting in The Journal of Physiology, researchers assessed three diet variations in elite race walkers: high carbohydrate availability (g kg−1 day−1: 8.6 carbohydrates, 2.1 protein, 1.2 fat) consumed before, during and after training (n = 9); identical macronutrient intake, periodized within or between days to alternate between low and high carbohydrate availability (n = 10); and low carbohydrate, high fat (< 50 g day−1 carbohydrates; 78% energy as fat; 2.1 g kg−1 day−1 protein; n = 10). All three groups demonstrated increased peak aerobic capacity during race walking. And while the low carbohydrate, high fat group had significantly increased rates of whole-body fat oxidation, these participants also had a higher oxygen cost of race walking. Meanwhile, the high carbohydrate and ideal macronutrient groups both reduced their oxygen uptake and improved their 10 km race walk times. The low carbohydrate, high fat group did not improve their 10 km race walk times. Overall, the researchers concluded that the low carbohydrate, high fat diet “negated performance benefits in elite endurance athletes, in part due to reduced exercise economy.”

Interestingly, one study found that how a ketogenic works in the body may differ between sexes. Research recently appearing in the Journal of the International Society of Sports Nutrition evaluated how four weeks on the keto diet affects CrossFit-trained athletes, assessing differences between male (n = 11) and female (n = 11) participants, during an incremental cycling test (ICT). The researchers examined oxygen uptake and carbon dioxide exhalation during the ICT while participants were following their normal diet and while on the ketogenic diet. The results of this study differed based on sex: “In males, the [keto diet] led to an increase in fat utilization (g·min− 1·kgFFM− 1 and % oxidation). It was particularly noticeable at exercise intensities up to 80% of VO2max. An increase in the area under the curve (AUC) was seen in males but not in females at up to ≤65% VO2max of fat utilization.”

And while the hype surrounding keto may be recent, the diet itself has been around for quite some time—perhaps under a different name, noted David L. Katz, MD, MPH, FACPM, FACP, FACLM, founder and president of the True Health Initiative, in a recent blog post. The ketogenic diet is decades old and was once marketed as the Atkins’ Diet—which was first popularized in the 1970s.

Dr. Katz has also previously called into question how athletic performance may suffer under the ketogenic diet:

“As for athletic performance, there the evidence is rather damning. World-leading researchers in nutrition for optimal human performance have studied degrees of carbohydrate restriction, and found that they tend to compromise peak performance, compound muscle injury, and delay recovery. A prominent colleague in this area recently told me directly that the global community of experts in sports nutrition is uniformly opposed to carbohydrate restriction in general, and the ketogenic diet in particular, based on the data currently available.”

So what’s the verdict? It may be too soon to say, suggests Dr. Katz.

“There are no long-term human studies of the keto diet, and no free-living, healthy human population lives this way. We thus have no evidence that the diet can be maintained, that it fosters health over time, or even that it is safe,” he wrote, also noting, “To be fair, though: absence of evidence is not evidence of absence.”