Discussing endurance performance

Early keto studies suggest that a ketogenic diet can maintain or improve performance (Phinney et al., 1980, Phinney et al., 1983, reviewed in Phinney, 2004) in an overweight and untrained population (Phinney et al., 1980) and in endurance athletes (Phinney et al., 1983). However, these two studies had many limitations, such as the lack of a control group and small number of subjects. In fact, there are several keto studies that lack comparative control groups (Klement et al., 2013, Zinn et al., 2017, Davis and Phinney, 1999, Urbain et al., 2017).

It is also important to remember that a small experimental group could mean one person affects the overall mean (to be covered in future limitations article).



Figure by Brad Dieter and Adam Tzur, based on data from Phinney et al., 1983 were subjects maintained performance on average, yet that is not the case when looking at individual data.

It was later shown in a crossover study that two weeks of ketogenic dieting, compared to two weeks of high-carbohydrate dieting, benefited prolonged submaximal exercise (60% VO2max) without affecting high-intensity endurance performance (90% VO2max) in cyclists. However, nutrition and training data was not provided, which makes it difficult to know exactly what the participants were eating or doing (Lambert et al., 1994).

One study looked at maximal and mean power output after only three days of a ketogenic diet in untrained subjects. There was a significant decrease in mean power and a trending decrease in maximal power output which is likely due to a decrease in glycogen (Langfort et al., 1997).

In 2013, Klement et al., published a pilot case study on the impact of a self-prescribed ketogenic diet (Klement et al., 2013). Five weeks of a ketogenic diet did not impair running performance, but large inter-individual differences existed. Importantly, there was no significant difference in Vo2 max. There was no control group or training data in this study(see above).

In a well-designed study, Burke and colleagues followed race-walkers for 3 weeks over one or two competitions (Burke et al., 2017). They compared high CHO to a ketogenic diet while the athletes were in a slight caloric deficit. The main findings were a) all groups improved performance compared to baseline and b) those on a ketogenic diet failed to improve their 10km performance during the second competition. However, their exercise economy decrease.

In one of the longest ketogenic diet studies to date, Zinn et al, used aged endurance athletes (Zinn et al., 2017). They found a decrease in performance using several tests such as time to exhaustion, Vo2 max, and peak power.

The keto endurance studies are a mixed bag. Many of them have very small sample sizes and are relatively short in duration. Since there seems to be a keto adaptation period, subjects are expected to underperform after a week or two on the diet (Phinney et al., 1980). The results might give us the wrong impression that keto leads to underperforming, even though things might be different if the studies were longer.

Most studies did not control training. They either asked subjects to maintain their regular training program or do no training. This is good for ecological validity. In other words, it tells us how the diet plays out in practice when we don’t control the variables of a person’s life.

For example, if people naturally exercise less when on a keto diet because of fatigue or weakness, then a drop in physical performance would probably follow. But if the participants were “forced” to exercise during the study, then they might improve their performance. However, when people in real life decide to go on a ketogenic diet, they won’t have scientists, doctors, and personal trainers watching over them. So the first scenario is more realistic. The second scenario is necessary when trying to figure out how keto affects people who train.

Lastly, there are many ways to measure aerobic and anaerobic performance (average power, peak power, time to exhaustion, race performance, speed in timed trials, VO2max, etc.). We must remember that one test isn’t necessarily enough to make conclusions about overall “endurance performance”. The term itself is somewhat vague and depends on the sport or activity in question.

In summary, most of the studies suggest that measures of aerobic and anaerobic short-term performance are either decreased or maintained with a ketogenic diet.

How keto affects strength

Note: when calculating averages, the highest numbers were used when ranges were given.