Even if elite race walkers don't change their diet, their stamina can still improve after a few weeks of intense training, as long as they're eating enough calories.

After four to six weeks on a low-carbohydrate, high-fat diet, trained athletes maintain the same endurance performance during high-intensity exercise tests, including time trials and prolonged cycling, and their bodies can sustain exercise up to 85% of VO2max without relying on muscle glycogen stores.

Even though elite race walkers get better at using oxygen overall on a keto diet, the diet makes their bodies use more oxygen during race pace, so their performance doesn’t actually improve after 3 weeks.

In trained male triathletes, following a very-low-carbohydrate diet for six weeks does not reduce endurance performance during prolonged exercise at 70% of maximum oxygen uptake compared to a high-carbohydrate diet, and the body maintains high-intensity endurance capacity even with lower muscle glycogen levels.

Elite race walkers who eat lots of carbs or time their carb intake during intense training get about 5-6% faster in a 10 km race, but those on a keto diet don’t get any faster.

After 4 to 6 weeks on a low-carbohydrate, high-fat diet, trained athletes show no reduction in their ability to perform repeated sprint efforts or achieve maximum oxygen uptake during endurance tests.