Is keto good for endurance athletes?

No, at this time, limited research shows no clear performance benefits to athletes following a ketogenic diet — endurance or otherwise [1,2]. 

The keto diet is a high fat, very low carb diet, where carbohydrate intake is typically limited to 30-50g per day. The keto diet has two big downsides for athletes: limited glucose availability and increased oxygen demands. Although fat burning increases following a keto diet, endurance capacity in keto-adapted athletes still appears to be limited by glucose availability [2]. A keto-adapted athlete will simply have less glucose available to fuel a long ride than a carbohydrate-fed competitor, and research to date suggests that this impairs performance, even if carbohydrate is temporarily restored pre-ride [2,3].

Another reason why keto is inferior to higher carbohydrate diets for endurance performance is that ATP (energy) production from fatty acids requires more oxygen than ATP production from glucose [2, 3]. Higher oxygen demands during exercise equate to lower efficiency, thus limiting the potential performance benefits for athletes following a strict ketogenic diet.