Here’s an article about Tour de France cyclists using exogenous ketones to help recover:
This seems a little crazy to me, mixing high carbs and high ketones, but then cyclists are always after any edge they can get. And they seem to be getting results. And it would be interesting to see what their ketone levels are. They are high carb, but also exercising at a ridiculously high level. It makes you wonder if they dip into ketosis at some point.
I believe that Phinney and Volek have shown that carb-adapted athletes metabolize some fat during high intensity exercise, but I don’t recall whether they tested for ketones. I only recall mention of respiratory quotient. Given that hitting the wall is a known phenomenon in carb-adapted cyclists, I don’t suppose that fat oxidation can be supplying that much energy, or the whole athletic gel industry would never have taken off. Dr. Noakes has publicly apologized for helping develop the first gel.