: Longevity of Fat Adaptation for Endurance Athlete I'm an adult male endurance runner, and earlier this year took on a moderately low-carb diet and became fat-adapted. My runs at first were your predictable sufferfest but

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If you're talking about entering ketosis via a ketogenic diet that's pretty interesting that you felt problems lasting for weeks. Most research I've seen, and my own experience, shows ketones consumption happening ~48 hours after carbohydrate restriction. I'm sure there's a huge variety in there, but 7 weeks seems excessively long. In this 2004 study that does some other analysis, you can see a lot of references that entire studies are done in less time than 6-7 weeks:

The most obvious of these is the time allotted (or not) for
keto-adaptation. In this context, the prescient observation of
Schwatka (that adaptation to "a diet of reindeer meat" takes 2–3
weeks) says it all.

Regardless though to answer your question I've felt the same problems dropping in and out of ketosis although what I have found is that (for me) if I manage to have most of my days with long spells of extremely low carbs, and the carbs I take in tend to be higher GI, the re-adjustment is minimal if at all.

The rough number is ~50grams of carbohydrates per day, persisted for a few days, to switch your body into ketosis. Some folks are lower and some are higher. I'm at a loss to explain the time lag for ketosis with your results unless your carbs were much higher than you thought, your body is much different than most studied, or some other variables are going on (and since we're talking about human biology, that's entirely possible).