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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 after 6-8 weeks I got to a point where I ran a 50k ultra without taking in any calories. In previous 50k races I needed to eat carbohydrates to continue.

Then, after a couple of months of this, summer and vacations and stuff happened, and I discontinued the diet. Now it's October, and I'm resuming the low-carb - but my runs are, once again, suffer-fests. Which got me to thinking:

How long did the fat adaptation last after stopping the diet?

Since the fat-adaptation is apparently lost, should I expect it to take another 6-8 weeks to regain it, or will the process be faster the second time?

  • So...you "fat adapted", ate a specific diet to support it, and in the absence of a rigid unnatural diet your body "reverted". What does that tell you about what your body prefers for a fuel source? – JohnP Dec 2 '14 at 14:35
  • Count and John, If you want to debate low carb vs low fat vs whatever you think is best then there are plenty of places to do it. If I wanted to debate the issue I would be there. My question assumes that I've already decided what I want to do and had a question about methodology. – steve Dec 2 '14 at 15:24
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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).

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