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I am losing weight on a keto diet, staying under 20g of carbs every day.

Today I slept late and didn't have time to pack my lunch. Buying lunch at a gas station limited my options, and for lunch I went over my allotted carbohydrates. I now at ~50g, and I'm wondering if/how this will effect ketogenesis.

So how delicate is this diet? Will I break ketogenesis, and have to restart it over the course of days? Every time I do a few months of keto, it takes a few days to feel like it's gastrointestinally "normal". Will I have to go through this "keto flu" again? Can ketogenesis be undone in one meal, or is more cheating required?

  • Please don't answer in comments with links. If it is worthy of an answer, write it up as such. – JohnP Dec 31 '16 at 13:53
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It is going to depend on a number of factors. By how big the excess of carbohydrates was, exercise, how long prior to that you were actually in ketosis and protein consumption.

Upon depletion of glucose and glycogen, the body will first attempt to create glucose from protein via gluconeogenesis. This is why protein consumption must also be moderated on a ketogenic diet. If protein intake is not sufficient for this, the tissue protein stores are protected by switching to ketone consumption in the brain, supplied by fat stores.

If you resupply some carbohydrates and/or protein, you may exit ketosis. That will depend on the carbohydrate and protein consumption. Slam down a burger with a side of fries and a sugared soft drink and you'll likely have enough glucose to fuel the muscles and signal your body that the brain can switch back to glucose consumption. So yes, one meal can definitely break ketosis if it is high in carbs.

With a more limited amount, it depends. If you've been in ketosis for some time, with glucose levels low and glycogen strongly depleted, then eat 50 grams of carbs, chances are there's not much impact. Even if you were to get out of ketosis, it won't be long before blood glucose levels are so low again that the body has to switch back to ketosis. If you get any keto flu at that point it will likely be less profound, or at least shorter, than before.

If you ate some excess carbs and then shortly after that went for a workout, whether it was endurance sport or strength training, you'll have used up the glucose very quickly, meaning its impact may have been almost negligible. Your blood glucose level and insulin level would have increased temporarily, but ketosis is moderated not only by a low insulin level but also by a high glucagon level, and breakdown of fatty acids is also promoted by epinephrine (adrenalin) which exercise increases.

Don't go eating extremely low protein in conjunction with low carbs to make up for the fluke meal, though, because protecting your muscle protein stores is important, and ketosis is also induced as a result of high gluconeogenesis. Just go back to what you did before.

If you'd like to take the guess work out of it, you can by some ketosticks which can detect levels of ketone bodies via urine (not sure if that's directly or indirectly via waste products). They can give feedback on your ketosis state in various contexts, so you'll get a better feeling of how your body responds to what you do.

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