Physiologically, your brain needs about 500 kcal/day. It can only get that from glucose or ketones, so it's easy to do the math -- anything below about 125g of carbs is guaranteed to make ketones.

So it depends on what you mean by "ketosis" to determine how you "fall out" of it. Some people measure a certain level of ketones in the blood or urine, but that makes no sense to me -- those are all in excess of the ketones consumed by your brain, and your brain is the main consumer.

To get back into ketosis is easy -- deplete glycogen. Either by restricting carbs or by burning them off with exercise. Fasting will speed up the process because nothing restricts carbs like fasting.

Oh were it as simple as Mr. Wabs suggests.

Abundant anecdotal evidence suggests otherwise.

It is a very, very individual thing. Some people doing as your example says "eating some carbs at dinner" (whatever that is - I could have extra veggies, or I could have a baked potato, garlic bread, tiramisu, and cheesecake) just keep right on trucking, stay in ketosis, and that dinner isn't even a blip on the radar.

Others knock themselves out of ketosis, take days or more to readapt, and get the keto flu all over again - plus gain 5 to 10 lbs of water weight and throw their bodies all out of kilter.

Bodies are very individual. I stay below 20 or 25 total grams per day - because I have to. Again, all cheat days are not created equal, and if I had a "cheat day" where I ate maybe 50 or 75 carbs it wouldn't be a big deal. If I ate 2 or 300 it would be another story entirely.

Best way to find out is to experiment on yourself.

Ketostix are fun. That's how I first discovered that you don't need to restrict to very low levels to become ketotic.

I was eating around 100g/d carbs, and ketostix were telling me I was in moderate ketosis. Exercise was probably boosting my ketone levels.

One of my favorite charts is from 2 days that Peter Attia spent in a metabolic chamber. If you're at all interested in how this stuff works, it's pure gold. 48 hours of glucose and ketone measurements, including how the levels respond to eating and exercise.

I'd guess that the end of day 1 ketone sample might be erroneous, but I don't really know. If you compare the end of day 1 to the start of day 2, I'd expect them to be pretty close, but it looks like he goes from 5 mmol/l to less than 2 mmol/l in 2 hours, which doesn't make much sense. The level at the end of day 2 makes more sense to me.

I'm assuming the end of day corresponds to when he wakes, but I'd have to go back to his presentation to verify.

The take-home message is that levels vary a LOT during the day and are affected a lot by eating and exercise, so be careful how you determine "in" vs "out" of ketosis, and you can use exercise as a tool to increase your level of ketones.

In any case, "falling out" of ketosis doesn't have any big consequences AFAIK. Worst case, you have a carb fest that completely refills your glycogen stores. That could require 1-2 days to make ketones again, depending on your level of carb restriction and exercise.

@vikashsinha - ketostix measure the concentration of ketones in urine, which are excess ketones that your body isn't using. The amount of ketones that you excrete may not correspond exactly with the amount that you use for energy.