24hr Fast confusion

(Alan) #1

Well folks I’m new here and I just want to say hi…so…aahh…hi.

I’m a little bit confused about a 24hr fast. If I have my last meal at say 5pm…and don’t eat again until the next night at 5pm, why is that considered a 24hr fast? I understand it’s a 24hr time period but foods can take a long time to digest. Is a “real” 24hr fast about 5 hours (in this scenario 10pm) after your last meal until 10pm the following night?

Thank you,


(John) #2

Well yes, technically you are in the “fed” state until your body has completed digesting the last meal, which is typically 8 - 12 hours after you stop eating, assuming normal meal size.

So if you want to do 29 hour fasts, go for it. Best way to do that would be breakfast on day one, skip lunch, dinner, skip breakfast the next day, and then eat a decent sized lunch and dinner.

(Robert C) #3

Technically it is a 24 hour fast but - as you rightly point out - that is really a meaningless description.

For example, having a regular breakfast that will take until about 5 PM to really clear your digestive system followed by a 5 PM intake of a few ounces of raw spinach and then not eating for 24 hours is also a 24 hour fast.

But, the differences in how long insulin would be low (the point of fasting) would be huge.

The way I like to think of fasting is that it starts when you actually skip a meal you would have eaten (vs. the last moment you had food).

So, if you wake up on Monday and skip your normal breakfast - you are about 4 hours fasted at lunch time (which probably corresponds to about 4 hours of low insulin). Tacking on the previous 12 hours that included digesting and taking in the nutrients from your previous night’s dinner so you can call it a 16 hour fast doesn’t make sense to me.

(Justin Jordan) #4

I mean, fast just means period without eating, so yes, that’s a fast.

But in practical terms, you have no idea how long it takes for stuff to digest, so trying to measure according to that is basically just going to be an arbitrary guessing game, whereas you do know when you last ate something. But you can always fast longer if you want.

(Alan) #5

Thanks folks.

I wonder how long it takes after a large meal for your insulin to return to normal. I’m not sure if this is true and they’re just arbitrary numbers but would your insulin settle down much sooner than it takes to digest the whole meal…or does your insulin stay spiked until the meal is fully digested?

For example: in an 8 hour digestion period would insulin return to normal with in 4 hours…or would it take the entire 8 hours of digestion before it returns to normal?



This an argument in favor of the effectiveness of the popular 36-40 hour fast which I normally aim for.

(Bunny) #7

Looks like the ”7 hour heavy digestion window” part is what is most important. And also the social act or dominance over natural circadian hooks does not fair well with the ‘3 meals a day ideation’ unless the body is under severe physical exertion conditions and historically speaking was probably a very small snack turned into a large breakfast and large lunch through the ages:

E.g. alternating between 12 hour & 24 hour eating window cycles seems appropriate and safe for humans! You have to work your way to that gradually (3-2-1: 12-to-24) coming off a high carbohydrate dominant diet and not something you start doing immediately? If you are feeling severe hunger pangs (hypoglycemic) when doing this then you are initiating starving-mode (means eat now); the reward centers and glycemic control centers of the brain go into lock-down-mode meaning you are not ready for those kind of caloric reductions yet?

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(charlie3) #8

What I aim for is eating less frequently. I have a weekly habit to eat two meals a day for 6 days, eat nothing one day and no snacks–ever. I’ve adhered to that with near perfection. It’s sustainable for me. I believe lower freequency is at least as important as timing. If I was retired I might eat breakfast and an early dinner so hunger pangs are blunted or eliminated by sleep.


You hit the nail on the head: the objective is to get insulin levels down. How long that takes is in essence, the definition of insulin sensitivity. Which as you know, varies greatly. If we both eat the same meal, it might take 8 hours for your insulin levels to return to normal, while my body takes 12 hours.

We can’t easily measure insulin, so we use blood glucose. Diabetics determine the appropriate exogenous insulin to administer based on this. Though I’m not diabetic, I use blood glucose as a proxy also.

(So much bacon . . . so little time . . .) #10

The people talking about insulin have a point, but “to fast” has always meant “to abstain from food,” I think it would introduce a lot of confusion to start referring to a period of 32 hours between one meal and the next as a “24-hour” fast. Besides, no one really knows how long it takes his or her own insulin level to subside after a meal, eight hours is a guess at what the average time might be. The things I’ve heard and read from Dr. Fung and Megan Ramos seem to be based on hours since the last meal, in any case, and I’d be willing to bet that they’ve already taken insulin levels into account when drawing up their recommendations.


This makes a lot of sense to me. So just for clarity we can call 5pm - 5pm a 24-hour fast but with the understanding that we might get (say) 23 hours of insulin benefit and 12 hours of rest of the digestive system, and different times for whatever other processes are happening in a fast - and in any case that all those numbers will be highly variable for different people.

(Katie the Quiche Scoffing Stick Ninja ) #12

I used to consider those a 23 hour fast with a 1 hour eating window.