Fasting Before Feasting

Suppose you are concerned about your weight and have a time of feasting coming up: a holiday, friends or family coming to town, or as I do, a retreat that will have a lot of good food. Suppose also that you have had a good experience with fasting in general, heeding all of the cautions about fasting that I repeat at the beginning of my post “Increasing Returns to Duration in Fasting.” (By “fasting,” I mean not eating food, but continuing to drink water.) Is it better to fast before feasting or fast after feasting? The answer is “Fasting before feasting is better,” and follows from an interesting logic.

In “Increasing Returns to Duration in Fasting” I write:

I theorize that when you end your fast and resume eating, you will have an enhanced appetite in order to replenish your glycogen stores. By contrast, I think of the amount of body fat having a weaker effect on appetite.

Everything I say from here on is predicated on that theory. If the body’s desire to replenish glycogen stores enhances appetite after a sustained period of fasting, then it will feel natural to eat somewhat more than usual after a period of fasting. Thus:

  • If you fast right before a time when you were planning to feast anyway, that isn’t extra at all.

  • But if you fast after feasting, then you will have an additional period when you are like to eat extra after your fast. That would be over and above the feast before your fast.

So, if your appetite is enhanced by the body’s desire to replenish glycogen stores after a fast, you will probably end up at a lower weight if you time that extra appetite to coincide with the feasting that you were going to do anyway instead of having that extra appetite come later, when it is likely to lead to an additional time of heavy eating.

Another totally equivalent way of looking at things that will make more sense if you read “Increasing Returns to Duration in Fasting” is that if you time a fast so it comes before a time of feasting you were going to do anyway, you escape the “fixed cost” of a period of fasting coming from the extra appetite to rebuild glycogen stores after your fast is over. In saying this, I am using the calories in/calories out identity, but treating calories in as something highly endogenous that depends on your appetite. And I am assuming that you are eating low on the insulin index so that calories out don’t change much while you are fasting. (See “Forget Calorie Counting; It's the Insulin Index, Stupid.”)

Note that eating low on the insulin index leaving calories out at a normal level is closely related to eating low on the insulin index making fasting easy: if your body were trying to reduce calories out it would likely make you feel sluggish at best and truly crummy at worst, which is no fun. But if your body is expending just as many calories as normal, you have a good chance of feeling fine during fasting, at least once you have adapted to the new way of eating when you do eat. (See “David Ludwig: It Takes Time to Adapt to a Lowcarb, Highfat Diet.”)

For annotated links to other posts on diet and health, see: