How does fasting work?

Since this is a blog, not a textbook or medical seminar, I’d like to keep things simple and try to explain with an analogy we can all relate to: money.  Think of glucose (“blood sugar”) as cash—we need some of it on hand for spending (metabolism), but we don’t want to keep too much on hand.  Therefore, we need to store excess glucose and are able to do so in a variety of forms.  One major place we deposit excess glucose is in the liver and muscle, where it is stored as glycogen.  Think of this as the checking account.  It’s there and readily available when you need it, but still it’s a relatively small amount.  During exercise or after a period of fasting we dip into that “checking account” and use glycogen first, but we can only store enough for shorter workouts or about 24 hrs of fasting.  And just like a checking account, you can “bounce a check” too; have you ever hit the wall or bonked during a particularly challenging workout?  That’s what it feels like to be a carb-adapted athlete and run out of glycogen.

Fortunately we have a second, much larger reserve set aside when we run out of glycogen: body fat.  Think of fat like the high-yield savings account or investment account.  You only put money into it when you have enough “cash” to cover everyday expenses and some in the “checking account.”  You see, we only store fat when you have excess energy/glucose.  I guess you could say that, by this analogy, all of us are millionaires!  But just like a real savings/investment account, the “money” stored in fat isn’t readily accessible for immediate spending; there is some waiting period while it is converted to cash.  As it turns out, your body can be trained to do this faster and more efficiently; this is what is referred to as being “fat adapted.”

Now let’s imagine what happens when you are fasting.  In the analogy, this means all of your cash and checking are depleted (ie, low blood glucose and depleted glycogen) and there are no more funds coming in.  Your only option is to liquidate some of that savings by burning fat or muscle for energy—don’t worry, so long as you have fat its the preferred source.  In prolonged fasts (or wasting conditions such as cancer cachexia), the body eventually runs out of fat and starts to break down muscle protein and convert that into glucose in a process called gluconeogenesis.  I liken this process to liquidating a retirement account or 401k: it can be done, but only in dire emergencies.  Now, things are obviously a lot more complicated than I’ve made it out to be with this analogy, but can you think of a better way to kick start fat burning than fasting?  On the other hand, just stop and imagine for a moment what happens when you are constantly snacking—especially on high carb foods.  Why would you ever tap into the savings account with the constant influx of cash?

So, in summary I guess we can all agree that it’s good to be broke!  Let me pause here and repeat that this is an over-simplified analogy that I think helps to conceptualize fasting.  It’s obviously not biochemically precise; after all, fat is not converted into glucose, but rather into ketones  and lipolysis (fat burning) doesn’t occur exclusively in the fasted state.  If my analogy didn’t help you to conceptualize this, maybe some other folks will do a better job than me.  Check out this video by Dr Jason Fung, where he makes a different analogy using the concept of the freezer and refrigerator.  If you’re not into analogies, just read Dr Fung and Jimmy Moore’s book “The Complete Guide to Fasting” for a more in-depth look at fasting.

That’s it for now, but check back soon for some more food for thought as the series on fasting continues.