I've mentioned before that I practice intermittent fasting. If you're familiar with the topic, you can probably skip this post. If not (and many people aren't), I thought I'd write a brief introduction to the idea with some basic facts about what it is and why people do it.
We all sort of know what fasting means - it means not eating for a period of time. Most people include an 8-12 hour fast nearly every day of their lives - when they sleep. Some people talk about juice fasts or maple syrup fasts or "cleanses" - I don't want to discuss those too much other than to say that a juice fast or a syrup fast is like an eating program designed to give you type II diabetes as quickly as possible while cannibalizing as much muscle tissue as possible. I can't imagine a quicker way to destroy your health and performance without using illegal substances. When I talk about a fast, I mean something like a water fast. In other words, eat no food (or very little food) and drink water, or possibly other low or zero calorie drinks. For some people this means they'll include diet soda or tea or coffee, sometimes even with a small amount of cream or sugar - it has to be a very small number of calories. I, for example, drink tea sweetened with stevia during my "fast," and I take some fish oil in the middle of it. Otherwise no calories pass my lips.
Intermittent fasting describes a lifestyle where someone periodically fasts. I'd say that the fasts have to be longer than 12 hours - most people fast 12 hours a day (from after dinner until the next morning), so it doesn't make much sense to call that by a special name. Fasting periods go from 16 hours (leangains) to 36 hours (ADF, or alternate day fasting, where fast from day 1 after dinner until day 3 breakfast). Fasts longer than 36 hours are problematic because the chance of breaking down lean body tissue is too high. Fasting frequency varies from daily (fast 16-22 hours; condense all eating into a shortened period, which is what I do) to weekly or even monthly (usually used with all day fasts from 24-36 hours).
Intermittent fasting is done primarily for two reasons: weight loss or health/longevity. I'll talk a little bit about each.
Fasting for weight loss is a fairly simple idea. To lose fat you have to establish an energy deficit - your body has to burn more energy than it uses. People usually do this by trying to reduce their energy intake on a daily basis. You can do the same thing with fasts - going periods of time each day or each week without eating. Some find this easier to deal with - I personally find it easier to skip meals than to eat a small amount. I'd rather just be hungry, and eat to satiation when I do eat, than graze all day.
Fasting for health/longevity is a more complicated issue. Forget the cleanse myth that not eating will somehow help your bowels clear out fecal matter that's been stuck there for days or weeks - that's just not how your bowels work, as every surgeon knows. On the other hand, periodic nutrient deprivation may force your body to do a whole bunch of things that improve health and extend life. For example, when you don't eat your insulin levels tend to go down. Insulin is pro-inflammatory and is implicated in a whole host of health problems. Less insulin, less inflammation, less aging. Another example is cellular autophagy. Your cells accumulate junk organelles - damaged proteins and other molecules that don't do anything. This junk is also implicated in the aging process - as the junk builds up the cells run less efficiently. How do you force your cells to break down and clear out the junk? Stop feeding them. Without a flow of nutrients your cells will metabolize the junk for fuel.
The science behind fasting for health/longevity is a lot weaker than that behind fasting for weight loss. There is some evidence that fasting can reproduce some or all of the benefits of calorie restriction (which is pretty much the gold standard for extending life in anmials) without the drawbacks (loss of muscle and energy, lethargy, etc.) I don't want to start quoting hundreds of studies here, but trust me that the evidence is mixed, but convincing enough for me.
At this point you might be somewhat interested but worried about all the things you've heard that are bad about fasting. Let me guess - eating less often will screw up your metabolism, cause you rmuscles to waste away, kill your energy levels, make you miserably hungry, cause you to faint, and wreak havoc with your performance. All untrue. If you want, check out some of the sites I'm going to include at the end of this post to read up on the science. Or, just think about this for a minute. We evolved from hunter gatherers. They often went periods of time without food - they didn't have refrigerators or supermarkets to shop at, and we know they weren't always successful in their hunts. Do you think they evolved to pass out or waste away any day they didn't manage to get in 3 (or 4 or 5) square meals? Do you think a species that got faint and weak at the first signs of hunger, right when they needed to be able to go out and hunt effectively, would be competitive biologically?
I fast to help manage weight, to save time (I don't cook breakfast or lunch, and cooking a big dinner is not as time consuming as cooking three separate meals), and for longevity (I believe it will extend my life). If I have a very hard day of physical activity planned - say, an 8 hour promotion or something - I'll snack during the day, but otherwise I train all the time without having eaten anything for 16-20 hours. There was definitely an adjustment period when I first started this (which was about 3 years ago) but now there's no problem.
Fasting is absolutely one of those things that I recommend with reservations. I believe it's good, but I find the evidence sketchy. That just means the subject hasn't been explored fully, not that the idea is wrong. As I learn more I'll share, through this blog, what I learn. In the meantime, if you want to learn more check out some of these sites and resources:
The first stops:
For some more help:
I think I'm missing some, but that's a good start.