Intermittent fasting (IF) is an eating strategy that's been around for a long time, but has been more prominent in the news the last few months for some reason. I'm a fan (the blog description even lists IF as one of its cornerstones), so I thought I'd discuss it for anyone who is curious.
What is Intermittent Fasting?
This is sort of tricky, because almost everyone practices a kind of IF. Most people go at least 8-10 hours a day without food - while they're sleeping. I know there are some people who get up in the middle of the night to grab some kind of snack, but they're in a minority.
So for an eating plan to 'count' as IF the fasts should be longer than 8-10 hours. Here's a pretty good start:
IF is a plan whereby one deliberately and on a regular basis goes without calorie containing food (so water is fine) for stretches of time that are at least 12 hours (NOTE: for most IF strategies the fasts are at least 16 hours long, so consider 12 the very shortest fast that might barely count as a fast) but NOT longer than 36 hours.
The MOST COMMON strategies I've seen are:
1. Restricted feeding window - Eat every day, but only for a period of time between 4 and 8 hours. So maybe every day eat normally between noon and 8 PM, then fast until noon the next day - this is a 16 hour fast, and an 8 hour eating window, sometimes called 16:8. You can shorten the feeding window, to get a 18:6 or 20:4 or whatever.
2. Every other day eating - one or more days per week, don't eat or eat MUCH LESS food than normal. So if your fast day is Monday, you'd eat normally on Sunday, stop at night, on Monday consume either NO calories at all or many fewer calories than normal - perhaps 20-40% of your 'normal' daily food intake. Obviously, you couldn't do this more than three days a week (or alternate 3 and 4), but most plans like this seem to average around 2 'fast' days per week.
What is IF good for?
This is the question.
In my opinion, you have to look at IF, and at the evidence supporting it, in 3 ways:
1. IF (probably) helps you create and maintain a caloric deficit, and caloric deficits have benefits;
2. IF may have health/performance benefits even compared to a diet with the same calories that are more spread throughout the day;
3. IF has lifestyle advantages that are completely separate from 1 and 2.
The reason I split these up is that the evidence in support of each point is different, and people comparing IF to other eating plans often get these confused. I'll address each one.
#1: IF helps you create and maintain a caloric deficit
Not everyone wants a caloric deficit (the leangains guys will use IF to build muscle), and this is NOT the only reason to do IF, but a huge selling point for IF is that it makes it much easier to eat fewer calories, and to sustain that lower caloric intake for a long period of time.
Suppose someone eats 4 times a day, about 500 calories per meal (that's like, a small sandwich and a Diet Coke), for 2000 total calories a day (much more likely is a 350 calorie breakfast, a 450 calorie lunch, 2 snacks for 100 calories each, and a 1000 calorie dinner, or something like that, but I'm trying to keep my math simpler). Suppose that person wants to create a 500 calorie deficit, because they're trying to lose bodyfat or increase insulin sensitivity or for some other health/appearance goal.
That person COULD shrink all their meals by 125 calories each. Eat 3/4 of a sandwich instead of a whole sandwich. OR that person could just skip one of the 4 meals, and eat 'regular' meals the rest of the time.
This probably isn't universally true, but for many (possibly most) people it is much easier to skip a meal every day than to eat the same number of smaller, less satisfying meals. Think about how people overeat - most people eat a large dessert or something at the end a meal, we are less likely to just go have a huge dessert in the middle of the afternoon. Eating triggers more eating for many of us, especially those of us who are overweight (those of us who need that caloric deficit the most).
Now there is definitely an adaption period to IF - if you're used to eating all day, then going all morning or an entire day without food is not easy. But in my experience it takes at most 2 weeks to adjust, and most often less than that. Your body 'learns' to go without food. Obviously, 36 hour fasts are a little more challenging than adopting a 16:8 eating window, but almost everyone I've known who has tried it has been surprised at how easy it is, and how much easier it is than eating the same number of meals but with smaller portions.
Now, what's the point of having a caloric deficit? That's primarily (though not entirely) about body composition. Meaning, in order to lose bodyfat, you pretty much have to cut calories. There are also probably benefits for everybody to be in a caloric deficit at least some of the time, to improve insulin sensitivity and increase autophagy, but that's less clear (in other words, I'm really sure that if you have too much bodyfat, you should be in a caloric deficit, and I suspect that periodic caloric deficits are good for everybody, but I'm less confident in the latter).
#2: IF may have health/performance/longevity benefits even compared to an isocaloric diet
There is a ton of research on the health impact of caloric deficits, but IF proponents often claim that IF has benefits separate from the caloric deficit. In other words, an eating plan where someone eats 1800 calories a day in a 4 hour window (maybe 2 large meals) will have a different outcome than someone who eats the same number of calories spread out through the day (3 small meals and a couple of snacks, covering 12-16 hours of the day).
I'm going to sum up the research on this. Basically, there are a few reasons why we think regularly going without food for long-ish periods of time does things to your body that don't happen when you trickle in a constant supply of food, even if the total amount is the same.
A: Longevity - there is some evidence that IF makes small animals live longer. This may or may not cross over to humans. I find the evidence compelling but not certain.
B: Insulin Sensitivity - IF may improve insulin sensitivity. I have heard diabetes researchers (actual MD's working in clinics, not the trainer at the gym) who claim IF does wonders for patients with type II Diabetes. If you're overweight you probably have poor insulin sensitivity, so this would be a good thing. I find the evidence here more compelling but still not absolutely conclusive.
C: Preserving Lean Tissue - IF, counter-intuitively, seems to make it easier to hold onto your muscle mass. This is a good thing. When you lose 'weight,' you really want to lose bodyfat while NOT losing muscle. IF may help with that. The evidence here is moderately convincing.
D: Autophagy and cancer prevention - When you don't eat for a while, your body starts 'looking' for places to get nutrients. One of those ways is to ramp up the cleanup processes that normally happen, clearing damaged proteins out of cells, looking to recycle nutrients. This is probably a good thing. Some research suggests that periods of fasting can help prevent cancer by putting your body in a position to kill cancerous cells very early in their formation. Evidence? More than none, but don't skip your chemo just because you didn't have breakfast.
#3: Lifestyle benefits of IF
Once you get used to IF, the benefits to everyday life are pretty large, especially when compared to an isocaloric (same total food in take) but more spread out plan.
A: Time - Eating fewer meals means saving time - less time preparing, less time eating. Cooking a 1000 calorie meal does not take much longer than cooking a 700 calorie meal, so removing a meal or two from your day is a big savings.
B: Social - On IF it is MUCH easier to eat out and socialize than on a 'regular' calorie-restricted diet. For better or worse, a lot of modern social life revolves around eating. It is VERY hard to be on a regular diet and go to a restaurant or a family function without either cheating or having a miserable time staying away from all the food. On IF, as long as you schedule social events during your eating window (which is usually pretty easy), you can often eat pretty much everything you want and still stay on track. For example, last night I knew I was going out to my favorite restaurant, so I ate only a light snack, about 320 calories, at 3PM and really pigged out on Korean food at 7. If I'd had a 'normal' breakfast and lunch I would have had to really restrain myself at dinner to stay in a deficit for the day. If your style of IF means fasting 2 full days a week, just pick days when you aren't out with friends.
C: Building resilience to fasts - IF builds up your ability to skip meals (because you're doing it every day). Suppose you're traveling and can't find a place you like for lunch. Just... don't eat lunch! If you're used to getting 3 or 4 squares a day, skipping lunch can be daunting, but for an experienced IFer it's no problem. And traveling is MUCH easier if you know you don't have to find places to eat all day long. It is very liberating to not worry about getting all your meals in.
But can I train while fasting?
Short answer: yes, especially once you get used to it (that is, workouts while fasted might be tough for you for a couple of weeks, but you'll adapt quickly).
If you're doing longer fasts, followed by longer workouts, you might have a hard time. So don't plan any 36 hour fasts that lead up to, for example, a belt promotion, or a weekend of camp where you're training 6 hours a day. Skip the fasts during times right before exceptionally long training sessions. But I've often followed 20 hour fasts with 2 hour martial arts classes, including sparring, and seen very little downside.
IF definitely frees up some of your time and makes travel easier.
IF is a really, really good way for many people to help manage a caloric deficit (in other words, if you're trying to cut calories, IF is a good way to make that easier to maintain).
IF is very likely a good way to improve insulin sensitivity. So if you're pre-diabetic, have a history of diabetes, or are Type II Diabetic (in which case talk to your doctor before making any dietary changes), IF is probably going to help.
IF might improve your immune system, might increase your lifespan, and might help prevent cancer.
IF might help you increase or preserve muscle mass, especially when in a caloric deficit.
If you need to/want to lose bodyfat IF is probably a really good choice. If not, it might be a good choice for the other reasons, but I can't say that with as much confidence.
Last point: if you're pregnant or still growing or breastfeeding, don't fast. If you have a metabolic disorder (like diabetes), talk to your doctor first. Otherwise, remember that if you're not used to fasting, you'll probably feel like crap when you first try it, but the adaption is really fast. As in, you'll be okay within a week or two.