The Short Answer:
Ah, intermittent fasting! Yet another attempt by modern humans to suffer the way our ancestors did in hopes of becoming more awesome. Technically, it’s been around ever since the first time Neanderthal Jack didn’t leave his cave to pick berries for dinner because he was scared of the lightning gods. It simply means not eating for short stretches of time, typically somewhere between 15 to 24 hours, in hopes of triggering positive physiological changes.
For Neanderthal Jack, that meant living to pick berries another day (and not becoming a giant ember). But when done right, there’s plenty of anecdotal evidence to indicate that intermittent fasting may help promote weight loss, fitness, and a host of other health-related issues—even if the science backing it up isn’t all that compelling. With this in mind, the short answer is “Why not? You have nothing to lose” (except maybe invitations to Sunday brunch).
The Long Answer:
Many religions have featured what is essentially intermittent fasting for centuries, including Judaism with Yom Kippur and Islam with the holy month of Ramadan. But it has also become a secular trend in recent years with some fitness and nutrition professionals claiming it can do everything from burn fat and build muscle to cleanse the body and boost longevity. The only thing is, the research supporting those claims is a little muddy.
One small study in 2005 showed that non-obese subjects did burn more fat by fasting every other day, but they were also really hungry, which begs the question of whether it’s worth being miserable just to lose weight a little faster. Another oft-cited study showed that rodents that were only allowed to eat during a daily eight-hour window ended up weighing 28 percent less than rodents that grazed at will—despite both groups consuming the same amount of calories each 24-hour period. Unfortunately, this study suffers from the fact that the mice were fed a high fat, junk food diet. While that makes it easier to pack a day’s worth of calories into an eight-hour window, such calories effect the body differently than those in nutrient-dense foods. Indeed, if you eat a fibrous, balanced diet, trying to digest that much food that quickly would be interesting.
If you want to eat junk food eight hours a day, that’s your call. But if you want to lose weight, there’s a far more common sense solution. Considering that most dietary transgressions occur after dinner, when television watching and mindless eating go hand-in-hand, try a 12- to 15- hour daily fast, meaning that the kitchen closes after dinner and doesn’t open again until breakfast.
If you’re a morning exerciser, the next logical question is whether you should wait until after you work out to break your fast. On one hand, research shows that doing so can promote fat burning. On the other, your muscles will be low on glycogen, reducing your exercise performance and making it difficult to maintain a high level of intensity. If you can’t “bring it” the way you normally do, are you truly benefiting? Likely not. Bottom line: Unless you’re an elite endurance athlete trying to train your body to better mobilize fat stores for fuel, mixing fasting and exercise—what experts call “fasted state training”—probably won’t do much for you.
Beyond weight loss, the fitness blogosphere occasionally pushes intermittent fasting as a way to promote the release of growth hormone (GH), which technically should lead to muscle gains. One study from 2001 showed that while the GH secreted during fasting does have an anabolic (protein-building) effect, it’s released more to protect muscles from breaking down—as happens when you train fasted—than to make them bigger. Excessive under eating can even cause growth hormone resistance, which can lead to all kinds of other hormonal issues, according to a study in the Journal of Endocrinology. This isn’t to say that the IF/GH connection should be ignored, but the science to support its use for training just isn’t there yet. That said, fasting for 15 hours or so can hardly be considered under-eating, so if you’re thinking about advising intermittent fasting, this, again, may be the best range.
There are also a number of studies showing intermittent fasting may have additional benefits. One study showed that 20-hour fasts may help improve insulin sensitivity. Other research shows that intermittent fasting promotes longevity in animals, including a study on rhesus monkeys that found that intermittent fasting decreased their chances of dying from age-related illnesses by more than a third. There are also animal studies showing alternate-day fasting may prevent cancer. But like the mouse study mentioned previously, such studies should be viewed with healthy skepticism. Humans, after all, are entirely different animals.
If you do your research and determine intermittent fasting might benefit you, by all means go for it, but remember—you’re not a doctor (unless you are, then I apologize). From a holistic perspective, intermittent fasting can be an effective way to clean out the system. And accomplishing a task like this can be a strong psychological motivator. But if you’re looking for more detailed prescriptive answers, you’re better off strategizing with a healthcare professional.
On a final note, intermittent fasting can take many different forms. There’s the 15-hour night fast mentioned above, the five normal days and two ultra-low calorie days model, the 24-hour fast model, and several more. If you come across an expert touting a particular method, it’s highly unlikely that all the research he or she uses to back it up actually aligns with their plan. For example, several plans point to the eight-hour feeding window used in the mice study, but few of them actually limit you to eight hours a day of eating. Given the various ways studies can be interpreted, it’s beneficial to visit PubMed.com and read the research firsthand.
Intermittent fasting may one day become for nutrition what high-intensity interval training became for fitness—a technique used for years that science eventually catches up to and validates. But for the moment, nothing is concrete, so you’re better off staying cautiously optimistic. Dig into the research, and make sure the model you have in mind is sane. When it comes to fasting, a little digging and a lot of common sense go a long way.