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There are many different forms of fasting, however, ranging from going extended periods of time without food to consistently eating less (perhaps cutting caloric intake by 20%) to intermittent or periodic fasting.
The hard part about classifying "intermittent fasting" is that there are a number of different forms of this kind of fast. Intermittent fasting regimens range from only allowing yourself to consume calories within a certain span of the day, likely between six and 12 hours; to eating normally five days a week and dramatically cutting calories on two fasting days; to taking a 36-hour break from food every week.
The different forms these fasts can take mean that much of the research showing benefits might be true for one of these fasts but not necessarily others. But there is good research on several of these fasts indicating that the benefits of intermittent fasting go beyond weight loss. There may be real long-term disease-fighting health improvements.
A recent study suggests that intermittent fasting can do more than help people lose weight — it also may improve blood pressure and help the body process fat.
For this small study, researchers had overweight participants either cut calories every day or eat normally five days a week and only consume 600 calories on their two fasting days.
Both groups were able to lose weight successfully, though those on what's known as the 5:2 diet did so slightly faster (though it's not clear the diet would always help people lose weight faster).
More significantly, those from the intermittent fasting group cleared fat from their system more quickly after a meal and experienced a 9% drop in systolic blood pressure (the "regular diet" group had a slight increase in blood pressure).
This was a small study and researchers say participants had a hard time following the diet, but these are promising results.
Other studies indicate intermittent fasting could reduce risk for forms of cancer, but more research is needed.
Other small studies on a similar 5:2 diet and on other intermittent fasting diets have shown that this form of intermittent fasting is associated with physical changes that could lead to reduced cancer risk, particularly for breast cancer.
Much more research on this area is needed, but these are promising results, Mark Mattson, a neuroscientist at the National Institute on Aging at the National Institutes of Health, previously told Business Insider.
There may be evolutionary reasons why depriving ourselves of food for some time makes us feel energetic and focused.
"Hungry," from an evolutionary perspective, isn't lifeless or drained. It's when our bodies and brains need to function at maximum capacity.
"It makes sense that the brain needs to be functioning very well when an individual is in a fasted state because it's in that state that they have to figure out how to find food," Mattson previously told Business Insider. "They also have to be able to expend a lot of energy. Individuals whose brains were not functioning well while fasting would not be able to compete and thrive."
Periodic fasting may make it easier for us to burn fat and enter ketosis.
Blood samples have shown that people who fast from 12 to 24 hours at a time enter a state called ketosis, when their bodies start to derive more energy from fat, Mattson told Business Insider in another interview.
The more you enter this state, the better your body gets at using fat as fuel. For that reason, some people try to trigger ketosis with "keto" diets that involve consuming a lot of fat. But according to Mattson, fasting is a significantly more effective way of boosting ketone levels.
Intermittent fasting may strengthen neural connections and improve memory and mood.
Many people who fast intermittently say that at times, they feel more clear and focused while fasting.
There's real science to back up the idea that being "hungry" gives you a sense of focus. Entering ketosis triggers the release of a molecule called BDNF, which strengthens neurons and brain connections linked to learning and memory.
That's one of the reasons researchers have suggested that ketogenic diets (both the fasting kind and the fat-heavy kind) could be useful for people fighting degenerative brain diseases like Alzheimer's. That also could explain the clarity or focus some people feel after fasting. It may provide a mood boost as well.
Research indicates that some forms of intermittent fasting may help with diabetes.
Both in mice and in people, there's evidence that certain forms of intermittent fasting can improve the body's response to sugar. In mice, researchers have basically been able to reboot the pancreas, which produces insulin, reversing diabetes with periods of fasting like the 5:2 diet.
Intermittent fasting works at least as well as other forms of dieting for weight loss.
No form of restricting food is necessarily easy, and people who get started with intermittent fasting for the first time agree that it's no picnic. On the one hand, it's nice to eat whatever your want when your diet isn't restricted — but it's also very hard to know you are still hours away from food when struck with a craving.
But research does indicate that intermittent fasting is at least as good as other forms of dieting for weight loss. That plus the other health benefits might make it a preferred candidate for many.
Certain forms of fasting are associated with anti-aging health effects, though it's not clear whether intermittent fasting does this for humans.
Various forms of fasting have been associated with significantly improved lifespan and healthspan — the time an organism is healthy — in several different studies.
This has mostly been demonstrated with caloric restriction in animals, which cuts the amount of calories these animals are provided by 20-30%. There's limited evidence that this may work for humans too.
But that sort of fast doesn't sound necessarily safe or pleasant.
Valter Longo, an anti-aging researcher at the University of Southern California, has published research and written a book about a diet he's developed that he says provides the health and anti-aging benefits of fasting while still letting people eat normally 25 days a month (the other five are pretty rough).
It's unclear whether intermittent fasting would trigger the same benefits, though it's possible.
More research is still needed on the different forms of intermittent fasting.
It's appealing to think that fasting might be an ancient survival mechanism that triggers healing processes in the body, as many fasting researchers suggest.
But that doesn't mean all forms of fasting are the same or that they have the same health effects — many will vary from person to person, and you should always consult your doctor before trying any severe dietary changes.
In his new book, "The Longevity Diet," Longo cautions against using the term "intermittent fasting" too broadly. We know various forms of fasts — only eating during certain hours, restricting eating one or two days a week — are associated with health benefits. But we don't know that all these health benefits are the same for all fasts.
But even so, many of these intermittent fasting regimens are considered relatively safe for a healthy person. So if they appeal, they could be worth a shot. And they may come with a host of health benefits.
