But there are problems with intermittent fasting, too. Besides leaning heavily on animal studies, the approach may have several benefits that only arise as an incidental result of the fact that it tends to lead to weight loss.
With that in mind, researchers have recently been trying to pin down whether some of those benefits could emerge even if intermittent fasters don't lose any weight.
And for the first time, a rigorous, but small study published last month has hinted at a positive answer. They discovered that people who tried the IF approach but were given enough calories to prevent them from losing weight still saw boons like reduced blood pressure. That's a promising sign for future studies of the plan.
"We found that there were benefits to this approach that were completely independent of losing weight," Courtney Peterson, the lead researcher on the study and an assistant professor of nutrition science at the University of Alabama at Birmingham, told Business Insider.
Intermittent fasters can eat whatever they want, within reason — there are no strict limits on carbs, fat, or anything else. In general, most intermittent fasters stick to their normal diet; all they change when they begin the eating plan are the hours in which they eat. Having temporarily tried the IF lifestyle myself, I can tell you it's not for everyone. That said, I also understand why some people love it. When I fasted, I found myself thinking about food less, working out more, and even unintentionally curbing my caffeine intake.
So far, the most well-researched benefit of intermittent fasting is weight loss. Krista Varady, a nutrition professor at the University of Illinois who wrote a book about IF called "The Every-Other-Day Diet" in 2013, published a study last year in the Journal of the American Medical Association showing that obese participants who intermittently fasted lost roughly the same amount of weight as those on a traditional diet that involved strict eating and calorie counting.
But a handful of recent papers suggest that in animals, intermittent fasting is linked with other more vital boons like improvements in blood sugar control and some antiaging effects. With that in mind, some researchers have been hard at work trying to suss out whether those benefits might also apply to people. At the same time, they also want to know something even more important: whether those perks are just a result of weight loss, or if they might have something to do with intermittent fasting itself.
Nutrition studies are hard to design and even harder to carry out. In many cases, scientists must rely on self-reports from participants, who often vastly underestimate the amount of food they actually ate.
So for Peterson's recent study, she and her colleagues decided to take a far more intense approach: they supervised everything their participants consumed, and only allowed them to eat the food that was given to them.
These sorts of studies are called supervised controlled feeding trials, and because of the difficulty involved in designing and performing them, they're rarely done. But the sort of insight they provide into a specific diet or eating plan is unique and high-caliber.
"Aside from locking up people in a hospital room and not letting them leave that room for weeks, supervised controlled feeding trials are the most rigorous type of nutrition study," Peterson said.
The difficulty of this kinds of research also means that large pools of people often get whittled down to just five to 25 individuals. After receiving interest from nearly 400 people who wanted to participate in the study, Peterson and her colleagues ultimately ended up with just eight men.
Still, the paper is the first study of its kind, and it hints at some surprising potential perks of fasting.
Although Peterson's study was small, some of its results were surprisingly positive. After doing intermittent fasting for five weeks, all of the eight participants showed improvements in blood pressure and insulin sensitivity, the body's response to sugar.
Notably, the participants in the study who had the worst sensitivity at its outset saw the biggest improvements, Peterson said. The blood pressure findings were also significant: many people showed drops of roughly 10-11 points, a difference that's roughly equal to the benefit someone might get from trying out a blood pressure reduction medication, according to a 2008 study published in the Cochrane Database.
"These were huge differences for a 5-week study," Peterson said. "I was very surprised by that."
To do the study, Peterson and her colleagues had eight men who showed early signs of diabetes restrict their eating window to just six hours. During this time, they ate only food provided by the researchers, and only while under supervision. Importantly, in order for the researchers to home in on the potential benefits of fasting that were not linked with weight loss, the study participants were not supposed to lose any weight. With that in mind, they were given just enough food to maintain their current weight. .