In the great melée of wellness trends, diets, fitness plans, and sleekly packaged miracle products vie for your attention.
Amid the Instagram influencers, TikTok videos, and plain old hype, it can be hard to cut through the haze to find a food regimen that actually does what it claims to do.
Intermittent fasting positions itself specifically as a not-a-diet diet plan — limit not what you put into your body but when. The not-diet diet claim is backed by hordes of intermittent fasting devotees who say it works, but what exactly it works to do is unclear.
Ask a researcher who studies fasting or ask the people who practice fasting themselves, and you will quickly realize: How they know intermittent fasting works depends on who you ask.
Researchers can measure changes in the body of fasting organisms at the most precise level and compare them with non-fasting organisms and compare the two.
But everyday people incorporating intermittent fasting into their lives don’t typically come with a control group. Instead, they look for signs of change, which can range from weight loss to psychological shifts. To complicate things further, people who start to fast tend to make other lifestyle changes beyond the timing of their meals.
How the experts measure intermittent fasting results
Scientists who study fasting use certain biomarkers — features in the body — that can pinpoint whether a fast is doing something or not. These can include:
- Body fat changes
- Chemical and hormone blood level changes
Valter Longo, who’s been called the “godfather of fasting,” tells Inverse that intermittent fasting is “working” if one sees changes in “weight, improving insulin sensitivity, reducing cholesterol and inflammation and blood pressure and sleeping better.” Longo is a professor of gerontology and biological sciences at the University of Southern California, where he directs their Longevity Institute.
Mark Mattson tells Inverse that fat loss in specific areas of the body can also be a sign of intermittent fasting at work. Mattson is a professor of neuroscience at John’s Hopkins School of Medicine and has extensively researched fasting and the brain.
“[Intermittent fasting] is working if there is a reduction in abdominal fat — waist size, an increase in insulin sensitivity as indicated by reduced fasting glucose and insulin levels, reduced resting heart rate and blood pressure.”
But beyond academia, droves of intermittent fasters seem sold — so is it really all about the timing? Or the food reduction that the time restriction might help facilitate?
Intermittent fasters: Two ways to know
Inverse talked to five intermittent fasters on Reddit to find out: how do you know your intermittent fasting regimen works?
Their responses echo some of the researchers’ replies, too: They observed changes within their body and these changes tended to include weight loss. Here’s what they had to say:
- “I started for weight maintenance, and it’s been good for that,” writes Patrick, a 30-year-old based in Australia.
- “My resting heart rate has gone down by 40 points (might have been on verge of high BP when I started, I ain’t anymore),” writes Reddit user u/mogli_170, who is 27 years old. “My weight has reduced a significant amount (~ 10kg ).”
- “The reasons that I feel that [intermittent fasting] is working for me are that I’m losing weight in a steady way,” writes Reddit user u/RS_Stylish, who is a 24-year-old man.
- “I guess I know it’s working because it’s the steadiest, most consistent weight loss I’ve ever achieved,” writes Reddit user u/Miszteek, who is a 35-year-old woman. “I fasted for two weeks sans exercise and lost the same amount of weight as the two weeks in which I did exercise.”
Essentially these responses boil down to two quantifiable things: Steady weight loss and a lower resting heart rate.
But much of the fasters’ enthusiasm for the regimen seems to stem from other, more qualitative factors — intermittent fasting was something people could realistically maintain, and intermittent fasting resulted, perhaps most crucially, in a change of their relationship with food and dieting.
“Working on an empty stomach is really a game-changer. I came to understand that what I had felt before wasn’t really hunger and that resting my stomach for long periods makes me feel very energetic,” Jesus Sato, 42, who lives in Mexico, tells Inverse.
Sato was overweight as a child and went to “countless nutritionists” who gave him the same advice: eat three meals a day with two snacks and do two hours of exercise.
“After a month, I wouldn’t see results, despite doing everything,” Sato says. “I lost maybe two pounds, but it was a constant suffering, eating lots of lettuce, tuna, whole-grain bread, no mayo, and so on.”
“Fast forward to seven months ago, started reading about [intermittent fasting] and all of a sudden began losing weight,” he says.
“[Intermittent fasting] is just easier for me. I like to eat a lot. It allows me to eat a lot over a shorter time frame without exceeding calories,” Patrick tells Inverse.
“I was obese as a child and teen. I’m very fit now but still haven’t shaken that ‘food as an emotional outlet’ mentality,” he says.
Sato even describes a “special feeling” he gets during fasts — especially extended ones. “When passing around the 36-hour mark, there is an even stranger feeling, [a] sort of ‘spiderman sense,’” he says.
“You get energized and more ‘aware’ of things, sounds, smell, etc. It is hard to explain, but it is like that.”
Intermittent fasting vs. calorie restriction
Here’s where it gets tricky: calorie restriction can also lead to weight loss and changes in cardio-metabolic markers.
Some mouse studies have kept calories constant to evaluate intermittent fasting independently and only changed the feeding schedule. They have compared a regime of caloric restriction with intermittent fasting to a regime of calorie restriction alone. These studies have yielded somewhat mixed results, so there are no clear answers to be had there.
One human study on intermittent fasting, which involved changing only the timing of people’s meals without any calorie restriction, found very negligible results for participants.
This interplay can make it hard to know what’s really behind the weight loss, blood pressure changes, insulin and glucose regulation, and other effects seen in research and people who follow intermittent fasting regimens. Is it the timing, or the reduction in calories that yields results?
“I think that’s mainly just driven by people eating less.”
Three of the five fasters Inverse interviewed also practice a ketogenic diet, have cut out certain foods (one says he stopped eating unhealthy meals his mother made, for example), or exercise while intermittent fasting, which may contribute to their results.
“Intermittent fasting has become popular over the past five years, and people go for weight loss, but I think that’s mainly just driven by people eating less,” Krista Varady, a professor of nutrition and kinesiology at the University of Illinois, Chicago who studies intermittent fasting, tells Inverse.
Three out of the five fasters also adhere to extremely tight eating windows: 19 hours of fasting and 5 hours of eating, or 20 hours of fasting and 4 hours of eating. One faster also claims to follow the one meal a day or OMAD regime. With only four hours to eat, it’s perhaps impossible to eat much more than one regular meal’s worth of food.
But Longo tells Inverse that this kind of drastic restriction isn’t recommended.
“I think people should stay away from the idea of just eating once a day,” he says. “It’s very, very risky. And in fact, based on the data, problematic.”
Ultimately, people themselves do not follow the same rigorous, controlled schedules as participants in scientific research. It is reasonable to suspect that what they’re experiencing might be a calorie deficit brought on by the mental game intermittent fasting sets in play.
One of the intermittent fasters Inverse spoke to, Patrick, agrees.
“For me, it’s primarily a form of calorie restriction.”
If you ask fasters, intermittent fasting is not strictly a diet — but you might end up eating less.