Intermittent fasting may not offer dramatic benefits — here’s why

What defines success depends on who you ask.

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Intermittent fasting encompasses a few approaches to food: eating during only certain hours of the day, refraining completely on certain days, or drastically reducing calories on a few days of the week. And yet, scientists and practitioners alike insist it’s not a diet. It’s an eating pattern.

The appeal of intermittent fasting to a general audience is arguably just that: it’s non-dietness. If you limit your eating to certain hours of the day, the reasoning goes, there’s no need to restrict calories.

It makes sense then that intermittent fasting is often pitted against calorie restriction — specifically reducing calories consumed per day — in scientific studies. The question is: Does it actually help people achieve their health goals, would they be better off trying calorie restriction, or should they choose something else entirely?

Intermittent fasting versus calorie restriction

What’s better: intermittent fasting or calorie restriction?

Implied in these comparisons is a debate about which is healthier, “better,” or more successful - and one about losing weight. But scientists and consumers define success very differently when it comes to the much-hyped eating practice. For intermittent fasting to be “better” than calorie restriction could mean more than just shedding fat. The comparison also assumes that one is to be practiced without the other. The reality, of course, is more complicated.

What’s more, we don’t yet have a complete picture of the long-term effects of fasting. Animal studies have shown compelling evidence that various forms of intermittent fasting confer significant health benefits to mice and rats, including longer lives.

But humans aren’t mice and rats. People aren’t fed precise amounts by researchers in a lab, and in nutrition studies that rely on people to recall what they ate, the information isn’t always accurate. It’s no surprise that when it comes to human trials of intermittent fasting, scientists themselves aren’t always in agreement.

It’s not always clear if intermittent fasting is a “diet” like calorie restriction or an “eating practice.”

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What does calorie restriction do?

Calorie restriction is the act of reducing your intake of daily calories below what your average might be, while still getting the nutrition your body needs. Many studies have suggested that calorie restriction imparts health benefits: For example, in a long-term study called CALERIE, scientists found that in normal and slightly overweight adults, a 25 percent calorie restriction led to reduced fat mass and better overall markers for heart and metabolic health. These include LDL cholesterol, blood pressure, insulin sensitivity, and others.

Many people are told by physicians (or choose by themselves) to restrict calories for weight loss and these potentially connected health benefits. Of course, calorie restriction for weight loss linked to appearance — and, some might argue, a narrowly defined standard of aesthetics — is also common. Almost every calorie-restricting fad diet — from Weight Watchers to juice cleanses — promises to transform waistlines, not blood glucose levels.

What does intermittent fasting do?

Intermittent fasting is about when you eat, not what you eat. It’s essentially blocking out times during the day or during the week when you fast or refrain from eating. Some popular versions of intermittent fasting include

  • The 16:8 schedule (16 hours without food, eating for 8)
  • The 5:2 (eating normally for 5 days, fasting or eating very few calories on 2)
  • Alternate-day fasting, or periodic fasting with occasional consecutive fasting periods

Scientists think when we stop eating for extended periods of time, our bodies undergo a “metabolic switch,” where the body moves from using glucose for energy to fatty acids and their descendants — molecules called ketones (which are featured in the reasoning behind a ketogenic or “keto” diet).

Matthew Phillips, a neurologist at Waikato Hospital in New Zealand, tells Inverse the switch gives our body’s cells a “mild bioenergetic challenge of nutrient deprivation by hunkering down and focusing more on maintenance, stress resistance, repair, and metabolic efficiency, rather than growth.”

Intermittent fasting may or may not result in an intake of fewer calories naturally, and people who fast intermittently may or may not have weight loss in mind. Proponents of intermittent fasting talk about their reduced inflammation, improved energy, mental clarity, and improved diabetes management - but research on humans in the long term is not yet conclusive.

What experts say about intermittent fasting vs. calorie restriction

For scientists, the “success” of intermittent fasting as opposed to calorie restriction might look like increased insulin sensitivity and blood glucose for diabetes management (indicators of reduced risk for heart disease), reduced symptoms of Alzheimer’s disease, or a boost in other brain function.

But many studies in humans focus on people who are overweight, and compare intermittent fasting to calorie restriction with the calories (both reduced from their normal intake) kept constant in both groups. As a whole, these generally suggest that both calorie groups lose body fat and improve blood pressure and cholesterol levels.

This implies that intermittent fasting and fewer calories could be a good alternative to just calorie restriction, simply because it’s easier for people to do. One can only eat so much within a short window of time.

“... there are health benefits that go beyond this calorie restriction.”

But proponents of IF argue it offers other benefits besides weight loss: reduced insulin resistance, for one, and that even without weight loss, in one study, early time-restricted eating (only eating in the morning) was effective at reducing blood pressure, oxidative stress and insulin resistance in prediabetic men.

Even the man who has been referred to as the “godfather of intermittent fasting” says you can’t lose weight without a calorie deficit.

“For people who are overweight, they have to reduce their calorie intake unless they increase their activity and exercise in order to lose weight,” Mark Mattson, tells Inverse. Mattson is a professor of neuroscience at John’s Hopkins School of Medicine.

“But studies we've been involved with... show that there are health benefits that go beyond this calorie restriction,” Mattson says.

Intermittent fasting is about when you eat, not what you eat.

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Other studies have found no real difference: In a 2018 JAMA Open Network study that compared intermittent energy restriction to continuous calorie restriction, there was no advantage observed for people with Type 2 diabetes in their glycemic control. The fasters had two days of low-calorie intake and five days of an unrestricted regular diet, much like the popular “5:2” regimen, and the calorie restrictors had 1,200 to 1,500 calories a day.

In another paper, this time a 2018 study published in JAMA Internal Medicine which observed otherwise “healthy” adults with obesity, alternate-day fasting did not offer any advantage to calorie restriction in weight loss or markers of heart health.

Meanwhile, a study released Tuesday in Science Translational Medicine specifically examined whether fasting offers “nutritional benefits” in “lean individuals” compared to daily calorie restriction, ultimately probing the idea of whether or not it is an effective way to lose weight. The study team found alternate-day fasting, in turn, was less effective, stating there is “nothing special” about fasting compared to standard diets.

This doesn’t mean that intermittent fasting doesn’t “work.” It means that when it’s used for losing fat mass and improving health markers like blood pressure, cholesterol, and diabetes, the science just isn’t conclusive on whether or not it’s superior. And if intermittent fasting is easier to stick to, it might be a satisfying alternative for some to counting calories.

Intermittent fasting and “real” people

All one has to do is go online to learn that when it comes to what “regular” people - as opposed to what nutrition scientists - are looking for, intermittent fasting is largely seen as a way to lose body fat. On Reddit, r/intermittentfasting is home to almost 800,000 members. Of the first ten “hot” posts, seven feature “before and after” pictures — visual representations of people who got thinner.

Over on Twitter, the “top” 4.5 million posts hashtagged #IntermittentFasting are more about meal photos, with a healthy percentage still body comparisons. And it’s clear that intermittent fasters are often trying a host of other eating plans, often simultaneously: from #lowcarb to #keto to #paleo, yo-yo dieting appears to be alive and well.

An #IntermittentFasting post on Twitter.


But from a clinical perspective, determining what one eating practice truly does requires scientists to test it in isolation. Meanwhile, few studies have explored what seems to be the basic premise of intermittent fasting: that one does not have to reduce or count calories, only adjust meal timing, to see benefits.

Ethan Weiss, a researcher and cardiologist affiliated with the University of California, San Francisco, co-authored a 2020 study with a team of researchers that aimed to parse it out. In it, two groups of people ate what they were used to eating but one group ate in a time-restricted window of 12:00 pm to 8:00 pm and the other ate another with no restrictions across three meals.

“... the amount of improvement in weight, and or metabolic health, is minuscule, relative to other diets.”

Both groups lost a very small amount of weight — between .75 and 1.17 percent of body weight — but there were no significant differences between the two groups in weight loss or in other health markers.

Weiss tells Inverse he got criticism for not asking the groups to reduce calories. But to him, that was the point.

“We didn't want to do that when we thought this [intermittent fasting] was sort of positioned potentially as an intervention where you didn't have to count calories or think about what you ate,” Weiss says. “You just had to think about when you ate.”

“I think the evidence continues to show that in well done large studies that last a decent amount of time that the amount of improvement in weight, and or metabolic health, is minuscule, relative to other diets,” he says. After the study, Weiss stopped intermittent fasting himself.

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