Fast facts

Intermittent fasting: the 3 things nobody tells you before you begin

The relationship between fasting, weight loss, and fat is complicated.

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From Ramadan to Karwa Chauth to Yom Kippur, fasting intermittently has been part of centuries-long traditions before it ever reached the Explore page of Instagram.

But beyond its spiritual associations, intermittent fasting is currently popular as a diet (though it’s technically an “eating pattern”) designed to help people live longer and healthier. The question is: Can it actually deliver?

This is something scientists are working toward figuring out. It is known that, whatever your reasons for abstaining from food are, your body chemistry undergoes a number of changes when you fast. New research is helping to make sense of them, especially when it comes to the type of body fat actually affected.

Here’s what the science says so far regarding:

What does fat have to do with losing weight?

The answer’s not as obvious as you might think. Consider the meaning of “losing weight” and it’s just that: seeing the numbers go down on a scale. But that can happen in a number of ways.

For example, you can lose weight from muscle mass from, for example, a long stay in a hospital. Or you could gain muscle from strength training and lose fat — but still weigh in at a higher number.

“Generally, when we consider being overweight and losing weight, people are referring to visceral fat,” Dylan Harney tells Inverse. Harney is a Ph.D. candidate at the University of Sydney.

Harney is the first author of an intermittent fasting study published in March in the journal Cell Reports. His team found feeding mice every other day led to different responses from different types of fat. Visceral fat is fat that surrounds the organs in the visceral cavity or the belly.

“It’s the fat that we commonly associate with morbidities such as heart disease, diabetes, and cancer,” Harney says.

Subcutaneous fat, meanwhile, sits just under our skin — think thighs, butt, and the top layer of fat on the abdomen. It changes less with weight gain, and typically females have more of it than males.

This study suggests if “visceral fat loss” is what we’re really talking about when we say “weight loss,” there are a number of factors to consider before trying intermittent fasting.

Intermittent fasting, in the strictest sense, means eating at certain times — certain hours of the day, or five days of the week — rather than restricting calories over time, though the two are often combined. If it causes the same effects as calorie restriction, the implications would be significant. Caloric restriction involves consuming fewer calories than is typical for an individual, without malnutrition or deprivation of essential nutrients.

The thing is, there’s no definitive answer yet.

Adipose fat, or white fat tissue cells, which include visceral and subcutaneous fat. Researchers studied these types of body fat in mice.

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How does intermittent fasting affect fat?

Short answer: there’s more to find out before we know for sure. A review of studies on intermittent fasting from 2015 found that mice eating on an alternate day schedule did have a reduction in visceral fat. However, in the human studies reviewed, the effects of intermittent fasting on fat were not as straightforward — because they mostly paired calorie restriction with intermittent fasting. Meanwhile, a 2019 review did find modest reductions in fat mass and weight circumference in humans.

In the new Cell Reports mouse study, researchers found different fat types actually adapted to intermittent fasting. During fasting, fat tissues can provide energy to the rest of the body by releasing fatty acids in a process called lipolysis. Mouse visceral fat, however, became resistant to this process during intermittent fasting, as opposed to the mice with unlimited access to food.

“Different fat depots around the body can adapt and respond differently to intermittent fasting.”

The study also suggests intermittent fasting caused fatty acid synthesis enzymes to increase in both fats, suggesting both subcutaneous and visceral fat began to store more fat ahead of the alternate-day fasting period.

Mark Larance, the senior author of the study and a senior lecturer in the School of Life and Environmental Sciences at the University of Sydney, tells Inverse this data suggests “different fat depots around the body can adapt and respond differently to intermittent fasting.”

But he also says scientists still need to confirm if these findings apply to humans.

When it comes to intermittent fasting, mouse metabolism isn’t a perfect comparison.

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The work is ongoing. For example, a 2020 study published in JAMA Internal Medicine found that a group of participants who fasted from 8 p.m. to noon the next day actually lost lean mass and very little body fat when compared to a group of people eating a regular three meals a day.

Ethan Wiess, that study’s senior author and a professor at the University of California, San Francisco, tells Inverse the “ratio of fat loss to non-fat loss was inverse of what you see, typically, in diet studies.”

Weiss says studies of mice can be unreliable, and similar research on humans has remained somewhat inconclusive on the fat question.

“I study mice for a living and mostly, mouse metabolism is really different,” Weiss says. “We have to be very cautious about how we interpret these mouse metabolism studies.”

Is intermittent fasting healthy?

There are tentative signs intermittent fasting can offer some health benefits outside of fat loss — including dampened inflammation, better metabolism, and better brain health — but the results are not definitive enough for all science and nutrition experts to recommend it. Most endorsements stem from anecdotal experiences, as opposed to empirical evidence.

“I couldn’t recommend it to anyone.”

A handful of studies have found associations between intermittent fasting and reductions in markers for cardiovascular disease and diabetes. Other studies, however, have found no significant changes in key health indicators like blood pressure, heart rate, triglycerides, glucose, and insulin — or not enough data to make a call. Essentially, the results are mixed.

“In both humans and animal models, intermittent fasting has been shown to improve the metabolic health and energy levels of people who are overweight and at a healthy weight,” Harney says.

However, he adds that intermittent fasting has been “shown to perform best when incorporated into a lifestyle that includes healthier eating and regular exercise.” This means making time for habits beyond fasting that generally lead to calorie loss.

According to Weiss, research on intermittent fasting in humans just isn’t conclusive.

“I think even though fasting is sort of more ancient than these other kinds of diets, it's way behind in terms of the science,” he says. “I couldn't recommend it to anyone.”

Weiss, meanwhile, says he fasted for years but has now stopped.

“I’m much nicer,” he says. “My family likes me more.”

When should you worry about weight loss?

Whether it’s with intermittent fasting or another plan, losing weight is not the right choice for everyone. Low body fat, in turn, isn’t necessarily a marker of good health.

“Health” itself is a hazy concept, one vaguely gauged by a number of factors taken as a whole. BMI, or the body mass index, doesn’t measure health — and a high BMI doesn’t mean you’re unhealthy. Mental health is also an important factor to consider because the stigma of weight loss can be linked to diminished mental health.

That said, excess body fat or obesity can put people at major risk for a host of problems including heart disease — the leading cause of death in the United States — musculoskeletal diseases, diabetes, and some cancers.

The 2015 review on intermittent fasting did suggest the practice had “broad-spectrum benefits for many health conditions” and could be an option for “overweight” people or those with hypertension, high cholesterol, and insulin resistance. Those authors did advise against it for elderly people and children.

Generally, if you’re considering changing your diet or are worried about body fat, Harney says it’s worth talking to a doctor or nutritionist first. The right diet for someone else may not be the right diet for you.

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