The Distasteful Truth About Intermittent Fasting

If your goals include weight loss, better health or longer life, the latest research offers skeptical food for thought

Intermittent fasting has been widely touted as a healthy way to lose weight and improve health, increase brain and body performance, and extend life — all by creating long windows of time during which you don’t eat. Successful anecdotes abound. Yet while some studies suggest certain people may benefit, the latest research questions the effectiveness and even the safety of the various schemes.

Among the many popular approaches to intermittent fasting:

• Time-restricted eating: You stop eating in the afternoon or early evening and resume in the morning — confining food consumption to a 12-hour window, or even as little as six hours, during which you eat (in some plans, anyway) whatever you want.
• Whole-day fasting: You eat normally (unrestricted) most days, but for one or two days during the week you eat nothing, or alternately on those days you have one meal only, restricting total calorie intake to about one-fourth normal.
• Alternate-day fasting: Similar to whole-day fasting but you do it every other day, or perhaps Monday, Wednesday and Friday.

Proponents say it’s natural now and then, from an evolutionary perspective, to go without food for many hours or even a day or more. That makes sense. The idea is that the body benefits from the breaks by letting the digestive system relax, and also by causing some “good” biological stress that bolsters the immune system. In addition, during a long fasting window the body uses up sugar stores and begins to burn fat.

“Intermittent fasting contrasts with the normal eating pattern for most Americans, who eat throughout their waking hours,” says Mark Mattson, PhD, a neuroscientist at Johns Hopkins University. “If someone is eating three meals a day, plus snacks, and they’re not exercising, then every time they eat, they’re running on those calories and not burning their fat stores.”

However, at the risk of raising ire among those who swear by intermittent fasting—called IF for short—here’s what the non-anecdotal science says:

Intermittent fasting has been found effective in some animal studies and has shown some promise in humans, particularly for weight loss among obese people, but overall results in human studies are inconclusive, with notable risks to consider.

Weighty but inconclusive findings

A new study of 550 obese adults followed for six years finds eating less overall, and fewer large meals, works better for managing weight than just restricting eating to a narrow time window. Writing in the Journal of the American Heart Association, the scientists contend that other rigorous research has not determined with any certainty that intermittent fasting helps with weight control.

A 2021 study concluded intermittent fasting can help people with weight loss, but:

“Intermittent fasting is not better than regular dieting,” said study leader Krista Varady, PhD, a professor of nutrition at the University of Illinois Chicago. “Both produce the same amount of weight loss and similar changes in blood pressure, cholesterol and inflammation.”

Still, it’s hard to know how much weight to give any single study, since most are done over short time periods, with relatively small numbers of people — often only people who are obese — and with widely varying research methods. So scientists turn to meta-analyses in an effort to synthesize existing research.

A review of multiple studies on the topic, published last year in Frontiers in Nutrition, found intermittent fasting can help obese people lose weight, but the effect was deemed no greater than simply eating less (what scientists call calorie restriction). This and other studies have suggested, but not proven, that intermittent fasting might be easier to stick with compared to long-term daily calorie restriction. On the other hand, there’s concern that intermittent fasting might encourage overeating on unrestricted days, especially for anyone prone to doing so.

There’s also outright potential harm in time-restricted eating.

Fasting can be dangerous for people with diabetes, or people taking certain medications that should be accompanied by food, according to a lengthy analysis by Harvard nutritionists who also caution against intermittent fasting for pregnant people or growing kids and adolescents.

“Although certain benefits of caloric restriction have been demonstrated in animal studies, similar benefits of intermittent fasting in humans have not been observed,” they write. “It is unclear that intermittent fasting is superior to other weight loss methods in regards to amount of weight loss, biological changes, compliance rates, and decreased appetite.”

New research, published last month in the journal Eating Behaviors, linked intermittent fasting to higher rates of eating disorders in women, including binge-eating and vomiting. The study involved 2,700 adolescents and young adults in Canada. Among both males and females in the analysis, intermittent fasting was linked to higher rates of compulsive exercise, an above-and-beyond addiction that can lead to injury or illness.

Finally, the notion that you should “eat whatever you want” during the consumption windows is grossly wrongheaded. Ample research confirms that eating highly processed foods, sugar-laden stuff and otherwise missing out on fruits, vegetables and other healthy foods is the fast track to lousy health and premature death.

Can fasting extend your life?

Beyond any possible near-term benefits such as weight loss, one important measure of the benefits of intermittent fasting — for people in any weight class — ought to be longevity. If a certain behavior is good for health, then those who adhere to it should live longer, on average, compared to those who don’t. So what do we know?

Despite many armchair scientists’ anecdotal claims, intermittent fasting has not yet been proven successful for extending lives. Time may tell a different tale, but the current fad of time-restricted eating hasn’t been around long enough to enable thorough tests of long-term effects on aging and mortality.

The most recent study on this suggests intermittent fasting might not increase lifespan. In the research, published last month in the journal Nature Communications, scientists used mice to examine the long-term effects of intermittent fasting.

I know, I know: mice. I don’t normally report on health studies done on non-humans, because they do not always reflect what’s ultimately found to be the case in people. But rodents can be helpful analogues for the biology of humans, and it’s worth making exceptions when the results can help inform important health topics that otherwise lack robust research or firm conclusions — as with intermittent fasting.

The study was highly complex, so I’ll stick to the conclusion: Intermittent fasting “proved largely ineffective in [the] supposed impact on aging,” the researchers stated.

Better approaches to a long, healthy life

Instead of silver bullets, anyone wishing to manage weight, be healthy and live long would be wise to consider a range of lifestyle and behavioral choices, including of course getting daily physical activity. (Some studies on intermittent fasting indicate that it works best when accompanied by regular exercise.) Plus there are two aspects of diet known to help with weight management and improve the chances of good health and longer life: Develop a healthy way of eating, and eat less.

As I’ve written previously:

Most adults should consume somewhere between 1,600 and 3,000 calories a day (here’s a calculator for you). But let’s not kid around: Most of us eat more than we need. The average American takes in more than 3,600 calories daily, 24% more than in 1961 — even though we’re much less active nowadays.

To properly digest all this, let’s add one more factual ingredient to the mix: Late-night snacking, particularly on junk food, has been shown to pack on extra pounds and, ironically, leave people hungrier the next day, fueling a distasteful cycle of further weight gain and worse health.

So here’s what we might safely and responsibly say about the value of intermittent fasting:

• It might be a helpful way to manage weight and improve health for some people who find it more palatable than daily calorie counting.
• It can be dangerous for people with certain disorders or conditions.
• A modest version — simply avoiding late-night snacking — could be the most advantageous approach for many people.

Restricting your eating to reasonable daytime hours, starting with a healthy breakfast, and ending with an evening meal, can help you eat less overall and perhaps avoid some of the worst calories you ingest, particularly at the worst time. Such a scheme, other research shows, would help you sleep better. And here’s your bonus fact: Better sleep is a well-proven path to improved weight management, better overall health, and longevity.

Your support makes my health and wellness writing possible. You can sign up for emails when I publish on Medium, or join Medium to directly support me and gain full access to all Medium stories, get my health news briefs on Mastodon, or check out my book: Make Sleep Your Superpower: A Guide to Greater Health, Happiness & Productivity (paperback or Kindle version). — Rob

Note: A brief version of this article first published in my Age Wise newsletter on Substack.