Intermittent Fasting: What the Research Actually Shows (and Who Should Skip It)

Intermittent fasting sounds like a hack, but it’s really just a schedule. Instead of changing what you eat, you change when you eat, and leave longer gaps between meals than most of us are used to.

It has a lot going for it. It’s simple, it costs nothing, and for some people it’s the first eating plan that actually sticks. It also gets oversold. Here’s what intermittent fasting does, what the research really shows, and who should leave it alone.

What Intermittent Fasting Actually Is

Intermittent fasting means switching between set periods of eating and not eating on a regular schedule. As Johns Hopkins Medicine puts it, the focus is on when you eat rather than what or how much.

The idea behind it is a “metabolic switch.” For several hours after a meal, your body runs mostly on sugar from that food and from the glucose stored in your liver. Once those stores run low, it shifts toward burning fat and making ketones. Most of us never get there, because we eat from the moment we wake up until we go to bed (de Cabo and Mattson, NEJM).

The Most Common Schedules

• 12:12: eat within a 12-hour window and fast for 12. Finish dinner by 7 p.m., eat breakfast at 7 a.m. Plenty of people already do something close to this, which makes it a good place to start.
• 16:8: eat within an 8-hour window, such as noon to 8 p.m., and fast the other 16 hours. This is the most popular version, also called time-restricted eating.
• 5:2: eat normally five days a week. On two non-consecutive days, eat one small meal, often around 500 to 600 calories.
• Alternate-day fasting: switch between normal days and very low-calorie days. It’s the hardest version to stick with.
• OMAD (one meal a day): popular online, but there’s very little research behind it, and it’s hard to fit enough protein and nutrients into one sitting.

What the Research Shows for Weight Loss

Short version: intermittent fasting works about as well as regular calorie cutting. Not better, not worse.

The most useful trials compare fasting against a normal diet, and they keep landing in the same place:

• In a 12-month trial in the New England Journal of Medicine, 139 people with obesity either cut calories and ate only between 8 a.m. and 4 p.m., or just cut calories. The time-restricted group lost 8.0 kg on average and the calorie-only group lost 6.3 kg, a difference that wasn’t statistically significant (Liu et al., 2022).
• A year-long trial of alternate-day fasting in JAMA Internal Medicine found weight loss similar to daily calorie restriction, but more people quit: 38% of the fasting group dropped out, compared with 29% of the calorie-restriction group (Trepanowski et al., 2017).
• A 12-week trial of 16:8 eating with no other diet changes found it was no more effective for weight loss than eating three regular meals a day. The fasting group also lost slightly more lean mass, which includes muscle (Lowe et al., 2020).

So why do so many people swear by it? Mostly because a shorter eating window makes it harder to overeat. It cuts out late-night snacking without you having to count anything. If that structure helps you eat less, it works. If you squeeze the same amount of food into eight hours, it won’t.

Other Possible Benefits

Researchers are interested in fasting for more than weight. A major review in the New England Journal of Medicine pulled together human studies showing improvements in insulin sensitivity, blood pressure, blood fats and inflammation, some of them partly independent of weight loss (de Cabo and Mattson). Animal studies go further, with longer lifespans and protection against several chronic diseases.

Now for some honest context. Many of the most exciting findings come from mice, and most human trials are short. The U.S. National Institute on Aging says there isn’t enough evidence yet to recommend fasting diets to the public, and that scientists still don’t know whether these eating patterns are safe or doable over the long run.

One more result is worth knowing about. In 2024, researchers presented an analysis of more than 20,000 U.S. adults at an American Heart Association meeting. People who ate all their food within less than 8 hours a day had a 91% higher risk of dying from cardiovascular disease than people who ate across 12 to 16 hours (American Heart Association). It was a preliminary abstract based on people’s own food reports, and it shows a link, not cause and effect. Still, it’s a good reason not to assume a narrower window is always better.

What the First Few Weeks Feel Like

The start is the hardest part. Hunger, crankiness, headaches and low energy are common at first, and it can take two to four weeks for your body to adjust (Johns Hopkins Medicine). Hunger tends to come in waves around your usual mealtimes rather than building all day.

A few things make it easier:

• Start with 12:12 and narrow the window gradually instead of jumping straight to 16:8.
• Drink plenty of water. Black coffee and plain tea are fine during the fast.
• Eat earlier if you can. Several studies suggest an earlier eating window works better for weight and blood sugar than skipping breakfast and eating late (Tinsley et al.).
• Make the meals count. Build them around protein, vegetables, whole grains and healthy fats. A fasting window doesn’t cancel out a drive-through dinner.
• Protect your muscle. Given the lean mass loss seen in some trials, get protein at every meal and do strength training two or three times a week.

A Sample 16:8 Day

• Morning: water, black coffee or plain tea
• Noon (first meal): two eggs scrambled with spinach, whole-grain toast and an orange
• 3:30 p.m.: Greek yogurt with berries and a handful of nuts
• 7:30 p.m.: salmon or tofu, roasted potatoes and a big serving of vegetables
• 8 p.m. to noon: fasting, with water, black coffee and plain tea only

Want more fiber in those meals? Our guide to resistant starch has easy options like cooled potatoes and rice.

Who Should Skip Intermittent Fasting

Fasting isn’t for everyone. Skip it, or only try it with your doctor’s guidance, if you:

• Are pregnant or breastfeeding
• Are under 18
• Have a history of eating disorders, since strict fasting windows can trigger restrictive or obsessive eating
• Have type 1 diabetes or take insulin, because of the risk of dangerously low blood sugar
• Have type 2 diabetes, heart disease or kidney disease, or take medications that need to be taken with food

Talk to your doctor if you notice unusual anxiety, headaches, nausea or other new symptoms after you start (Johns Hopkins Medicine).

Common Questions

Does black coffee break a fast? Not in any way that matters. Black coffee and plain tea have almost no calories, and Johns Hopkins lists zero-calorie drinks like these as fine while fasting. Milk, sugar and creamer do add calories.

Can I work out while fasting? Yes, and lots of people do. Light to moderate exercise is usually fine. If you lift hard, a protein-rich meal soon after training helps you hold on to muscle.

Is it safe long term? For healthy adults, trials lasting up to a year haven’t turned up major safety problems. Longer-term data is still thin, which is one more reason to favor a moderate window like 12 or 14 hours over an extreme one.

The Bottom Line

Intermittent fasting is a tool, not magic. In good trials it helps people lose about as much weight as regular calorie cutting, and its biggest advantage is simplicity: one rule instead of a food log. If a 12- to 14-hour overnight fast makes it easier for you to eat well, it’s a reasonable thing to try. If it leaves you obsessing about food, worn out or overeating the moment your window opens, a regular eating schedule will serve you just as well.

Informational only, not medical advice. Talk to your doctor before making big changes to how you eat, especially if you have a health condition or take medication.

Sources

• de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 2019.
• Liu D, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. New England Journal of Medicine, 2022.
• Trepanowski JF, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection. JAMA Internal Medicine, 2017.
• Lowe DA, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters (TREAT trial). JAMA Internal Medicine, 2020.
• Tinsley GM, Peterson CM, Horne BD. Caution Against Overinterpreting Time-Restricted Eating Results. JAMA Internal Medicine, 2021.
• Johns Hopkins Medicine. Intermittent Fasting: What Is It, and How Does It Work?
• National Institute on Aging. Calorie Restriction and Fasting Diets: What Do We Know?
• American Heart Association. 8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death, 2024.

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