Intermittent fasting limits when you eat, using plans such as an eight-hour daily window (16:8) or two very low-calorie days a week (5:2). In general, people lose about the same amount of weight as they would with standard calorie cutting. Shifting your eating window earlier in the day may offer a slight edge. Talk to a clinician first if you take diabetes medication, are pregnant, or have a history of eating disorders.[1],[2],[3]
What Is Intermittent Fasting and How Does It Work?
Intermittent fasting is an eating pattern that alternates between set times for eating and longer stretches without food. Some plans fit every meal into a set number of hours each day, while others use whole days with little or no food.[1],[2]
| Method | How it works | Example schedule |
|---|---|---|
| Time-Restricted Eating (e.g., 16:8) | All daily calories fit within a set window (usually 6 to 8 hours).[2],[3] | Eat between 10:00 a.m. and 6:00 p.m.; fast overnight and through mid-morning.[4] |
| 5:2 Method | Five days of regular eating, plus two days capped at about 500 calories.[1] | Very low-calorie days on Monday and Thursday; standard eating on the remaining five days. |
| Alternate-Day Fasting | Days of normal eating alternate with days of very few or no calories.[1],[2] | Eat normally on Monday, Wednesday, and Friday; fast on Tuesday, Thursday, and Saturday. |
Why does it work? For most people, the main reason is simple: a shorter window usually means eating less.[5]
Researchers are also studying whether long gaps between meals change how your body handles blood sugar. But much of the excitement, especially about living longer, comes from animal studies.[6] Results in animals don’t always hold up in people, so human trials are the better guide to what you can expect.
Does Intermittent Fasting Work for Weight Loss?
In short, yes. Fasting can help you lose weight. However, in head-to-head trials it works about as well as standard calorie cutting, not better.[1],[2]
Many people like fasting because it feels simple. Setting a firm cutoff, like 6:00 p.m., can be easier than logging every meal. That’s the real advantage: a window is easier to stick with than counting.[5]
The results tend to match, too. In one two-month study, adults with overweight or obesity lost about 15 pounds whether they fasted every other day or cut calories daily.[2]
Meal timing may offer a small boost. When researchers pooled 41 trials, people who ate earlier in the day lost about two to three pounds more than late eaters or standard calorie counters.[3]
Still, most studies lasted only weeks or months. We don’t know if that small perk lasts over time.[3] Everyday life matters, too. An early dinner cutoff has to fit your work hours, family meals, and evening plans. If a strict window clashes with your schedule, a flexible diet is easier to stick with.
Can Fasting Help Blood Sugar or Heart Health?
In the short term, a fasting schedule can help improve blood sugar control for people dealing with prediabetes or metabolic syndrome.[2],[7] That said, researchers still debate whether the credit goes to the weight loss, the meal timing, or both.[2]
An 8- to 10-hour eating window shows modest promise. In one three-month study of adults with metabolic syndrome, participants on this schedule lost an average of 6.6 pounds.[7] Most of that loss was fat rather than muscle.[7] Their A1C, a marker of average blood sugar, also dipped slightly.[7] All participants received nutrition advice and logged their food, so the timed window was an added tool rather than a standalone fix.[7]
Weight loss may not be the whole story. In a small study of men with prediabetes, eating earlier improved blood sugar and lowered blood pressure without any weight loss.[2] Still, better lab results don’t prove that fasting prevents heart attacks or lengthens life. We need longer trials to answer those questions.[3],[7] For now, there isn’t enough evidence to recommend fasting diets in general, especially for older adults.[6]
Is Fasting Safe, and Who Should Skip It?
A straightforward 12-hour eating window followed by a 12-hour overnight fast (for example, eating from 7:00 a.m. to 7:00 p.m.) is likely safe for most people.[2] Pushing that to a 16:8 schedule adds four more hours of fasting, and feeling fine at 12 hours doesn’t guarantee 16 hours is right for you.
Always check with a clinician before you start fasting if any of the following apply:
- You take insulin or another medicine to manage diabetes.[2] Ask your clinician how fasting could affect your medication schedule. Don’t change doses on your own.
- You’re pregnant or breastfeeding.[1],[2]
- You’ve had an eating disorder or disordered eating.[1]
- You take medication that must be taken with food.[2]
- You’re younger than 25.[2]
- You have a seizure disorder.[2]
- You work the night shift or operate heavy machinery at work.[2]
Even if you’re in the clear to try it, pay attention to side effects. With alternate-day fasting, some people struggle with intense hunger and poor focus on their fasting days, or they end up overeating on their regular days.[1] There are also physical risks to consider. Studies have linked regular fasts of more than 16 or 18 hours a day to a higher risk of gallstones.[2] A 16:8 schedule sits right at the edge of that range, which is worth knowing if you plan to fast that way regularly.
Does What You Eat Still Matter on a Fasting Schedule?
What you eat probably matters more than when you eat.[2] A shorter eating window won’t turn a poor diet into a healthy one.
To get the most out of fasting, focus on meals packed with protein, vegetables, whole grains, and other fiber-rich foods. It helps to decide what you’re going to eat before your window opens so your first meal isn’t just a rushed response to hunger. Water and other calorie-free drinks are fine during fasting hours.[1]
It’s best to build your schedule around balanced meals you actually have time to prepare and enjoy, rather than forcing your food into a strict window that leaves you hungry or short on nutrients.
Are longer fasts better? Probably not. A 2026 review found no clear link between window length and metabolic benefits.[3] Windows under eight hours trimmed fat mass a little more than daily calorie counting, but they also raised total cholesterol compared with an eight-hour window.[3]
Is Intermittent Fasting Worth Trying?
Intermittent fasting is one tool for eating less. It isn’t necessary for good health. An eating window works well if it makes meal planning easier. But if fasting leaves you drained or obsessing over food, standard calorie cutting offers similar weight loss.[1],[2]
Related terms
Macronutrients: the carbs, protein, and fat that supply calories.
Metabolic health: how well your body manages blood sugar, blood pressure, and blood fats.
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Sources Cited
- Academy of Nutrition and Dietetics. What is intermittent fasting? Eatright.org. July 2025.
- National Institutes of Health. To fast or not to fast. NIH News in Health. December 2019.
- Chen YE, Tsai HL, Tu YK, Chen LW. Effects of timing and eating duration of time restricted eating on metabolic outcomes: systematic review and network meta-analysis. BMJ Med. 2026;5(1):e001071.
- Gabel K, Hoddy KK, Haggerty N, et al. Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: a pilot study. Nutr Healthy Aging. 2018;4(4):345-353.
- National Institute of Diabetes and Digestive and Kidney Diseases. What can you tell your patients about intermittent fasting and type 2 diabetes? Diabetes Discoveries and Practice. May 2024.
- National Institute on Aging. Calorie restriction and fasting diets: what do we know? August 2018.
- National Institutes of Health. Time-restricted eating for metabolic syndrome. NIH Research Matters. October 2024.
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