Intermittent fasting: what the evidence says

Intermittent fasting is not a magic method, but it isn't a bad one either. Clinical trials show that approaches such as 16:8, 5:2 or alternate-day fasting lead, overall, to about the same weight loss as cutting calories every day. Their main advantage is simply that some people find them easier to stick to. If the pattern fits your life and nothing rules it out for you, it can be one option; if it doesn't fit, you haven't missed anything.

This article is educational and does not replace an examination or advice from your own doctor.

What exactly is intermittent fasting?

Instead of focusing only on "what should I eat", intermittent fasting sets when you eat and when you don't. Three patterns are common:

• Time-restricted eating (16:8): each day you have an eating window of about 8 hours and fast for about 16, having nothing but water or unsweetened tea or coffee. For example, from noon to 8 pm.
• 5:2: five days a week you eat normally and in a balanced way, and on two non-consecutive days you eat very little, usually about a quarter of your daily needs.
• Alternate-day fasting: one normal day, one day of very little or no food, and so on.

The idea behind all of them is the same: when the chance to eat is limited, many people end up eating less overall.

What have the studies actually shown?

There are many claims about intermittent fasting on social media, from "incredible fat burning" to "rejuvenated cells". Let's turn to randomized controlled trials, since they are the most reliable kind of evidence here.

Compared with an ordinary low-calorie diet

• The TREAT trial (Lowe and colleagues, JAMA Internal Medicine, 2020): for 12 weeks, adults with overweight either followed a 16:8 pattern without counting calories or ate three regular meals. There was no significant difference in weight loss between the groups, and part of the weight the 16:8 group lost was lean mass, which shows the importance of protein and movement.
• Liu and colleagues (NEJM, 2022): over one year, both groups cut calories, and one group ate only between 8 am and 4 pm. At the end, weight loss and most metabolic markers did not differ significantly between the groups. The main factor was eating fewer calories, not the clock.
• Alternate-day fasting (Trepanowski and colleagues, JAMA Internal Medicine, 2017): weight loss was similar to a daily low-calorie diet, but more people dropped out of the alternate-day fasting group.
• The 5:2 approach (Harvie and colleagues, 2011): in women with overweight, weight loss was similar with intermittent and continuous restriction.

The big-picture review

A review published by de Cabo and Mattson in NEJM in 2019 describes possible mechanisms of fasting, such as better insulin sensitivity. Much of this evidence comes from animal studies or short human studies, and the authors themselves stress the need for longer studies in people.

The evidence in short: intermittent fasting is a way to eat less, not a shortcut. If it's easier for you, great; if it isn't, an ordinary diet works just as well.

The methods at a glance

Comparing common intermittent fasting methods

Method	Pattern	Advantage	Challenge
16:8	About 8 hours of eating each day	Simple, no calorie counting, fits a daily routine	You may overeat inside the window; late dinners and gatherings make it hard
14:10	About a 10-hour eating window	Gentler and good for starting	Its effect depends more on food quality
5:2	Very little food on two non-consecutive days a week	Five days of more freedom	Low-food days can be hard, with hunger and headaches
Alternate day	One normal day, one very low day	Weight loss similar to an ordinary diet	Dropping out is more common
Daily low-calorie diet	A little below your needs every day	Steady and predictable	Needs patience for tracking and measuring

Who shouldn't try it without a doctor?

For some people intermittent fasting isn't just unnecessary; it can be dangerous. If you are in one of these groups, talk to your doctor before making any change:

• Pregnancy and breastfeeding: energy and nutrient needs are high in this period.
• Diabetes treated with insulin or sulfonylurea drugs (such as glibenclamide or gliclazide): there is a serious risk of low blood sugar, and the drug dose may need adjusting. That adjustment is only for your doctor to make.
• A history of an eating disorder such as anorexia nervosa or bulimia: restricting eating times can restart the cycle of restriction and bingeing.
• Teenagers: the body is still growing, and restrictive diets aren't recommended for them.
• Being underweight or having unintended weight loss.
• Kidney disease, heart disease, high blood pressure, or taking medicines that must be taken with food.
• Frail older adults and people at risk of losing muscle.

If you have repeated episodes of binge eating in which you feel out of control, this may be a sign of an eating disorder. It is not a lack of willpower, and specialist help works.

If you notice signs such as tiredness after meals, sugar cravings or darkened skin on the neck, it may be worth first reading our article on the signs of insulin resistance and talking to your doctor.

Ramadan fasting and intermittent fasting

Many of us know the experience of fasting in Ramadan, and it's natural to ask whether it is the same as intermittent fasting. The short answer: they share some features, but they aren't the same thing.

• Ramadan fasting is an act of worship with spiritual and cultural meaning, not a diet method, and it deserves respect in its own right.
• In Ramadan fasting, you don't drink water either, whereas in intermittent fasting drinking water is allowed and even essential.
• Ramadan lasts only one month a year, and its hours are set by sunrise and sunset.

As for weight, many people find that they don't lose much in Ramadan, or even gain, because an elaborate iftar with zoolbia and bamieh (fried syrup sweets), dates, bread, cheese and herbs, followed by dinner and a heavy pre-dawn meal (sahari), can add up to more than an ordinary day. If you fast, start iftar with water, a few dates and a bowl of light soup or ash (thick herb and bean soup), eat sweets in small, planned amounts, and include protein and whole-grain bread at sahari to get through the day more comfortably. People with diabetes, or who take blood sugar medication, should plan their medicine and meals with their doctor before Ramadan.

If you decide to try it

If nothing rules it out for you and you want to try, these tips help:

1. Start gently. First leave 12 hours between dinner and breakfast, then 14, and if that's comfortable, 16.
2. Fit the window to your life. If family dinner is at 9, a window from noon to 9 pm may be more realistic than 8 am to 4 pm.
3. Don't forget food quality. An eating window is not a permit for fast food and soda. Include protein (eggs, chicken, legumes, yogurt) and vegetables in every meal; they keep you full and reduce muscle loss.
4. Drink enough water. Unsweetened tea and coffee are usually fine during fasting hours; sugar or sweets with them break the fast.
5. Don't overeat at your first meal. Breaking a fast with a huge plate of rice or sweets usually ends in tiredness and hunger again.
6. Listen to your body. Dizziness, shakiness, a racing heart, a severe headache or constant thoughts of food mean this approach isn't right for you for now.
7. Judge progress by the weekly average, not each day's number; if your weight stays flat after a few weeks, see our article on weight plateaus.

Remember that intermittent fasting, like any other diet, only works if you can keep it up. In our article on diet myths we also saw that most "golden rules" about meal timing have little solid evidence behind them.

Summary

Today's evidence says intermittent fasting is about as effective for weight loss as an ordinary low-calorie diet. For some people its advantage is simplicity and a clear rule; for others its drawbacks are hunger, overeating inside the window and a poor fit with family life and gatherings. If you are pregnant or breastfeeding, have diabetes treated with insulin or a sulfonylurea, have a history of an eating disorder, or are a teenager or underweight, don't try it without a doctor. The best diet is the one that fits your own body, medicines and lifestyle.

If you'd like to know which pattern suits you better, at Rejimist a doctor writes a personal diet based on your questionnaire and follows up with a short daily text message. You can begin at Start your diet.

Sources

• Lowe DA, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Internal Medicine, 2020.
• Liu D, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. New England Journal of Medicine, 2022.
• de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 2019.
• Trepanowski JF, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine, 2017.
• Harvie MN, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity, 2011.
• International Diabetes Federation (IDF) and DAR International Alliance. Diabetes and Ramadan: Practical Guidelines, 2021.