Intermittent Fasting: Does It Work? An Evidence-Based Guide

What is intermittent fasting?

Intermittent fasting is an eating pattern, not a specific diet. Instead of changing what you eat, it sets rules about when you eat: you alternate periods of normal eating with periods of fasting or very low intake. During the fasting period, most approaches allow water, black coffee, unsweetened tea and other zero-calorie drinks.

There are three main families of IF. They differ in how long the fasts last and how often they happen, and the best one is simply the one you can follow comfortably for months, not just a couple of weeks.

• Time-restricted eating (16:8, 14:10, 18:6): you eat all your food within a daily window, for example 12:00–20:00, and fast for the remaining hours, much of that time while you sleep.
• 5:2 diet: you eat normally five days a week and have about 500–600 kcal on two non-consecutive days.
• Alternate-day fasting: you alternate normal eating days with fasting days of zero or roughly 25% of your usual calories.
• Other versions, such as a weekly 24-hour fast or one meal a day (OMAD), are harder to sustain and make it harder to eat enough protein.

What does the research show?

Most IF research looks at weight loss in adults with overweight or obesity. A 2018 meta-analysis of randomized trials by Cioffi and colleagues found that intermittent and continuous calorie restriction produced similar weight loss and similar short-term changes in markers such as blood lipids, glucose and insulin. A systematic review by Harris and colleagues, also from 2018, reached a similar conclusion: IF worked better than no intervention, but was not clearly better than a standard calorie-controlled diet.

Longer and more rigorous trials point the same way. In a 12-month trial published in 2022, Liu and colleagues gave adults with obesity the same calorie target with or without an 8-hour eating window; both groups lost weight and the difference between them was not statistically significant. In the TREAT trial (Lowe and colleagues, 2020), a 16:8 schedule without any calorie guidance led to small weight loss over 12 weeks that was not significantly different from a group eating three meals a day, and part of the weight lost was lean mass.

In short, IF is a legitimate tool, but the evidence does not show that it beats other ways of eating less. Most studies are short, include mainly people with excess weight and measure body weight rather than long-term health outcomes, so claims that go beyond weight control remain uncertain.

Why IF works – and why it isn't magic

IF mostly helps because a shorter eating window or a few low-calorie days make it easier to eat less overall. Dropping a late-night snack or a breakfast you didn't really want can remove a few hundred calories a day without counting anything. For some people, a clear rule such as 'I don't eat after 8 pm' is easier to follow than portion control at every meal.

What IF does not do is cancel out calories. If you eat more than you burn inside an 8-hour window, you won't lose fat. Popular claims that IF 'boosts metabolism', 'resets hormones' or dramatically increases fat burning through autophagy come largely from animal studies or short lab measurements, and they haven't translated into clearly better fat loss in human trials when calories are matched.

Who might intermittent fasting suit?

IF tends to work best for people who aren't very hungry in the morning, prefer fewer and larger meals, or find it easier to avoid food completely for a period than to eat small portions all day. It can also fit a busy work schedule or cultural and religious fasting practices.

It tends to work less well if long gaps without food leave you irritable, lightheaded or likely to overeat later, if you train early and feel weak without breakfast, or if it clashes with family meals and social life. Adherence is the strongest predictor of success on any diet, so an approach you dislike is the wrong approach for you.

• Good signs: you naturally skip breakfast, most of your snacking happens at night, and you like simple rules.
• Warning signs: you feel out of control when you finally eat, you think about food all day, or your sleep, mood or training suffer.

Protein and training around your eating window

If you want to keep or build muscle, IF adds one challenge: fitting enough protein into fewer meals. Aim for the same daily total we recommend for active people, about 1.6–2.2 g per kg of body weight, and split it into at least two or three protein-rich meals of roughly 0.4 g/kg each. For a 70 kg person, that's about 28 g of protein per meal – for example, a portion of chicken breast, Greek yogurt with eggs, or a tofu dish.

Resistance training matters as much as protein. A 2020 systematic review by Keenan and colleagues found that when people combined IF with resistance training, lean mass was generally maintained, although the studies were small and short. When possible, schedule hard sessions inside or just before your eating window so you can have a protein-rich meal afterwards.

Easy cardio or light sessions in the fasted state are fine for most healthy people. If you feel dizzy, weak or notice your performance dropping, shift your window so you train after a meal, or have a small snack before training.

How to start: a practical 16:8 example

Start gently. Many people begin with a 12-hour overnight fast, then shorten the eating window to 10 hours and finally to 8 hours if it still feels good. Keep the same food quality you'd want on any eating plan: lean proteins, vegetables, fruit, whole grains, legumes and some healthy fats.

Here is an example day for someone eating between 12:00 and 20:00 and training in the late afternoon. Adjust portions to your own calorie target.

• Morning: water, black coffee or unsweetened tea.
• 12:00 – First meal: rice, beans, grilled chicken, salad and a drizzle of olive oil.
• 16:00 – Snack: Greek yogurt with fruit and oats.
• 17:30 – Training session.
• 19:30 – Last meal: salmon or tofu, potatoes, vegetables and a piece of fruit.

Who should avoid IF or get medical advice first

IF is not appropriate for everyone. Long fasts can cause low blood sugar in people who take insulin or certain other diabetes medications, can interfere with medications that must be taken with food, and can trigger or worsen disordered eating.

If any of the situations below applies to you, talk to your doctor or a registered dietitian before trying IF. And if fasting leaves you feeling unwell, obsessed with food or guilty about eating, stop: there are many other ways to eat for your goals.

• Pregnancy or breastfeeding.
• Diabetes, especially if you take insulin or other glucose-lowering medication.
• A current or past eating disorder.
• Being under 18.
• Being underweight or frail, or having another medical condition or regular medication.