16:8, OMAD, 36-hour fasts: does intermittent fasting really work?
Intermittent fasting means eating within set hours or on set days. It can help you lose weight, but it isn't magic, and it isn't safe for everyone.
Medically reviewed by Dr LJ Ncube, MBChB · Updated November 2026
- Intermittent fasting helps some people eat less, and that's why they lose weight
- In trials, it works about as well as cutting calories every day, not better
- Longer fasts aren't proven to work better and carry more risk
- Check with a doctor first if you take diabetes or blood pressure medicines
The jargon, in meals skipped
Fasting plans have confusing names. Here's what each one means in everyday terms, assuming you'd normally eat breakfast, lunch and supper.
| Name | What it means | Meals skipped |
|---|---|---|
| 12:12 | Stop eating after supper, eat breakfast 12 hours later | None, just no late snacks |
| 16:8 | Eat only within 8 hours, for example 10am to 6pm | Usually breakfast |
| 18:6 or 20:4 | Eat within 6 or 4 hours | Breakfast, and a smaller lunch or supper |
| OMAD (one meal a day) | Eat once a day, about a 23-hour fast | 2 meals every day |
| 24-hour fast | Supper one day to supper the next | Breakfast and lunch, once or twice a week |
| 36-hour fast | Supper one day to breakfast two days later | A whole day of meals (3 meals) |
| 42-hour fast | Supper one day to lunch two days later | 4 meals in a row |
| 5:2 | 2 days a week of about 500 to 600 calories | Roughly 2 meals on those days |
| Alternate-day fasting | Every other day is a fast or very low-calorie day | Most meals every second day |
Fasts of 24 hours or longer aren't proven to work better for weight loss than shorter ones, and they're harder to stick to. Don't do them without medical advice, especially if you take medicines.
Does it work?
Yes, for some people, mainly because it helps them eat less overall. In a year-long trial, people on alternate-day fasting lost about the same as people who cut calories daily (about 6% of body weight), and more of them dropped out. In another year-long trial, adding an 8-hour eating window to a calorie-controlled diet didn't lead to meaningfully more weight loss.
Show as a table
| Alternate-day fasting | 6% of body weight |
| Cutting calories every day | 5.3% of body weight |
Source: Trepanowski JF, et al. JAMA Intern Med. 2017. Differences were not statistically significant.
Eating fewer meals without eating less doesn't help much. In one study, healthy adults who ate one meal a day without cutting calories felt hungrier and had small rises in blood pressure and cholesterol.
Tracking helps
Whatever plan you choose, writing down when and what you eat, and weighing yourself regularly, makes it easier to see what's working. Our WeightHabits app lets you log meals, weight and habits, and your doctor can see your progress between visits.
Who shouldn't fast without a doctor's advice
- People taking insulin, gliclazide or glibenclamide, because of the risk of low sugar
- Anyone with type 1 diabetes
- Pregnant or breastfeeding women
- Anyone with an eating disorder, now or in the past
- Under-18s and frail older adults
- People taking medicines that must be taken with food
How to do it well
- Start gently, for example a 12-hour overnight fast, and build up
- Drink plenty of water. Black tea and coffee are fine
- Don't make up for it by overeating in your eating window
- Include protein and vegetables at each meal, to protect muscle and stay full
- Stop if you feel faint, shaky or unwell
Fasting with weight-loss injections
Weight-loss medicines already reduce appetite, so many people naturally eat less often. Strict fasting on top can make it hard to get enough protein and fluids. Ask your doctor or nutritionist before combining them.
Answer a few questions online and our doctor will reply on WhatsApp, usually within 24 hours.
Common questions
How much weight will I lose with 16:8?
It varies. Most people lose a modest amount over a few months, similar to other ways of cutting calories.
Can I fast if I have type 2 diabetes?
Only with your doctor's advice, especially if you take insulin, gliclazide or glibenclamide, which can cause low sugar.
Is breakfast the most important meal?
Not necessarily. What matters most is the total amount and quality of what you eat over the day.
Does fasting speed up my metabolism?
Only briefly. Your energy use rises slightly in the first days of a fast, but over weeks fasting doesn't burn more than other ways of eating less.
Related
Sources
We base our guides on peer-reviewed research and guidance from health authorities. Where studies disagree or evidence is limited, we say so.
- Trepanowski JF, et al. Effect of alternate-day fasting on weight loss, weight maintenance and cardioprotection. JAMA Intern Med. 2017. PubMed
- Liu D, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med. 2022. PubMed
- Lowe DA, et al. Effects of time-restricted eating on weight loss (TREAT trial). JAMA Intern Med. 2020. PubMed
- Stote KS, et al. A controlled trial of reduced meal frequency without caloric restriction. Am J Clin Nutr. 2007. PubMed
- Templeman I, et al. A randomized controlled trial to isolate the effects of fasting and energy restriction. Sci Transl Med. 2021. PubMed
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019. PubMed
- Zauner C, et al. Resting energy expenditure in short-term starvation is increased. Am J Clin Nutr. 2000. PubMed
This guide is general information, not advice for your situation. Speak to a doctor before starting or changing any medicine.
