Written by Robbie Clark, BExSc (Nutrition), M (Nutr+Diet), Accredited Practising Dietitian (APD) and Exercise Scientist · Last updated 27 October 2026 · 6 references
The short answer
- The main types are 5:2 (two low-energy days a week), 16:8 (eating within an 8-hour window) and alternate-day fasting.
- They work about as well as cutting back a little every day – not better.1,3,4
- 16:8 on its own made little difference to weight in a trial unless people also ate less.2,4
- The best type is the one you can stick to while still eating a balanced diet.
- Fasting isn't for everyone. Avoid it if you're pregnant, breastfeeding or have a history of disordered eating, and talk to your doctor first if you take diabetes medicines.5
Intermittent fasting comes in many forms, and each comes with big promises. As a dietitian, I'm often asked which one works best. Here's how the main types compare, what the evidence says about weight loss, and who should give fasting a miss.
What are the main types of intermittent fasting?
- 5:2. Eat normally five days a week and limit yourself to about 2,100–2,500 kJ (500–600 calories) on the other two.
- 16:8 (time-restricted eating). Eat within an eight-hour window each day, such as 12pm to 8pm, and fast for the other 16 hours.
- Alternate-day fasting. Alternate between normal eating days and fasting days of about 2,100 kJ (500 calories), or around a quarter of your usual intake.
- 800-calorie programs. A commercial approach that starts with about 3,350 kJ (800 calories) every day, then moves to two lower-energy days a week with an overnight fast.
- The Warrior Diet. Eat very little for 20 hours a day, then eat one large meal at night.
Does intermittent fasting work for weight loss?
Yes, but no better than other ways of eating less. A review of randomised trials found that intermittent fasting led to similar weight loss to cutting kilojoules every day.1
The bigger individual trials point the same way. Over a year, alternate-day fasting and daily calorie restriction led to similar weight loss, but more people dropped out of the fasting group (38%, compared with 29%).3 In another year-long trial, people who ate within an 8-hour window and also cut back lost about 8 kg, compared with about 6 kg for those who just cut back – a difference that wasn't statistically significant.4
Fasting mainly works because it helps some people eat less overall. Any improvements in blood glucose and insulin appear similar to those from other reduced-energy diets, and other possible benefits are still being researched.5
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Find a weight management practitioner →Is 5:2 better than 16:8?
Neither has been shown to be better. 5:2 and alternate-day fasting lead to similar weight loss to daily calorie restriction.1,3 16:8 seems to help mainly when it leads you to eat less: in a 12-week trial, people who simply ate within an 8-hour window without other changes lost about the same small amount of weight as people who ate three regular meals.2
The right choice depends on what suits your routine. Some people find two harder days a week easier to manage; others prefer a daily eating window. For more on timing, see is eating before bed bad?
What about 800-calorie programs and the Warrior Diet?
800-calorie programs start with a very low intake every day for weeks, which can lead to quick weight loss but is hard to meet all your nutrient needs on. If you want to try one, it's best done with support from a practitioner.
The Warrior Diet hasn't been studied directly. Eating very little all day and then one large meal at night can encourage overeating, and it isn't suitable for anyone at risk of disordered eating.
What are the downsides of intermittent fasting?
- Hunger, tiredness, irritability and headaches, especially at first. Drinking enough water can help with headaches.5
- Overeating in the eating window, which can cancel out the fast.
- Missing out on nutrients if fasting days or short eating windows leave little room for vegetables, fruit, wholegrains, dairy or alternatives and protein foods.
- It can be hard to keep up long term, and the long-term effects aren't yet clear.3,5
Who shouldn't try intermittent fasting?
Intermittent fasting isn't recommended if you're pregnant or breastfeeding, a child or teenager, frail or older, or have an eating disorder or a history of one.5 If food or weight is causing you distress, the Butterfly National Helpline (1800 33 4673) offers free, confidential support.6
If you have diabetes or another health condition, or take regular medicines, talk to your doctor before you start.5
Taking diabetes medicines? Talk to your doctor before fasting
Fasting can cause low blood glucose if you take medicines that lower it, such as insulin. Your doses may need to change, and you'll need to check your blood glucose more often.5
Get support from a practitioner who helps with weight and metabolic health
Find practitioners on HealthBank who work with weight management, metabolic health and type 2 diabetes. Many offer a free discovery call, online, by phone or in person.
Find a weight management practitioner →Frequently asked questions
Which type of intermittent fasting is best for weight loss?
None has been shown to be clearly better. They lead to similar weight loss to daily calorie restriction, so choose one you can stick to.1
Does 16:8 work without counting kilojoules?
Not on its own, in trials. Eating within an 8-hour window without eating less made little difference to weight.2
Is intermittent fasting safe?
It appears well tolerated in the short term for most healthy adults, but it isn't suitable during pregnancy, for children or for people with an eating disorder.5
Can I fast if I have diabetes?
Possibly, but talk to your doctor first, as diabetes medicines may need adjusting to prevent low blood glucose.5
How many kilojoules do you eat on a 5:2 fasting day?
About 2,100–2,500 kJ (500–600 calories).
References
Research (PubMed)
- Cioffi I, Evangelista A, Ponzo V, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16:371. DOI 10.1186/s12967-018-1748-4
- Lowe DA, Wu N, Rohdin-Bibby L, 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 Intern Med. 2020;180(11):1491–1499. PubMed 32986097
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Intern Med. 2017;177(7):930–938. PubMed 28459931
- Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med. 2022;386(16):1495–1504. PubMed 35443107
Australian guidelines
- Diabetes Australia. Intermittent fasting. View source
- healthdirect. Butterfly Foundation. View source
This article is general information, not medical advice. Speak with your practitioner about your own situation before changing your diet.
Robbie is a Sydney-based Accredited Practising Dietitian (APD) and Exercise Scientist with over 17 years of experience. He holds a Bachelor of Exercise Science (Nutrition) and a Master of Nutrition and Dietetics, and is the Director and Co-founder of HealthBank.