Does Intermittent Fasting Work for Weight Loss? Evidence, Types and Who Should Avoid It

Intermittent fasting (IF) is one of the most talked-about ways of eating. As a dietitian, I'm often asked whether it's worth trying. Here's how it works, what the research shows, and who should steer clear.

What is intermittent fasting?

Intermittent fasting changes when you eat rather than what you eat. You alternate between periods of normal eating and longer periods of eating little or nothing. There's no single right way to do it.⁶ The most common approaches are:

• 16:8 (time-restricted eating): eat within an eight-hour window each day and fast for 16 hours, for example eating between 12pm and 8pm. Gentler versions include 12:12 and 14:10.
• 5:2: eat normally five days a week. On two days, eat about 2,100 kJ (women) or 2,500 kJ (men). The two days can be together or apart.
• Alternate-day fasting: swap between normal eating days and very low-energy or fasting days.
• Eat-stop-eat: a full 24-hour fast once or twice a week, such as dinner to dinner.

Drink plenty of water during fasting periods. Fasts longer than a day should only be done with medical or dietetic supervision.⁶

Does intermittent fasting work for weight loss?

Yes, but not better than other approaches. A 2025 review of 99 randomised trials with more than 6,500 adults found intermittent fasting led to similar weight loss and similar changes in heart and metabolic health markers as cutting kilojoules every day. Alternate-day fasting came out slightly ahead, by about 1.3 kg on average.¹

Fasting works mainly because people don't fully make up for the food they skip, so they eat less overall.⁶ Some people find it easier than counting kilojoules at every meal. Others find it harder. As with any diet, weight is often regained over time, so choose an approach you can keep up.

How does intermittent fasting work in the body?

When you eat, insulin rises and helps store glucose as glycogen in your liver and muscles. Glycogen stores are limited, so extra energy is stored as fat.

When you fast, insulin and blood glucose fall, and your body draws on its stored energy: first glycogen, then increasingly fat. Fat burning rises within hours of your last meal. Liver glycogen is largely used up within about 24 hours, after which your body relies even more on fat for fuel.

What are the other possible health benefits?

Research has looked at several benefits beyond weight loss. Some come from human trials, others mainly from animal studies:

• Heart health markers: trials have linked IF with lower LDL ("bad") cholesterol, triglycerides and blood pressure, but most of this evidence is low quality.²
• Blood glucose and insulin: IF is linked with lower fasting glucose and insulin, and may improve insulin sensitivity.^2,6
• Inflammation: some studies have found lower inflammatory markers in the blood.⁶
• Ageing and brain health: studies, mostly in animals, suggest fasting may affect processes linked with ageing, cell repair and brain health. We don't yet know whether this applies to people.³

It isn't clear whether these changes come from fasting itself or simply from eating less and losing weight.⁶

Who should not do intermittent fasting?

Intermittent fasting isn't for everyone. Avoid it, or only try it with guidance from your practitioner, if you:

• are pregnant or breastfeeding;
• are a child or teenager, as it may affect growth and development;
• are underweight;
• have a history of disordered eating. Studies in the general population haven't shown major changes in disordered-eating scores,⁵ but fasting could be a trigger for someone vulnerable to an eating disorder;⁶
• take medicines for diabetes, especially insulin or sulfonylureas. In one trial of 5:2 fasting in people with type 2 diabetes, fasting doubled the rate of low blood glucose (hypoglycaemia), even after medicines were reduced.⁴ Talk to your doctor before you start;
• are having active cancer treatment. Speak to your treating team first.⁶

What are the side effects of intermittent fasting?

Hunger is the most common side effect. Time-restricted eating is linked with higher hunger at bedtime.⁵ You may also feel weak or find it harder to concentrate, which often eases as your body adapts. If side effects keep disrupting your day, stop and get advice.⁶

• Caffeine: it's easy to lean on coffee to get through a fast, which can add to anxiety and disrupt sleep.
• Exercise: there's no clear performance benefit from training while fasted.⁶ If you train hard, plan your eating window around your sessions so you can refuel and recover.
• Nutrition: with fewer meals, it's harder to meet your nutrient needs. Fill your eating window with vegetables, fruit, wholegrains and lean protein.

Frequently asked questions

Does intermittent fasting work for weight loss?

Yes, but it works about as well as cutting kilojoules every day, not better. It works mainly because you eat less overall.

Is 16:8 or 5:2 better for weight loss?

Neither has a clear advantage for most people. Alternate-day fasting showed slightly more weight loss in a large review, but the best plan is the one you can stick with.

Who should not do intermittent fasting?

People who are pregnant or breastfeeding, children and teenagers, people who are underweight or have a history of disordered eating, and anyone taking diabetes medicines without medical advice.

Does intermittent fasting cause muscle loss?

It's linked with some loss of lean mass, as with most weight-loss diets. Eating enough protein and doing strength training can help protect muscle.

What can I drink while fasting?

Water is the best choice, and you should drink plenty of it. Go easy on coffee, as too much caffeine can affect anxiety and sleep.

References

Research (PubMed)

1. Semnani-Azad Z, Khan TA, Chiavaroli L, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. PubMed 40533200
2. Patikorn C, Roubal K, Veettil SK, et al. Intermittent fasting and obesity-related health outcomes: an umbrella review of meta-analyses of randomized clinical trials. JAMA Netw Open. 2021;4(12):e2139558. PubMed 34919135
3. de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541–2551. PubMed 31881139
4. Corley BT, Carroll RW, Hall RM, Weatherall M, Parry-Strong A, Krebs JD. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabet Med. 2018;35(5):588–594. PubMed 29405359
5. Vizthum D, Katz SE, Pacanowski CR. The impact of time restricted eating on appetite and disordered eating in adults: a mixed methods systematic review. Appetite. 2023;183:106452. PubMed 36610542

Australian guidelines

6. Dietitians Australia. Intermittent fasting. View source

This article is general information and isn't a substitute for personalised advice. Speak to your practitioner about your situation.