Does the 5:2 Diet Work? What the Evidence Says and How to Do It Safely

The 5:2 diet is one of the most popular forms of intermittent fasting. It's simple to explain, but it isn't right for everyone. Here's what the evidence says, who should skip it, and how to do fasting days safely if you decide to try it.

What is the 5:2 diet?

On the 5:2 diet, you eat normally five days a week and eat much less on the other two. Australian guidance describes fasting days as about 2,000 to 3,000 kilojoules.⁶ The original plan suggests about 2,100 kJ (500 calories) for women and 2,500 kJ (600 calories) for men. Choose two non-consecutive days, such as Monday and Thursday.

It's an eating pattern rather than a set menu: it sets when you cut back, not what you eat. "Eating normally" on the other five days doesn't mean eating anything you like. If you make up for fasting days by eating more, you probably won't lose weight.

Does the 5:2 diet work for weight loss?

Yes, but it doesn't beat other approaches. A 2026 Cochrane review found that intermittent fasting led to little or no difference in weight loss compared with regular dietary advice over up to 12 months.¹ A large 2025 analysis of 99 trials found that all intermittent fasting and calorie-cutting diets reduced weight compared with eating as usual. Whole-day fasting diets like 5:2 performed about the same as everyday calorie cutting.²

In other words, 5:2 works because you eat less over the week. Some people find two strict days easier than cutting back every day; others find fasting days too hard. Short-term weight loss is often followed by regain, whichever approach you choose.⁷

Does intermittent fasting have other health benefits?

Possibly. An umbrella review of 130 trials linked intermittent fasting, including 5:2, with improvements in body weight, LDL cholesterol, triglycerides, fasting glucose, insulin and blood pressure. But most of these findings rested on low or very low quality evidence, and fasting was also linked with a loss of fat-free mass, which includes muscle.³ It's still unclear whether the benefits come from fasting itself or simply from eating less overall.⁷

You may have read that fasting protects against cancer or Alzheimer's disease. That research is mostly in animals, and it hasn't been shown in people.⁵

What are the downsides of the 5:2 diet?

• Side effects: hunger and headaches are common at first and usually ease. Stop and get advice if you feel weak, anxious or irritable, can't concentrate, or can't manage your usual activities.⁶
• Nutrients: on low-kilojoule days it's harder to get enough vegetables, fruit, wholegrains, dairy or alternatives, and protein.
• Habits: it doesn't teach you what to eat on the other five days, which is where most of your nutrition comes from.

In my experience, the swing between very hungry days and normal days can feel all-or-nothing for some people. If that sounds like you, a steadier approach may suit you better.

Is the 5:2 diet safe? Who should avoid it

Trials haven't shown clear harms in adults with overweight, but the evidence on side effects is very uncertain.¹ And it isn't recommended for:⁶⁷

• people who are pregnant or breastfeeding, or trying to conceive
• children and teenagers
• anyone with a current or past eating disorder
• people who are underweight, malnourished or have known nutrient deficiencies
• older or frail adults
• people who take medicines that must be taken with food
• people having cancer treatment, unless their medical team agrees.

If you have diabetes: don't start without talking to your doctor. Medicines such as insulin and sulfonylureas can cause low blood sugar on fasting days. In an Australian 12-month trial of 5:2 in people with type 2 diabetes, these medicines were reduced at the start, yet about a third of people using them still had low or high blood sugar episodes in the first two weeks.⁴ With medical supervision, the same trial found 5:2 lowered HbA1c about as well as a daily calorie-cut diet.

How to do the 5:2 diet safely

Before you start, check with your doctor if you have a health condition or take regular medicines, as doses may need adjusting. A dietitian can help you plan fasting days that still meet your nutrient needs.

• Pick your pattern: try three small meals (breakfast, lunch and dinner) or two slightly larger ones (lunch and dinner) and see which suits you.
• Make every kilojoule count: build fasting-day meals around vegetables and lean protein such as eggs, fish, chicken, tofu or legumes. High-fibre, low-GI foods help you stay full.
• Drink plenty of water throughout the day, including while fasting.⁶
• Eat well on the other five days: vegetables, fruit, wholegrains, dairy or alternatives and protein, in normal portions.
• Keep active and keep protein up to help protect muscle while you lose weight.

Frequently asked questions

Does the 5:2 diet work?

It can help you lose weight, but trials show it works about as well as everyday calorie cutting, not better.

How many kilojoules can you eat on a 5:2 fasting day?

Australian guidance is about 2,000 to 3,000 kJ on each of the two fasting days, and normal eating on the other five.

Is the 5:2 diet safe?

Trials haven't shown clear harms in adults, but the evidence is limited. It isn't recommended in pregnancy, for under-18s, or for anyone with an eating disorder history.

Can you do the 5:2 diet if you have diabetes?

Only with your doctor's advice. Insulin and some other diabetes medicines can cause low blood sugar on fasting days and may need adjusting.

What are the side effects of the 5:2 diet?

Hunger and headaches are common at first. Stop and get advice if you feel weak, irritable or can't concentrate.

References

Research (PubMed)

1. Garegnani LI, Oltra G, Ivaldi D, et al. Intermittent fasting for adults with overweight or obesity. Cochrane Database Syst Rev. 2026;2(2):CD015610. PubMed 41692034
2. 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
3. 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
4. Carter S, Clifton PM, Keogh JB. Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial. JAMA Netw Open. 2018;1(3):e180756. PubMed 30646030
5. 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

Australian guidelines

6. healthdirect Australia. Intermittent fasting. Last reviewed December 2025. View source
7. 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.