You've probably heard friends or colleagues talking about 16:8 or the 5:2 diet. But what is intermittent fasting, and does it actually help with weight loss? In short, intermittent fasting means cycling between set periods of eating and fasting. Rather than focusing on what you eat, it focuses on when.
This guide explains how intermittent fasting works, what the latest research says, the most popular methods, and who should avoid it. We'll also look at how it compares with other ways to lose weight.
Intermittent fasting isn't magic. It works mainly by making it easier for some people to eat fewer calories overall.
What is intermittent fasting?
Intermittent fasting is an umbrella term for eating patterns that alternate between fasting and eating. Some limit the hours you eat each day. Others cut calories sharply on certain days of the week.
It's less a diet and more a pattern of eating. You can follow it alongside almost any type of food preference, from vegetarian to high-protein.
How does intermittent fasting help with weight loss?
Like any weight loss approach, intermittent fasting works by putting you into a calorie deficit, where you take in less energy than your body uses. Limiting the hours you eat can naturally cut out late-night snacking and an extra meal or two.
Does intermittent fasting work?
It can. A large 2025 review in The BMJ pooled 99 clinical trials involving more than 6,500 adults. It found that all types of intermittent fasting led to weight loss compared with eating normally.1
But here's the important part: most types of intermittent fasting worked about as well as a traditional calorie-controlled diet. Alternate day fasting showed a small extra benefit, averaging just over 1kg more weight lost than standard calorie cutting, mostly in shorter trials.1
So the best approach is the one you can stick to. If fasting windows suit your routine, great. If they leave you ravenous and grumpy, a steady calorie-controlled diet may work better for you.
Fasting and steady calorie cutting lead to similar results for most people. Pick the pattern that fits your life, not the one that sounds most impressive.
How to do intermittent fasting
There are several popular methods. It's worth trying one that fits around your work, family and sleep.
The 16:8 method
You fast for 16 hours and eat within an 8-hour window each day, for example between 12pm and 8pm. Many people find this the easiest to start, especially if they already skip breakfast.
During the fasting window, stick to water, black coffee, plain tea and other drinks without calories. In one small study of resistance-trained men, a 16:8 pattern combined with weight training reduced fat mass while maintaining muscle.2
The 5:2 diet
You eat normally for 5 days a week and cut right back on 2 non-consecutive days, usually to around 500 to 600 calories. Research comparing this approach with a daily calorie-controlled diet found similar weight loss with both.3
Alternate day fasting
You alternate between normal eating days and fasting or very low-calorie days. This was the method with a small edge in the 2025 review, but many people find it hard to keep up long term.1
24-hour fasts
Sometimes called eat-stop-eat, this involves fasting for a full 24 hours, for example from dinner to dinner, once or twice a week. It's one of the tougher approaches, so build up gradually if you want to try it.
Starting with a 12-hour overnight fast and stretching it gradually is a gentle way to see if intermittent fasting suits you.
Tips for sticking with intermittent fasting
- Stay hydrated - water and herbal teas help with hunger and keep you feeling better during fasting windows.
- Make meals count - fill your eating windows with protein, vegetables, wholegrains and healthy fats rather than processed snacks.
- Plan your day - keep busy during fasting periods so you're not thinking about food.
- Don't overcompensate - eating very large meals when your window opens can cancel out the calorie deficit.
- Stay active - adults should aim for at least 150 minutes of moderate activity a week, plus strength exercises on 2 or more days.4
Who should avoid intermittent fasting?
Intermittent fasting isn't suitable for everyone. Speak to a GP before you start if you:
- Have diabetes, especially if you take insulin or other medicines that can cause low blood sugar
- Are pregnant, trying to get pregnant or breastfeeding
- Have a history of an eating disorder
- Are under 18
- Have a long-term health condition or take regular medication
Common side effects when you start include hunger, headaches, tiredness, irritability and trouble concentrating. These often ease after the first couple of weeks.
If fasting leaves you feeling anxious or out of control around food, it isn't the right approach for you. Speak to a GP for support.
What else can help you lose weight?
Intermittent fasting works best as part of a wider plan. Cutting back on foods high in sugar and saturated fat, eating plenty of fruit, vegetables and fibre, and staying active all make a difference.
If you're struggling to lose weight with diet and exercise alone and your BMI is in the overweight or obese range, a medical weight loss treatment may be an option. Treatments range from Orlistat, which reduces the fat your body absorbs from food, to weekly injections such as Wegovy. Our pharmacists can help you find out what's suitable. If you're already using a weight loss injection, check with your prescriber before you start fasting, as going long periods without food can make side effects such as nausea and dizziness harder to manage.
You can browse our full range of weight loss treatments to see what's available.
Frequently asked questions
What can you drink during intermittent fasting?
During fasting windows, stick to drinks with no or almost no calories. Water, sparkling water, black coffee and plain or herbal tea are all fine. Drinks with sugar, milk or juice add calories and technically break your fast. Staying well hydrated also helps with hunger and headaches while you’re getting used to fasting.
Can you drink coffee while intermittent fasting?
Yes, black coffee is fine during a fast as it contains almost no calories. Adding a splash of milk adds a small number of calories, which some people accept and others avoid. Sugar, syrups and creamy coffee drinks do break your fast. Be careful not to overdo caffeine on an empty stomach, as it can make some people jittery.
Is intermittent fasting good for women?
Many women use intermittent fasting successfully, and the research shows similar weight loss to other calorie-controlled diets. But it isn’t suitable if you’re pregnant, trying to conceive or breastfeeding, or have a history of an eating disorder. If you notice changes to your periods, mood or energy, speak to a GP.
How long does intermittent fasting take to work?
Most people notice some weight loss within the first few weeks, as long as they’re eating fewer calories overall. Early changes may partly be water weight. A steady loss of around 0.5kg to 1kg a week is a realistic target. Results depend on what you eat during your eating windows, not just when.
Can you do intermittent fasting on Mounjaro or Wegovy?
Check with your prescriber first. Weight loss injections already reduce your appetite, and adding long fasting windows can make it harder to get enough nutrients and may make side effects like nausea and dizziness worse. Regular, balanced meals are usually recommended while you’re on treatment.
References
- 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. doi:10.1136/bmj-2024-082007
- Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine. 2016;14(1):290. doi:10.1186/s12967-016-1044-0
- Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity. 2011;35(5):714-727. doi:10.1038/ijo.2010.171
- NHS. Physical activity guidelines for adults aged 19 to 64. NHS website; 2024. nhs.uk
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.