Short answer: Intermittent fasting means limiting when you eat: within a daily window (such as 16:8), on two low-calorie days a week (5:2) or on alternate days. In randomized trials it has led to about the same weight loss as ordinary daily calorie restriction, not more. If a fasting timetable suits your routine, it is one reasonable option. Talk to your doctor first if you are pregnant or breastfeeding, have diabetes or have had an eating disorder.
Intermittent fasting changes the clock rather than the menu. Instead of deciding what to cut, you decide when to eat. The US National Institute on Aging (NIA) notes the practical effect: with less time for regular eating, you may simply eat fewer calories.
What is intermittent fasting?
The NIA separates two ideas. Calorie restriction means eating less on average every day, without missing essential nutrients. A fasting diet means eating nothing, or very little, at set times of the day, week or month, whether or not you also eat less the rest of the time.
| Pattern | How it works | Example | Worth knowing |
|---|---|---|---|
| 16:8 (time-restricted eating) | All meals within a limited window each day, such as 6–8 hours; 16:8 means an 8-hour window | Eat between 10:00 and 18:00, nothing with calories outside it | You choose the window, so you can fit it around work and family meals |
| 5:2 | Normal eating on 5 days, restricted intake on 2 days a week | Lighter days on Monday and Tuesday | In an NIA study of 5:2, restricted days were limited to 500–600 calories |
| Alternate-day fasting | No or very little food every other day, unrestricted eating in between | One trial used 25% of energy needs on fast days, 125% on the days between | The pattern with the most dropouts in that trial |
Does intermittent fasting work better than calorie restriction?
For weight loss, the best trials so far point the same way: about as well as eating a little less every day, not better.
Time-restricted eating (16:8)
In a 12-month trial in the New England Journal of Medicine (2022), 139 adults with obesity were all asked to eat 1,500–1,800 kcal a day (men) or 1,200–1,500 kcal (women); half also ate only between 8 a.m. and 4 p.m. The time-restricted group lost 8.0 kg (17.6 lb) on average and the calorie-only group 6.3 kg (13.9 lb), a difference that was not statistically significant. Body fat, metabolic risk factors and side effects showed the same picture.
5:2 and other two-day patterns
A six-month trial of 107 women with overweight or obesity compared two ways of eating 25% less energy: about 2,710 kJ (roughly 650 kcal) a day on two days a week, or a smaller cut every day. Average weight loss was 6.4 kg (14.1 lb) and 5.6 kg (12.3 lb), and the researchers judged the approaches equally effective.
Alternate-day fasting
A one-year trial in JAMA Internal Medicine (2017) assigned 100 adults with obesity to alternate-day fasting, daily calorie restriction (75% of energy needs every day) or no diet. Both diet groups lost 6.8% of body weight at six months relative to the no-diet group. More people dropped out of alternate-day fasting (38% vs 29%), the fasting group tended to eat more than planned on fast days, and by 12 months their LDL cholesterol was higher than in the daily group.
What it adds up to
A 2026 Cochrane review of 22 randomized trials with 1,995 adults with overweight or obesity found that, compared with regular dietary advice, intermittent fasting may make little to no difference to weight loss or quality of life; the evidence on side effects is very uncertain, and the trials covered up to 12 months only. The NIA adds that there is not yet enough evidence to recommend any calorie-restriction or fasting diet to the public.
So the useful question is which pattern you can keep up comfortably while eating well. Weight change still comes down to overall energy balance; see our guide to calorie deficits and plateaus.
Who should not fast without medical advice
Talk to your doctor or a registered dietitian before trying any fasting pattern if any of these apply to you:
- Pregnancy or breastfeeding. Diabetes guidance from Know Diabetes, including a British Dietetic Association verdict, says not to attempt intermittent fasting while pregnant or breastfeeding.
- Diabetes, or medicines that lower blood sugar. With insulin or medicines such as sulfonylureas or glinides, fasting could significantly increase the risk of hypos (low blood sugar). Your diabetes team can help adjust your medication.
- A history of eating disorders, such as anorexia or bulimia. The same guidance advises against intermittent fasting.
- Children and teenagers. Fasting trials have mostly recruited adults aged 60 or younger, so the NIA notes their results don’t necessarily apply to children.
- Older adults. The NIA says more needs to be learned about the safety of these diets, especially in older people.
- Other conditions: regular medication, being underweight, a heart, kidney or other long-term condition, or recovery from illness or surgery.
If counting hours or calories starts to make you anxious or preoccupied with food, stop and talk to a health professional.
How to start intermittent fasting, step by step
- Check the list above. If any point applies, start with a conversation with your doctor, not with a fast.
- Pick one pattern that fits your week. A daily eating window is the most flexible; place it around meals you share with others.
- Start with a gentle overnight gap. For example, if you usually eat from 7:00 to 22:00, first finish after dinner at 20:00 and keep breakfast at 8:00. Once that feels normal, move breakfast later in small steps.
- Set a calorie target, not just a clock. In the 16:8 trial above, both groups had one. A window doesn’t guarantee a deficit if you eat more inside it.
- Drink water through the fasting hours. Intermittent fasting limits food and drinks with calories, not water.
- Review after a few weeks: weekly average weight, hunger, energy, sleep and mood. If it isn’t working for you, stop; daily calorie restriction gave similar results in the trials.
What to eat in your eating window
Fasting changes the timing, not your nutritional needs. The NIA stresses that any pattern should give you a safe level of nutrition, and the British Dietetic Association verdict quoted by Know Diabetes warns against nutritional deficiencies, dehydration and overeating on non-fasting days.
- Build each meal around protein: eggs, yogurt, fish, chicken, tofu, lentils, beans or chickpeas. See how much protein you need per day.
- Add vegetables, fruit and whole grains such as oats, brown rice, bulgur or whole-wheat chapati; fiber and volume help you stay full.
- On 5:2 low days, choose filling, lower-calorie foods: vegetable or lentil soup, eggs, salad with chicken or beans.
| Time | Meal | Why it helps |
|---|---|---|
| Morning | Water, unsweetened tea or black coffee | Fluids without energy |
| 12:00 | Lentil soup, whole-grain bread, a bowl of yogurt | Protein and fiber after the overnight fast |
| 16:00 | Fruit and a small handful of nuts | Bridges the afternoon |
| 19:30 | Grilled fish or tofu, rice, stir-fried vegetables | A filling last meal before the fast |
Common side effects and when to stop
In the six-month two-day-a-week trial above, there were no major adverse effects. Still, 15% of women on the fasting pattern complained of hunger (none in the daily group), 8% reported minor symptoms such as lack of energy, headaches, feeling cold or constipation, and 15% reported poor concentration, bad temper or preoccupation with food, compared with 7% on daily restriction.
To ease hunger, put protein and fiber in the last meal before a fast and drink water. If you feel irritable, tired or unfocused, try a wider window or move low days to quieter days.
Stop fasting and get medical advice if you feel dizzy, faint, shaky or confused, if you have diabetes and notice signs of a hypo, or if symptoms don’t settle.
How intermittent fasting differs from Ramadan fasting
- No water. From dawn to sunset, people fasting for Ramadan take no food or drink, including water, so hydration has to fit between iftar and suhoor. Most intermittent fasting plans limit calories, not water.
- The timing isn’t yours to choose. It follows dawn and sunset where you live, so the fast’s length changes with the season and location.
- The purpose is religious, not weight loss, and meals are often shared and festive.
If you have diabetes or another condition, talk to your doctor well before Ramadan. Our guide to eating well during Ramadan covers suhoor, iftar and hydration.
How Portiva helps
Portiva puts an intermittent fasting timer next to your food log, so you see your eating window and what you eat in it in one place.
- The fasting timer and basic Ramadan tools are free; Ramadan mode shows imsak and iftar times for your city.
- Log meals by photo, voice, text or barcode. When oil, sauce or portion size would change the calories a lot, Portiva asks one short question instead of guessing.
- Calorie and macro targets adapt to your weight trend, so you can see whether your pattern is creating a deficit.
Frequently asked questions
Is intermittent fasting better than calorie counting for weight loss?
Not according to the trials so far. Adding an 8-hour eating window to a calorie target did not lead to significantly more weight loss over 12 months, and alternate-day fasting matched daily restriction over a year. A 2026 Cochrane review found little to no difference compared with regular dietary advice. Choose the approach you can sustain.
Can I drink water, coffee or tea while fasting?
Yes. Intermittent fasting limits food and drinks with calories, not fluids, so water matters. Unsweetened tea and black coffee add little or no energy. Milk, sugar, juice and soft drinks do contain calories, so keep them for your eating window. Ramadan fasting is different, because it excludes water too.
How many calories do you eat on a 5:2 fasting day?
Plans vary. In an NIA study of the 5:2 pattern, restricted days were limited to 500–600 calories, and a UK trial used about 2,710 kJ (roughly 650 kcal). Low days are a big change from normal eating, so if you have a medical condition or take regular medication, agree on a plan with your doctor first.
Do I still need to count calories with intermittent fasting?
Not necessarily, but it helps to check. A shorter window often means eating less, yet larger meals can make up the difference. Logging for a few weeks shows whether your total really fell. Our guide on how to count calories accurately covers the oils, drinks and sauces that are easiest to miss.
Sources
- Calorie restriction and fasting diets: What do we know?, National Institute on Aging, NIH (2018).
- Intermittent fasting for adults with overweight or obesity, Garegnani LI et al., Cochrane Database of Systematic Reviews (2026).
- Calorie Restriction with or without Time-Restricted Eating in Weight Loss, Liu D et al., New England Journal of Medicine (2022).
- Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults, Trepanowski JF et al., JAMA Internal Medicine (2017).
- The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers, Harvie MN et al., International Journal of Obesity (2011).
- Intermittent fasting: 5:2 eating plan, Know Diabetes.