Yes, intermittent fasting works for weight loss, and about as well as cutting calories every day. A 2025 BMJ review of 99 trials found that only alternate-day fasting beat daily calorie cutting, and only by about 1.3 kg (3 lb). Fasting works because a shorter eating window helps many people eat less, not because of a special fat-burning effect.
That makes it a scheduling tool. If a fixed eating window makes it easier for you to stay in a calorie deficit, it's a good choice. If you end up eating the same amount in fewer hours, the scale won't move.
Key takeaways
- Similar results to daily calorie cutting: head-to-head trials of 16:8, 5:2 and alternate-day fasting found little or no difference when calories were similar.
- The deficit does the work: 16:8 with no calorie limits lost no more weight than three meals a day in a 12-week trial.
- Alternate-day fasting has a small edge: about 1.3 kg (3 lb) more than daily cutting, but more people quit it.
- Protect your muscle: fit 20-40 g of protein into each meal and lift weights.
How intermittent fasting causes weight loss
Intermittent fasting (IF) tells you when to eat, not what to eat. You lose weight when the eating window removes more calories than you add back later.
Say your usual breakfast is a 400-calorie bowl of cereal with milk and a latte. With a 16:8 window from noon to 8 pm, that breakfast disappears. If lunch and dinner stay the same size, you've cut 400 calories a day, or 2,800 a week. If you make up for it with a bigger lunch and an evening snack, the deficit shrinks to nothing.
That's why the first step is knowing roughly what you burn. The TDEE calculator gives you a starting number, and our guide on how to calculate TDEE for weight loss shows how to turn it into a target.
Intermittent fasting vs calorie counting: what the trials show
| Trial | Who and how long | Fasting approach | Daily calorie cutting | Difference |
|---|---|---|---|---|
| TREAT, 2020 | 116 adults, 12 weeks | 16:8, no calorie advice: -0.94 kg | 3 regular meals: -0.68 kg | -0.26 kg, not significant |
| Liu, 2022 | 139 adults with obesity, 12 months | 8 am-4 pm window plus calorie limit: -8.0 kg | Calorie limit only: -6.3 kg | -1.8 kg, not significant |
| Trepanowski, 2017 | 100 adults, 12 months | Alternate-day fasting: -6.0% vs control | Eat 75% of needs daily: -5.3% vs control | Similar; 38% vs 29% dropped out |
| Harvie, 2011 | 107 women, 6 months | 2 days a week at about 650 kcal: -6.4 kg | Daily 25% cut: -5.6 kg | Not significant |
| BMJ review, 2025 | 99 trials, 6,582 adults | Alternate-day fasting | Daily calorie cutting | -1.29 kg in favor of alternate-day |
The BMJ network meta-analysis found that every fasting style and daily calorie cutting led to more weight loss than eating freely. Against daily cutting, only alternate-day fasting came out ahead, and in trials lasting 24 weeks or longer even that edge wasn't clear. The authors concluded that intermittent fasting has similar benefits to daily calorie restriction.
Types of intermittent fasting
| Method | How it works | Good fit if you |
|---|---|---|
| 14:10 | Eat within 10 hours, for example 9 am-7 pm | Want a gentle start |
| 16:8 | Eat within 8 hours, for example noon-8 pm | Don't mind skipping breakfast |
| 5:2 | Eat normally 5 days, about 500-650 kcal on 2 non-consecutive days | Prefer a few hard days over daily limits |
| Alternate-day fasting | About 25% of your needs every other day | Want the method with a small edge in short trials and can handle hunger |
| 24-hour fast | Dinner to dinner, once or twice a week | Have a flexible schedule |
| One meal a day (OMAD) | All food in one sitting | Find big meals easier than small ones |
OMAD hasn't been tested much in long trials, and it's hard to fit enough protein and fiber into one meal. If you want to plan macros inside a short window, see intermittent fasting macros, or try the intermittent fasting calculator to plan your window.
How much weight can you lose with intermittent fasting?
In the 6- to 12-month trials above, people lost about 5.6-8.0 kg (12-18 lb). In the alternate-day trial, both diet groups lost 5-6% more of their body weight than the control group. In the alternate-day trial, most of the loss came in the first 6 months and a little came back during the maintenance phase, the same pattern seen with daily calorie cutting.
Your result depends on the size of your deficit, not the method. If your weight stalls, check portions in your eating window before you lengthen the fast. Our guide to breaking a weight loss plateau walks through the usual causes.
Does intermittent fasting cause muscle loss?
Any weight loss can cost some muscle along with fat. In the TREAT trial's in-person group, people on 16:8 lost more lean mass in their arms and legs than those eating three meals. The trial didn't pin down why, but it's a good reason not to skimp on protein.
Two habits help:
- Protein at every meal. The International Society of Sports Nutrition suggests 20-40 g of protein per meal, spread every 3-4 hours. With two meals in an 8-hour window, aim for the top of that range. Set a daily target with the protein calculator.
- Strength training 2-3 times a week.
For more on protein while losing weight, read high-protein diets for weight loss.
What breaks a fast
Anything with meaningful calories ends a fast. These don't:
- Water, still or sparkling: 0 calories.
- Black coffee: about 2 calories per 8 fl oz (237 ml) cup.
- Plain tea, including herbal: about 2 calories a cup or less.
- Zero-calorie drinks: fine for most people, though some find sweet tastes make them hungrier.
A splash of milk or a teaspoon of sugar adds 10-20 calories. That won't wreck your results, but a latte or a smoothie in the "fasting" hours is really an early meal.
Who should not try intermittent fasting
Talk to your doctor before fasting if any of these apply:
- Pregnant or breastfeeding.
- Under 18.
- A history of eating disorders.
- Underweight.
- Diabetes treated with insulin or drugs that can cause low blood sugar, such as sulfonylureas.
- Medicines that must be taken with food.
Even healthy adults often feel hungry, irritable or headachy in the first week or two. If that doesn't ease, a shorter fast such as 14:10 may suit you better.
How to start 16:8 for weight loss
- Set your calorie target first. Fasting works best when you know your number.
- Move in steps. Start at 12:12, then 14:10, then 16:8 over two weeks.
- Pick a window that fits your life. Noon to 8 pm works for many people; 8 am to 4 pm is the window the Liu trial used.
- Build meals around protein and fiber. Eggs, Greek yogurt, chicken, fish, beans, vegetables and whole grains keep you full until the window closes.
- Plan the first meal. Opening the window with a planned meal stops the "I fasted, so I deserve it" overeating.
- Track for the first 2-4 weeks. You'll see whether the window is actually cutting calories.
- Judge it after a month. Check your weekly average weight, not single days.
If you cook ahead, meal prep for intermittent fasting has plans built around 8-hour windows. And if you track, Nutrify lets you log a meal from a photo, which makes the first few weeks quicker.
Sources
- 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.
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of time-restricted eating on weight loss and other metabolic parameters: the TREAT randomized clinical trial. JAMA Intern Med, 2020.
- Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med, 2022.
- 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. JAMA Intern Med, 2017.
- Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers. Int J Obes, 2011.
- Kerksick CM, Arent S, Schoenfeld BJ, et al. International Society of Sports Nutrition position stand: nutrient timing. J Int Soc Sports Nutr, 2017.
- USDA FoodData Central, brewed coffee, FDC ID 171890. Search at fdc.nal.usda.gov.
