Intermittent Fasting: What the Evidence Says About Weight Loss

Intermittent fasting works for weight loss about as well as an ordinary calorie deficit, but not clearly better. In randomized trials lasting up to a year, people who ate within a time window or fasted on some days lost similar amounts of weight to people who simply ate a bit less every day. What intermittent fasting really offers is a different way to eat less, and that suits some people and not others.

Below is what the evidence on intermittent fasting actually shows, which version might fit your life, and who should not try it.

What intermittent fasting is

Intermittent fasting (IF) means cycling between periods of eating and periods of not eating or eating very little. The three main types studied in trials are:

• Time-restricted eating (for example 16:8): you eat within a daily window, such as 12:00 to 20:00, and fast the rest of the day.
• Whole-day fasting (for example 5:2): you eat normally five days a week and eat very little, often around a quarter of your usual needs, on two days.
• Alternate-day fasting: you alternate “fast days” with about 25% of your energy needs and unrestricted days.

None of these tells you what to eat. That matters, because the food on your plate still shapes your health and how full you feel.

Intermittent fasting: what the evidence says about weight loss

The big question is whether fasting does something special beyond cutting calories. Here are the most informative trials.

16:8 with calorie counting: no clear extra benefit

In a 12-month trial published in the New England Journal of Medicine in 2022, 139 adults with obesity all followed the same calorie-reduced diet (1,200 to 1,500 kcal a day for women, 1,500 to 1,800 for men). Half also ate only between 8:00 and 16:00. After a year, the time-restricted group had lost about 8.0 kg on average and the calorie-only group about 6.3 kg. That gap was not statistically significant, and body fat, waist size, blood pressure and blood markers moved similarly in both groups.

16:8 on its own: very little change

The TREAT trial tested 16:8 without any calorie advice. Over 12 weeks, 116 adults ate from 12:00 to 20:00 or ate three structured meals a day. The fasting group lost about 0.94 kg and the comparison group about 0.68 kg, a difference of roughly a quarter of a kilo that could easily be chance. The authors concluded that time-restricted eating alone was not more effective than eating throughout the day.

Alternate-day fasting: similar results, more dropouts

A one-year trial compared alternate-day fasting with a daily 25% calorie cut in 100 adults with obesity. Weight loss was similar (about 6.0% vs 5.3% of body weight at 12 months compared with the control group). More people quit the fasting plan (38%) than the daily plan (29%), and people on fast days tended to eat more than prescribed. LDL cholesterol was somewhat higher in the fasting group at 12 months.

What the combined evidence shows

A 2025 network meta-analysis in the BMJ pooled 99 randomized trials with 6,582 adults. Every fasting approach, and plain daily calorie restriction, led to weight loss compared with eating freely. Only alternate-day fasting beat daily calorie restriction, and only by about 1.3 kg, mostly in shorter trials. In trials lasting 24 weeks or longer, the diets differed from free eating but not clearly from each other.

The Harvard T.H. Chan School of Public Health reaches a similar conclusion in its diet review: research does not consistently show intermittent fasting to be superior to continuous calorie reduction, and longer, high-quality trials are still needed.

Trial	Who and how long	Result
Liu et al., NEJM 2022	139 adults, 12 months, 16:8-style window plus calorie target vs calorie target alone	about 8.0 vs 6.3 kg lost, difference not significant
TREAT, JAMA Intern Med 2020	116 adults, 12 weeks, 16:8 with no calorie advice vs three meals	about 0.94 vs 0.68 kg lost, difference not significant
Trepanowski et al., 2017	100 adults, 12 months, alternate-day fasting vs daily 25% cut	similar weight loss; 38% vs 29% dropped out
BMJ network meta-analysis 2025	99 trials, 6,582 adults	all approaches beat free eating; only alternate-day fasting slightly beat daily restriction

Why fasting and daily restriction end up so similar

Your weight responds to the energy you take in over days and weeks, not to the clock alone. A simple worked example makes this clear.

Say your body needs about 2,000 kcal a day. In the alternate-day fasting trial above, fast days were set at 25% of needs and “feast” days at 125%:

• Fast day: 2,000 × 0.25 = 500 kcal
• Feast day: 2,000 × 1.25 = 2,500 kcal
• Two-day total: 500 + 2,500 = 3,000 kcal, or an average of 1,500 kcal a day

The daily-restriction group ate 75% of needs every day: 2,000 × 0.75 = 1,500 kcal. On paper, both groups ate the same amount. The results were similar because the calorie deficit was similar.

The same logic applies to 16:8. If skipping breakfast removes 400 kcal but lunch and dinner quietly grow by 400 kcal, your total has not changed. If you want to understand the deficit side of this, our guide to the calorie deficit walks through it step by step.

Possible benefits beyond the scale

Research on fasting and blood sugar, cholesterol and blood pressure is mixed. In the trials above, blood markers did not improve more with fasting than with daily calorie restriction. Two practical points stand out:

• It can match how you already eat. Harvard notes that people who usually eat only one or two meals a day may find this pattern easier to stick to.
• An evening cut-off can help night snackers. Harvard notes that people who eat a lot late at night may benefit, especially if late eating worsens reflux or sleep.

On the flip side, fasting is hard for people who like to eat every few hours, and long gaps can set some people up to overeat once food is back on the table. Prolonged food restriction can also increase a fixation on food in some people.

Who should not try intermittent fasting

The Harvard review lists groups who should avoid intermittent fasting:

• People with diabetes, especially if you take insulin or other medicines that can cause low blood sugar
• People who take medications that need to be taken with food
• Anyone with a history of eating disorders such as anorexia or bulimia
• Children and teenagers who are still growing
• People who are pregnant or breastfeeding

If you have kidney disease, a heart condition, or you take regular medication, talk to your doctor or a registered dietitian before changing when you eat. Very low intake days, like 5:2 fast days of around 500 kcal, go well below the 1,200 kcal (women) and 1,500 kcal (men) floor usually advised without medical supervision, so they are not something to experiment with casually.

How to try it sensibly if you want to

If you are a healthy adult and the structure appeals to you, start gently:

1. Begin with a 12-hour overnight window, for example 20:00 to 08:00. Most people already come close to this.
2. Stretch to 14:10 or 16:8 only if it feels easy. It is still unclear how long or how often you need to fast to see any extra benefit.
3. Plan real meals for the window. Aim for protein, vegetables, whole grains or legumes and some healthy fat. Our guide to how much protein per day helps you spread it over fewer meals.
4. Drink water, plain tea or black coffee during fasting hours.
5. Check your total intake for two weeks. This is the step most people skip. Logging your meals, for example by snapping a photo of each plate in PixNutri and checking the AI estimate, shows you whether the window actually lowered your daily calories or just moved them. Treat photo estimates as estimates and correct portions or hidden oils where you know better.
6. Stop if it makes you miserable, dizzy, or obsessed with food. A steady daily deficit works just as well.

If your weight stalls after a few months, that is normal with any approach. Our article on the weight loss plateau explains what to adjust.

FAQ

Does intermittent fasting burn more fat than a normal diet?

Not in a meaningful way, based on current trials. When total calories are similar, fasting and daily calorie restriction lead to similar losses of weight and body fat over 6 to 12 months.

Is 16:8 fasting effective for weight loss?

It can help if it leads you to eat less overall. Without any change in total intake, the TREAT trial found 16:8 produced about the same small weight change as eating three regular meals.

Can I drink coffee while fasting?

Black coffee, plain tea and water have almost no calories, so they are generally fine during fasting hours. Milk, sugar and syrups add energy.

Is intermittent fasting safe for women?

For most healthy adult women it appears as safe as other calorie-reduced diets. It is not recommended during pregnancy or breastfeeding, or with a history of eating disorders.

How long does it take to see results?

In the trials above, weight loss built up over months, as with any diet. Expect a similar pace to a moderate daily deficit, not faster.