Intermittent Fasting for Women: What the Research Shows

Intermittent fasting for women is one of the most talked-about ways to lose weight, and many women wonder whether it works differently for them. Perhaps you have heard that fasting affects your periods, that it helps during the menopause or that it is especially good for PCOS. This article goes through what the studies actually show, which groups have been studied and where the knowledge runs out. It is not a guide to doing it and contains no schedules. The Swedish Food Agency (Livsmedelsverket) sums up the evidence like this: "There does not seem to be any difference in weight loss between cutting calories on certain days and reducing energy intake a little every day" (our translation, Livsmedelsverket, Diets).

What is intermittent fasting?#

Intermittent fasting is an umbrella term for approaches that alternate between periods of eating and periods of eating little or nothing. The most common forms in research are 16:8, where all food is eaten within an 8-hour eating window, 5:2, where you eat 500–600 kcal on two days a week, and alternate-day fasting. What they have in common is that they govern when you eat, not what.

The terms are used slightly differently in studies and in popular books. In the TREAT trial, 16:8 meant an eating window of 8 hours a day (Lowe 2020). In a diabetes trial, 5:2 meant participants ate 500–600 kcal on two days a week and as usual for the rest of the week (Carter 2018). Alternate-day fasting means fasting days and normal days take turns.

The Swedish Food Agency also points out that "the evidence for saying anything about effects other than weight loss in humans is insufficient" (our translation, Livsmedelsverket). That is worth keeping in mind when you read about fasting's claimed effects on hormones, metabolism or ageing.

Do you lose more weight with intermittent fasting?#

No, not according to the studies so far. Intermittent fasting does lead to weight loss, but about as much as an ordinary calorie deficit. The 2026 Cochrane review covers 22 randomised trials with 1,995 adults with overweight or obesity. It found little or no difference compared with standard dietary advice: the difference in weight loss was 0.33 percentage points (21 trials, 1,430 participants), and the certainty of the evidence was rated as low (Garegnani 2026).

A randomised trial from China had a calorie target in both groups. 139 adults with obesity ate to a calorie target for 12 months, and half of them also ate within an 8-hour window. The fasting group lost 8.0 kg and the calorie group 6.3 kg. The difference of 1.8 kg was not statistically significant (P = 0.11) (Liu 2022, NEJM). The trial gave the women 1,200–1,500 kcal a day as part of its design. That is not a recommendation. What a reasonable deficit looks like for you is covered in our guide to how big a calorie deficit needs to be.

The BMJ network meta-analysis from 2025 (99 randomised trials, 6,582 adults) compared several forms against each other. Alternate-day fasting was the only form that gave a small advantage over daily calorie restriction, −1.29 kg, and only in trials shorter than 24 weeks (Semnani-Azad 2025). The conclusion is that fasting is one way of eating less, not a shortcut.

How long the result lasts is not known. The trials in the Cochrane review followed participants for 12 months at most. What affects your weight after weight loss is covered in the article on stabilisation after weight loss.

Intermittent fasting compared with an ordinary calorie deficit: what the studies show#

The table shows the most important studies side by side. The comparison group differs between the studies and is given in the column. In every comparison, the difference was small or not significant. Note the populations: TREAT was 60% men, Carter covered people with type 2 diabetes and Liu Chinese adults with obesity. Only Chen and Corapi studied women: Corapi only women, Chen trials in which at least 85% were women.

The TREAT trial fits the pattern. There, estimated energy intake did not differ between the groups, and neither did weight loss (Lowe 2020). The studies therefore suggest that how much you eat over time matters more than the clock.

Does intermittent fasting affect the menstrual cycle and hormones?#

There is too little research to say for certain. A review of human trials found that fasting "did not have any effect on estrogen, gonadotropins, or prolactin levels in women", but the authors also write that there are "very few studies" and that firm conclusions are hard to draw (Cienfuegos 2022). In premenopausal women with obesity, androgen markers fell and SHBG rose.

There are animal studies where an 8-hour eating window in animals of normal weight "may have adverse effects on reproductive function" (Mao 2024). This has not been shown in humans and cannot be transferred directly. The opposite claim, that fasting is especially good for women's hormones, has no support either.

What is known with more confidence concerns energy as a whole. If your periods become irregular or stop when you eat less, that is a signal to contact your GP. Why too large a deficit can affect your periods, and what energy availability means, is explained in the article on calorie deficits for women.

Intermittent fasting during the menopause: what is known?#

Very little. Many women thinking about their weight during the menopause come across fasting as a suggestion, but the research is thin. A systematic review of 15 randomised trials on diet for menopausal symptoms found that the evidence for intermittent fasting is limited and concerns cognition, not hot flushes (O'Neill 2026). There is no evidence that fasting relieves menopausal symptoms.

A small pilot study of 25 postmenopausal women compared two ways of cutting calories: five-week periods of calorie restriction alternating with five weeks of weight stabilisation, against continuous restriction. Weight loss was similar in both groups (Arguin 2012). These were diet breaks, not fasting within the day or week, so the study says nothing about 16:8 or 5:2 during the menopause. For weight during the menopause the same principle applies as at any other time: it is your total intake over the week that counts.

Does intermittent fasting help with PCOS?#

As well as calorie restriction, but not better, according to a randomised trial from 2026. 76 women with PCOS were followed for 6 months. Compared with the control group, which did not change its diet, the group with a 6-hour eating window lost 4.32 percentage points more of their body weight and the group on 25% calorie restriction 4.66 points more. The difference of 0.34 percentage points between the two was not significant (P = 0.79) (Corapi 2026, Nature Medicine).

A meta-analysis of five studies compared values before and after fasting, without a control group. Weight fell by 4.25 kg, the free androgen index (FAI) by 1.61 percentage points, and SHBG rose. The authors write that the evidence is still "preliminary" (Ranneh 2025).

Worth knowing: the Corapi trial and the Cienfuegos review come from the same research group, and according to the Nature Medicine paper co-author Krista A. Varady receives fees from publishers for books on fasting diets. That does not make the results wrong, but it is a reason not to base conclusions on a single study. PCOS is treated individually, so talk to your doctor before you change your diet.

Can you fast while pregnant or breastfeeding?#

There is no randomised trial of fasting during pregnancy. A meta-analysis found 19 studies, mainly on Ramadan fasting, and all were observational. Fasting was associated with an average birth weight 94 g lower, but not with a higher risk of low birth weight (RR 0.96) (Giorno 2025). An umbrella review concludes that clinicians "cannot provide firm recommendations" (Al-Taiar 2025).

Fasting during pregnancy has therefore been shown neither safe nor harmful; it simply has not been studied in randomised trials. Avoid it and ask your midwife.

For breastfeeding, the Swedish Food Agency advises: "On average, a breastfeeding mother needs about 500 extra kilocalories a day." The agency also writes that "a good goal is to return to your pre-pregnancy weight within about a year" (our translation, Livsmedelsverket, Breastfeeding). If you want to lose weight while breastfeeding, your midwife or GP is the right person to talk to.

Who should not fast?#

Pregnant and breastfeeding women, people under 18, anyone who has or has had an eating disorder, and anyone with diabetes who takes insulin or sulfonylureas without their doctor involved. For these groups there are either no randomised trials, or known risks that require healthcare to be involved.

The diabetes risk is concrete. In a 5:2 trial of 137 people with type 2 diabetes, medication that can cause low blood sugar was reduced from the start. Even so, 16 of 46 people taking sulfonylureas or insulin (35%) had hypo- or hyperglycaemia during the first two weeks. The figure covers both groups together, 5:2 and daily calorie restriction, and the number of events did not differ significantly between them (Carter 2018). A trial of 46 people with insulin-treated type 2 diabetes saw no severe hypoglycaemia over 12 weeks, but participants had dietary counselling and continuous glucose monitoring (Obermayer 2023). This suggests that fasting while on diabetes medication should only be done with your doctor.

The link to eating disorders has not been shown to be causal, but it exists. Among 2,762 Canadian adolescents and young adults, 47.7% of the female participants reported fasting in the past year, and fasting was associated with more eating disorder symptoms (Ganson 2022, cross-sectional study). In a Chinese follow-up of 491 adults over 8 months, fasting was associated with more eating disorder symptoms at follow-up (He 2025). If you have a difficult relationship with food, you do not have to handle it alone: talk to your GP or contact the National Alliance for Eating Disorders.

Can you drink black coffee while fasting?#

Black coffee provides almost no energy. According to Swedish Food Agency (Livsmedelsverket) values from its food composition database, brewed coffee contains 2 kcal and 0.0 g protein per 100 g, and espresso 9 kcal and 0.1 g protein per 100 g. It is milk, cream and sugar that provide the energy. Whether it "breaks the fast" depends on how you define the fast, not on a physiological limit.

None of the studies in this review compares coffee with and without additions during fasting. Claims that black coffee strengthens fasting's effects on blood sugar or fat metabolism are therefore not supported by them. The simple answer is that black coffee adds no energy worth mentioning, while a caffè latte does.

Do you lose muscle with intermittent fasting?#

The results point in different directions. In the TREAT trial (12 weeks), body composition was measured in a subgroup of 50 people, 25 in each group. There, a height-adjusted index of lean mass in the arms and legs fell more in the fasting group than in the control group, by 0.16 kg/m² (Lowe 2020). Chen's meta-analysis of 13 randomised trials, in which at least 85% of participants were women with overweight or obesity, by contrast found no statistically significant difference from the control groups in lean mass (Chen 2025).

Why the studies differ has not been explained. One possibility, which the studies have not tested, is how much protein participants ate: fewer meals can mean less protein if you do not pay attention to it. How to get protein in early in the day is described in the article on a high-protein breakfast for weight loss.

How Smaklig fits in#

Smaklig does not offer a fasting diet, and there is no need for one. The studies suggest that what matters is what and how much you eat over the week, not the clock. That is why Smaklig plans the week's dinners around the offers in the shop where you buy your food, across all seven Swedish chains the app follows, and writes the shopping list for you. If you shop at ICA or Coop, the items can go straight into your basket in the app. Whether you eat three meals a day or within a shorter window, a planned week helps you get nourishing meals without starting from scratch every evening. More on how planning helps with weight loss is in the guide to weight loss with meal planning.

The app works out a daily target for calories and protein from your weight, your goal and how active you are, and every recipe shows protein per portion. That makes it easier to see whether the day's meals give you enough protein, even when there are fewer of them. If a dinner doesn't suit you, you replace the recipe, and if more or fewer people eat, you change the number of portions. If any of the situations above apply to you, such as pregnancy, breastfeeding or treatment with insulin or sulfonylureas, your GP's or midwife's advice comes before the app's suggestions.

Common mistakes#

1. Believing the eating window itself gives extra weight loss. The effect comes from eating less, not from the clock (Garegnani 2026, Liu 2022).
2. Assuming the eating window works on its own. In TREAT, neither estimated energy intake nor weight loss differed between the groups.
3. Fasting during pregnancy, breastfeeding or diabetes medication without healthcare involved. There are either no randomised trials or known risks.
4. Reading missed periods as the body "adjusting". It is a signal to contact your GP.
5. Trusting promises that fasting cleanses the body or regulates hormones. Such effects are not found in the studies.
6. Letting protein slip when the number of meals falls. In the TREAT subgroup, the lean mass index for arms and legs fell more in the fasting group.

Checklist#

• Ask yourself first: are you pregnant, breastfeeding, under 18, do you have or have you had an eating disorder, or do you take insulin or sulfonylureas? Then your GP or midwife is the right first step, not fasting on your own.
• Compare with the alternative: a steady, moderate deficit gives the same results as fasting in trials.
• Think about content: protein, vegetables and wholegrains matter whenever you eat.
• Listen to your body: if your periods stop, or food starts to cause anxiety, it is time to seek support, for example from your GP.
• Choose what you can live with: the Swedish Food Agency writes that the most important thing is that a diet "can be kept up over a longer period" (our translation).
• Plan the week: a weekly plan with protein per portion makes it easier to eat well, whatever time of day you eat.