Maya declines the bread with a short speech. Her eating window opens at noon and closes at eight, she tells the table, and she has been watching the clock since the olives.

Across from her, Jo is on day two of a three-day water fast. She holds her sparkling water like a microphone and describes a mental clarity she can only call electric. You are thirty-eight, you lift three times a week, and you would very much like the bread.

Everyone at this table is quoting research, and almost none of it was done on a woman your age who lifts. For intermittent fasting in women over 35, the length of the fast decides whether it helps or backfires, and the evidence thins out as the hours grow. Call your friends Maya and Jo. You have met them.

Three women at a dinner party table debating intermittent fasting for women over 35, one waving away the bread

The short answer: The best-supported form of intermittent fasting for women over 35 is a 12- to 13-hour overnight fast, linked over years to better blood sugar in an observational study of women. A 16:8 window brought no extra weight loss in trials and keeps muscle only with enough protein and lifting. Fasts beyond a day need medical supervision.

  • The TREAT trial of a noon-to-8 p.m. window ran 12 weeks and enrolled 116 adults, 70 of them men.
  • Resistance-trained women eating about 1.6 g of protein per kg daily matched regular eaters for muscle inside an eight-hour window.
  • Among 2,413 San Diego women with early breast cancer, overnight fasts under 13 hours carried 36% higher recurrence risk.
The fast Who at the table What the evidence in women shows How sure are we
12 to 13 hours overnight You Linked, over years, to better blood sugar in a long study of women. Needs no willpower. Emerging: observational
16:8 Maya No extra weight loss over regular meals. Muscle holds only with enough protein and regular lifting. Emerging: short trials
18 to 24 hours, or alternate days The influencer Jo keeps quoting A year-long trial, mostly in women, found no advantage over daily calorie cutting, more dropouts and higher LDL cholesterol. Emerging: one good trial
Three days or more Jo Mostly one fasting clinic's own records of 1,422 guests, with no comparison group. Experiential

The first course: the scientist who stopped

Maya's favourite trial has become dinner-party legend in the wrong direction. It was run from San Francisco by the cardiologist Ethan Weiss, who had been eating inside a daily window himself for years. He went in hoping to show that it worked.

Plate of creamy risotto under grated Parmesan, with a woman's wrist and watch hovering beside the fork

His volunteers ate only between noon and eight for three months and lost no more weight than people eating three regular meals. Among those who had body scans, the window group lost more muscle from their arms and legs. "As soon as I saw the data, I stopped," Weiss told CNBC.

Maya points out, fairly, that the volunteers were mostly men, 70 of the 116. The table agrees this is the best point of the evening so far. A trial of mostly men leaves the case for women open, and the open question is muscle.

The main course: can women over 35 do intermittent fasting without losing muscle?

Maya lifts, and that changes the argument. In a Texas trial, resistance-trained women spent eight weeks on supervised weights. Some ate from breakfast onward and others inside an eight-hour afternoon window, all with whey and matched calories. Every group gained similar muscle and strength.

Slice of tarte tatin pushed away by one woman's hand as another hand pulls the plate back across the dinner table

The fine print was on their plates. Those women ate about 1.6 grams of protein per kilogram of body weight a day. For a 65-kilogram woman that comes to roughly 104 grams inside eight hours, or about 35 grams at each of three meals. The arithmetic is illustrative, and it assumes a trained, well-fed woman who is not pregnant and has healthy kidneys.

You watch Maya's risotto and do the sums in your head. Rice and a snowfall of Parmesan will not get her there before eight. If your protein cannot fit comfortably inside the window, the window is too short for you.

Natural Factors Whey Factors grass-fed whey protein, unflavoured, 1 kg tub

Natural Factors Grass Fed Whey Protein, Unflavoured (38 servings), $39.69

Unflavoured, it disappears into a noon latte or a bowl of overnight oats, which is where a short window gets its head start on the day's protein. At 38 servings a tub, it works out to about a dollar a scoop.

Shop now

Dessert: Jo and the doctor from Darmstadt

Jo waves away the tarte tatin with the serenity of a saint in a fresco. Her fast has an older pedigree than she knows. In 1917 a German naval doctor named Otto Buchinger was sent home an invalid with septic rheumatism of the joints. Two years later a colleague in Freiburg put him on a fast, and Buchinger credited it with saving his life.

Within a year he was running a fasting clinic of his own. His family still runs one on Lake Constance, where guests fast for days on a few hundred calories under medical supervision. The clinic has published its own records, which report very few problems and include no comparison group, a little like a restaurant reviewing its own soufflé.

Jo is doing hers on a sofa in Leslieville with an app. Past a day without food, fasting belongs where Buchinger kept it, under a doctor's eye. That goes double for anyone on glucose-lowering medication or with a history of disordered eating.

Coffee: why is the overnight 12 hours the one worth keeping?

The candles have burned low and the espresso cups are out. Somewhere between the cheese and the coffee you realize you have been fasting too. Dinner ended at nine, you will not eat again until breakfast, and nobody gave a speech about it.

Kitchen after midnight with espresso cups drying beside a protein shaker at the start of an intermittent fasting window

The best evidence in women for this gentle version comes from an unexpected place. Researchers in San Diego followed thousands of women treated for early breast cancer for about seven years. Those who went less than 13 hours overnight without food had a 36% higher risk of the cancer returning. Every extra two hours of overnight fasting also tracked with better long-term blood sugar.

Correlation is a slippery dinner guest, and a study that only watches people cannot prove the fast made the difference. Still, few fasting findings come from women, over years, at a length you can keep without anyone at the table noticing.

So close the kitchen after dinner, keep lifting, and make breakfast generous.

Regenerlife micronized creatine monohydrate powder, unflavoured, 300 g

Regenerlife Creatine Monohydrate, Unflavoured (300 g), $23.97

Stir a scoop into the coffee that breaks your overnight fast and it vanishes without a trace of flavour. It belongs to anyone who lifts and wants to keep what she has built.

Shop now

Maya goes home with a wedge of tarte tatin wrapped in foil for tomorrow at noon. Jo takes the long way home to avoid the bakery. You turn off the kitchen light a little after midnight, and the easiest fast of the week has already begun.

This article is general information and does not replace advice from your doctor or pharmacist.

Shop the story

Frequently asked questions

Will fasting disrupt my hormones or my cycle?

The human data are more reassuring than social media suggests. A 2022 review of trials found intermittent fasting did not change estrogen, prolactin or the pituitary hormones that drive ovulation. In premenopausal women with obesity it lowered testosterone. The authors stressed how few studies exist.

Is an early eating window better than a late one?

Not for weight, on current evidence. A year-long Chinese trial in the New England Journal of Medicine had everyone cut calories and half also eat only between 8 a.m. and 4 p.m. The early window added nothing significant to weight, body fat or metabolic markers.

Do I need a protein powder, or is food enough?

Food is enough if you reach your target. A McMaster University meta-analysis found protein supplements added about 0.3 kg of lean mass over months of training. The benefit was larger in people who already trained and smaller with age.

Read next

Sources