Fasting affects women differently from men. So should they still do it?

Humans didn’t evolve to eat all day long. We work to a rhythm where our bodies need fuel, but also need time, uninterrupted by digestive processes, to rest and repair. The strange environment we live in, with food available 24 hours a day and the ability to move through life while barely moving at all, has led to an exponential rise in obesity and chronic illness.

Fasting – in its various forms, including intermittent and time-restricted eating – has become a highly fashionable antidote, often pitched as a more sustainable way of eating less and returning our bodies to their “natural” rhythm and state.

Fasting and fasted exercise, especially intense exercise, is different in women and men. Credit: iStock

Adding fasted exercise (working out on an empty stomach) into the equation is an evolution of the trend that has become equally popular, especially off the back of suggestions it can accelerate fat burning and even improve endurance. But like most scientific health research, studies that look at fasting and fasting exercise are often done on men, then presumed to apply to women.

Yet there are sex differences in metabolism, likely to be evolutionary adaptations that allow women to maintain energy reserves for reproduction when food supplies are low. So what does this mean for fasting and fasted exercise among women?

The heart of the matter

“Based on what we know, there are no major differences between men and women for cardiometabolic health,” says Professor Luigi Fontana, scientific director of the University of Sydney’s Charles Perkins Centre.

And for relatively little effort – confining our food intake to a set period of time each day, eating a bit less on a couple of days of the week or fasting every other day – there may be good cardiometabolic returns. Research suggests intermittent fasting might benefit inflammation, blood pressure and insulin levels as well as weight.

Finding ways to consume a little less (while still giving our body the nutrients it needs) might even extend healthy lifespan.

However, a new paper presented at an American Heart Association conference created confusion about the beneficial effects, suggesting that people who followed an eight-hour time-restricted eating schedule had a 91 per cent higher risk of death from cardiovascular disease.

The paper, which was not peer-reviewed, was based on the participants’ recollection of just two days of eating and provided no information about their overall health, lifestyle and what or how they normally ate. But, it does highlight how limited the research still is. “We need more studies,” says Fontana.

What we eat when we break our fast is important too. The purported metabolic benefits of fasting may be vetoed if our overall diet is made up of mostly ultra-processed foods. And, it’s possible (for men and women) to go too far with fasting, depriving our bodies of what they need to function.

What about women?

As is often the case, the devil is in the detail. Preliminary research has found that time-restricted eating (TRE), specifically eating earlier in the day, offers promise in the treatment of hyperandrogenic conditions, such as polycystic ovary syndrome. It may also be effective in reducing the risk of type 2 diabetes and as a potentially more sustainable alternative to treat obesity.

But that doesn’t mean it’s good for all women, all the time.

In one 2022 study, premenopausal women practised an intense form of TRE (eating an unrestricted amount of food in a four-hour daily window) for eight weeks. While most of their reproductive hormones were unchanged, it did reduce levels of DHEA, a hormone related to ovarian function and egg quality.

In her 2022 book Fast Like a Girl, Dr Mindy Pelz (who has a bachelor of science degree and is a doctor of chiropractic), suggests that the time of month a woman fasts matters too. Specifically, she suggests that women should not fast (or reduce carbohydrate intake) the week before their period because we need more glucose to make progesterone. Once our period has started, and oestrogen levels are low, Pelz argues that it is a perfect time to fast.

“Oestrogen loves when we fast,” she told Dr Rangan Chatterjee on his Feel Better, Live More podcast in February. “When [oestrogen levels] are low, we can really fast as long as we want.”

But it’s still complicated. Dr Stacy Sims, an exercise physiologist and nutrition scientist, disagrees with Pelz: “I have no idea where she would have gotten this theory.”

Fasted exercise: A training tool with risks

Sims believes that women who are healthy, eating a nutritious diet and exercising regularly need to ask themselves why they want to fast in the first place.

“An endurance athlete might do some sessions with a ‘sleep low/train low’ strategy, in which they have depleted muscle glycogen with the previous afternoon/evening’s workout, then complete a recovery session in the morning without topping up carbohydrate stores,” explains professor Louise Burke, the head of Sports Nutrition at the Australian Institute of Sport.

“Such a training strategy (training with low glycogen and carbohydrate availability) may enhance some of the signalling pathways that are turned on by exercise to gain a greater muscle adaptation to the session (that is, stimulate more mitochondrial activity).”

Other athletes who do lengthy training sessions (for example, cyclists) may start the workout fasted, and then start eating some carbs as they go along. But these are strategies that are used occasionally and in a targeted way.

For the rest of us, reviews suggest that fasted exercise does not, in fact, accelerate fat burning,  alter body composition or enhance performance.

In fact, there is robust population data to indicate that fasted training is not beneficial for women of a normal weight, Sims says.

She worries the trend of intermittent fasting has become an acceptable proxy for eating disorder behaviour.

Sims argues that the differences in women’s metabolic function and the areas of the brain responsible for appetite control and endocrine function mean women who are doing fasted exercise – particularly endurance or high intensity activities – are more prone to endocrine dysfunction, lean mass loss and issues with bone metabolism.

Just a few tablespoons of Greek yoghurt, or a protein-fortified coffee, or a banana before exercise is enough to tell the brain that there’s some nutrition available, she says. “Then the body can rev up and do what it needs to do in the gym and benefit from that stress and adapt to it.”

Following intense exercise with a protein-rich meal signals to the brain that it can go to recovery mode, she adds, pointing out that intermittent fasting needs to be distinguished from time-restricted eating, which is a way of working with our circadian rhythm.

“You’re eating when your body needs it, and then you’re giving it a rest when it doesn’t,” Sims says. “Intermittent fasting is more severe, with large times of no food.”

University of Adelaide’s Professor Leonie Heilbronn, an expert in obesity and metabolism, is more hesitant about how different the effects are between men and women and the impact of the various forms of fasting. “I don’t think we have sufficient data to make conclusions around this,” she says.

Much remains unclear about the difference between sexes as well as differences in individuals. And while long-term research is key to unravelling a trend that shows promise as well as potential for harm, our primary focus should be on moving as much as we can and having a diverse diet, rich with minimally processed foods.

“Fasting with healthy nutrition during feasting days is a tool,” says Fontana, “but not the solution to the pandemic of abdominal obesity, metabolic syndrome and unhealthy ageing.”

Make the most of your health, relationships, fitness and nutrition with our Live Well newsletter. Get it in your inbox every Monday.