Is getting too lean risking your fertility?

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Is getting too lean risking your fertility?Hearst Owned

While unceasing muscle-bound, sweat-drenched Instagram stories and discussions over brunch tables collapsing under generous portions of hearty protein-packed dishes seem to confirm that the strong-not-skinny message is alive, well and motivating health choices across the UK, outside the wellness bubble, the statistics show a different story.

In a WH Global Naked Survey, a staggering 90% of readers surveyed said they wanted to reduce their levels of body fat, with nearly a third of UK women looking to shift more than 6kg. It points to a desire to get really lean. But as this body goal remains hugely popular, what isn’t immediately clear is the damage super-low-fat goals will make to your future gains – and it’s all down to a simple yet little-known concept called energy availability.


We’re talking about fertility specifically, and that monthly reminder that everything in the ovary arena is in working order. ‘Anyone who exercises is automatically lauded as healthy, even when they’re not,’ says Francesca Baker, 30, a writer.

She took up running at university to counterbalance boozy nights and burgers, but wound up hooked on daily gym sessions and restrictive diets. ‘I stopped taking the contraceptive pill around the same time, so I told myself that the abrupt stop in my periods was down to that, instead of what was quickly becoming an out-of-control desire to be “healthy”.’

It wasn’t. Francesca was suffering from relative energy deficiency in sports (RED-S, pronounced ‘reds’). ‘RED-S is a disparity between food intake (the energy and micronutrients you’re consuming) and the nutrition required to cover the energy demands of basic “housekeeping” tasks in the body and exercise,’ explains sports endocrinologist Dr Nicky Keay ( Like a smartphone on 9% battery, fail to top up its juice and it reverts to an energy-saving mode where automatic functions – in women’s case, periods – switch off.

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Unsurprisingly, humans aren’t quite as homogeneous as iPhone models, so the level at which these processes slow or halt is different for every woman. A regime or body fat percentage that one can maintain without saying goodbye to regular periods could affect the reproductive cycle of another.

‘When you’re focused on significantly lowering body fat, whether through following an extreme exercise regime and/or restricting calories, the body perceives you to be in a stressed state and decides this is not the time to reproduce,’ says Dr Meggie Smith, a fellow in reproductive endocrinology and infertility at the University of Southern California. ‘Signals from the brain to the ovaries, known as the hypothalamic-pituitary-ovarian axis, shut down. The luteinising hormone [which triggers ovulation] isn’t produced and the ovary neither releases an egg nor makes oestrogen and progesterone.’

While being overweight or obese disrupts your reproductive system by producing too much oestrogen, veer towards the other extreme and ‘you essentially enter a post-menopausal
state’, warns Dr Smith.


Given that RED-S could severely disrupt your fertility, it’s fair to wonder why this hasn’t been grabbing headlines. Until recently, the phenomenon was known
as the female athlete triad – or simply ‘the triad’ (we can’t imagine why the acronym
FAT didn’t take off). The triad linked energy deficiency to menstrual disturbances and loss of bone mineral density to explain once-puzzling period loss data, such as why 69% of female college-aged dancers* and 65% of female long-distance runners* didn’t have regular cycles. But in 2014, the International Olympic Committee renamed it, realising that the triad wasn’t actually a threesome after all.

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Energy deficiency was also found to slow metabolic rate, impair immunity, increase fatigue and affect cardiovascular and even gut health. Today, it’s broadening further:
in July 2017, the British Journal Of Sports Medicine published University of Virginia research that explored RED-S in undernourished male athletes, while Dr Keay believes the pressing issue for women is decoding its effect on non-athletes – you know, the rest of us.

‘Elite athletes have the expertise of a coach or team doctor. A dedicated amateur with no such support is therefore more at risk of RED-S if they’re taking measures
to significantly decrease body fat through exercise and dieting,’ she explains. ‘I see this in dancers: aspiring students are often at risk until they join a company, where there
is a support network.’

Dr Keay’s interest isn’t just professional, though. A fanatical gymnast as a child, she would train four times a week, alongside ballet classes that together left her incredibly lean. As a result, it wasn’t until after her second son was born – conceived with the help of an ovulation-inducing tablet and injection – that she got her first period. She was 32. ‘I was fortunate to work in a hospital where I could seek conception advice. If I hadn’t had this medical knowledge, who knows what would have happened. That’s why it’s important women are aware of RED-S.’


‘In my experience, those most susceptible to falling into difficulty are women in their twenties for whom motherhood feels a long way off,’ says registered dietitian
and sports nutritionist Laura Clark ( ‘They hit the gym six or seven times a week – sometimes twice a day. Intense training is one thing, but if the nutrition isn’t there to support that energy expenditure and body function, body fat drops significantly and you will run into real difficulties.’

To the dilemma of too much exercise versus too little food, Dr Smith says that calorie restriction is far more dangerous than high-intensity exercise. The study ‘Neuroendocrinology of Nutritional Infertility’ (not one to try to say while spinning) found that just one month of a significant drop in energy intake could mess with menstrual function.

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But as long as the body composition scales show a BMI in the healthy range, you’re golden, right? Well, the experts can’t seem to agree on how useful traditional measurements are. The late biologist Rose Frisch found that evaluating weight alone could be misleading – as muscles are heavy (80% water compared to 5-10% water in fatty tissue), many athletes appeared in the ‘normal’ weight range despite having no periods. Even with body composition scales, Clark points out that you’ll get different results depending on where you are during your menstrual cycle, as that affects how much water your cells hold.

Frisch was the first to link low body fat to infertility and suggested that women needed at least 17% body fat to fall pregnant – nicknaming it ‘sex fat’, as it provides the energy for reproduction. This is in line with numbers released by the Royal College of Nursing in 2015, stating that the healthy body fat percentage for a women aged 20 to 40 is between 15% and 31%.

It’s why energy availability – the numerical difference between energy intake and expenditure from exercise – is the new buzzword. ‘The energy requirement is 45 calories per kilogram of lean body mass for endocrine function,’ says Dr Keay. ‘However, it’s also about the quality of your diet. I had a dancer who got to that level by eating biscuits and sweets, but she still had no periods as she wasn’t covering the major food groups.’


Spotting whether you’re at risk of RED-S is best done via the process of elimination. ‘Even if your weight is steady, if your periods have stopped or become irregular and you’re not on a contraceptive method, don’t ignore it,’ urges Dr Keay. ‘Your doctor will exclude other causes first, like polycystic ovary syndrome and thyroid disorder, then look at your nutrition in line with your training load.’

Health and training experts also highlight other signs that your quest to become lean
is going too far, even before your periods show any changes. Luke Worthington, elite personal trainer at Third Space, London, recommends being alert to physiological changes like hair loss (a sign of inadequate nutrition), increased body hair (where the body develops a layer of soft downy hair as insulation) and a pot belly caused by intestinal gas.

Psychological warning signs are just as important to be mindful of. ‘If you find yourself obsessing over a missed workout, scheduling your entire life around exercise or meal prep and losing interest in sex, take them as cues that you may be too focused on your goal of getting leaner in an unhealthy way,’ he adds.

As to any long-term impact, loss of periods through too little body fat is reversible. Your fertility is informed by a whole host of factors, from your age to how much you drink and how stressed you are. Consultant sports gynaecologist Michael Dooley ( says that if you’re not underweight and not losing weight via restriction, then you’re likely to conceive unless other medical conditions are at play. His research looks into the possibility that unreleased eggs aren’t lost for good due to RED-S, but are stored, freezer-like, inside your body.

Getting normal menstrual function back is, in fact, more food science than rocket science. The International Olympic Committee’s report on RED-S found that the only strategy that has received scientific scrutiny – and worked – was to add an energy-rich supplement (in the study, it was a daily liquid meal product of around 300-600 calories) and a weekly rest day. The general advice when you’re exercising hard is to think about what you can add to your diet, rather than what you can take away.


Experts’ food tips to help future-proof your fertility


‘If you’re consuming fewer than 2,000 calories and doing endurance sport, your intake is likely to be inadequate,’ says registered nutritional therapist Jo Scott-Dalgleish. ‘Fewer than 2,250 calories on a day when you’re training for two hours puts you at an increased risk of RED-S.’


Compare a droopy, day-before-expiry pack of asparagus with a fresh bunch from a local market. The first isn’t offering you much energy. ‘It’s not only about a healthy diet, it’s about healthy food,’ urges Dooley. ‘Your gut microbes are important, so think about the food that your body’s bacteria eats.’


Don’t swerve carbs in favour of protein. ‘The body’s primary aim is obtaining fuel,’ says Scott-Dalgleish. ‘If there is a lack of fuel (carbs), then the body will convert protein into it – thus not using protein for repairing or building.’ Scott-Dalgleish suggests consuming more starches and sugars (including fruit and veg) on heavier training days.

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