Louise Watson ceased menstruation at 18 after taking up running. Pictured left, exercising in Blackheath, London, she said: 'I've never been unhealthily thin, nor have I exercised excessively. When my periods stopped, doctors put me on hormone replacement treatment. The risk of infertility never occurred to me: I'm completely healthy in every other way.'

Millions of healthy young women are making themselves infertile by surviving on low-fat diets that silently 'switch off' their reproductive systems, according to a controversial new book.

Women who keep themselves just slightly underweight are most at risk of unknowingly suffering from infertility because their borderline status means that, although their menstrual cycle appears unaffected, they have nevertheless lost the ability to conceive.

'There is a close connection to fat and fertility; we can think of it as "sex fat" because body fat provides the energy for reproduction,' said Professor Rose Frisch, author of Female Fertility and the Body Fat Connection .

'Many women who maintain the body shape made popular on catwalks throughout the world are completely infertile,' she said. 'These women can, however, continue having periods and not guess that a thing is wrong with them until they try - and fail - to fall pregnant.'

Frisch, who is Associate Professor of Population Sciences Emerita at the Harvard School of Public Health and a member of the research faculty of the Harvard Centre for Population and Development Studies, has spent the past 20 years researching the link between body fat and fertility.

'What I found most astonishing is that there is a razor-thin borderline where losing just 3lb in weight can tip a normal-sized woman over into infertility without any outward sign at all that such an enormous event has taken place,' she said.

Frisch's theory has been greeted with enthusiasm by other experts in the field. 'This is one of the most simple and elegant theories in biology,' said Robert Barbieri, the Kate Macy Ladd Professor of Obstetrics at Harvard Medical School.

'In women, body fat provides the over-arching control of the organs responsible for reproduction and Frisch's findings will help us answer some of the most vital hitherto unanswered questions concerning fertility and women.'

Frisch criticises the fear of fat shared by many women, pointing out that a young woman who is not overweight should have 35lb of fat in her body while a woman who is well-nourished should be made up of 40 per cent fat.

A well-nourished adult man, by contrast, is only 30 per cent fat.

'These figures initially amazed me,' said Frisch. 'It is not something to be scared or ashamed of: women's body fat converts the male hormone, androgen, into the female hormone, oestrogen: it is therefore a key part of a woman's reproductive ability and therefore something that should be celebrated.'

Frisch has crossed what she calls her critical-fatness theory with the official scientific chart of body mass index (BMI) - body weight in kilograms divided by height in meters, squared - to show at which weight women of differing heights and ages become infertile.

According to her research, for example, a premenstrual girl who is 5ft 3in tall must weight at least 90lb if she is to become fertile, while a premenopausal woman of the same height must weigh over 101lb if she is to continue to ovulate.

'My work proves scientifically how important body fat is to a woman's health,' said Frisch. 'It is a wake-up call to all those women who believe that keeping themselves super-lean is the right thing to do.'

A successful pregnancy costs about 50,000 calories over and above normal metabolic requirements, while nursing, or lactating, costs about 500-1,000 calories a day.

If the body does not contain the necessary calories, Frisch claims, the brain will simply switch off the body's ability to reproduce by gradually restricting the flow of a hormone called leptin, itself only discovered in 1994. If the level of body fat falls below the minimum stated in her table, even if only marginally, a woman will cease to ovulate but continue to menstruate. If the body fat level continues to fall, however, she will then experience a total cessation of her menstrual cycle.

'The super-slim, super-lean women in my studies represented a new phenomenon,' said Frisch. 'They ate, but only enough to be slim and lean, and their diet consisted mostly of non-fat yogurt, lettuce, pasta and gallons of diet drinks.'

According to Frisch, if low body-fat switches off a woman's reproductive system for too long, there may come a point where it is too late to regain fertility by sim ply putting on weight. Although doctors commonly claim that women need to have a BMI of between 20 to 25 to be healthy, Frisch's research enables her to be more specific.

'If a woman has a body mass of 18 or 19, she will continue to have periods but will be infertile,' she said. 'If her BMI falls below that point, she will cease to menstruate at all.'

Frisch's work has been welcomed by experts who are concerned at the lack of research into nutrition-based fertility problems.

'Despite the number of ill-informed celebrity diets, food fads and the modern-day obsession with being super-lean, Frisch is pretty much the only one who's looking at this area,' said Professor George Hall, from St George's Hospital in London.


From Guardian.co.uk