Fertility fears for thin women

Some women aim for model-thin figures

Women who live on low-fat diets could be risking their fertility, a US expert has warned.

Professor Rose Frisch, author of Female Fertility and the Body Fat Connection, said the problem affects women who are just slightly underweight, perhaps aiming to copy the body image of skinny models and celebrities.

She warns the problems can go unnoticed as a woman's menstrual cycle may appear to be unaffected even if she is infertile.

Professor Frisch, who is Associate Professor of Population Sciences Emerita at the Harvard School of Public Health, Boston, Massachusetts, argues a certain amount of body fat is crucial to the effective working of the reproductive system and sexual development.

My work proves scientifically how important body fat is to a woman's health

Professor Rose Frisch

She has devised a 'critical-fatness theory', which uses the scientifically recognised body mass index (BMI) - weight in kilograms divided by height in metres - to show at what point women of different heights and ages become infertile.

A successful pregnancy needs around 50,000 calories more than normal metabolic requirements.

She said women aiming for the lean look tended to eat low-calorie diets consisting of non-fat yoghurt, lettuce, pasta and large quantities of diet drinks.

Professor Frisch said if women did not have enough calories, the brain switches off the ability to reproduce by gradually restricting the flow of a hormone called leptin.

If body fat falls below a BMI of 18 to 19, she predicts ovulation will stop, although menstruation will continue.

If levels continue to fall, periods could stop altogether.

A premenstrual girl who is 5ft 3in tall must weigh at least 90lbs if she is to become fertile, according to Professor Frisch, while a 20-year-old of the same height must weigh over 101lbs to continue to ovulate.

'Borderline'

In research, she used magnetic resonance imaging to assess how much fat was in a woman's body.

A female rower's thigh was compared with that of a sedentary woman of the same age.

The rower had a lot of muscle, but 30 to 40% less fat and no menstrual cycle.

There may be other factors such as stress and exercise which may also contribute

Professor Ian Craft, London Fertility Centre

She said: "What I found amazing is that there is a razor-thin borderline where losing just 3lbs 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 added that women should not be ashamed of having body fat.

"It 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."

Professor Frisch added: "My work proves scientifically how important body fat is to a woman's health.

"It is a wake-up call to all those women who believe keeping themselves super-lean is the right thing to do."

Image cost

Professor Ian Craft of the London Fertility Centre told BBC News Online: "Women may be trying to copy these models on the catwalk, they may want that body image, but they do so at a cost."

But he said leanness may not be the only problem.

"There may be other factors such as stress and exercise which may also contribute."

"There is a very delicate balance in the body."

He added: "Girls who are overtly still menstruating may have stopped ovulating.

"It's too often assumed that if you're having periods, you're ovulating."

Mr Adam Balen of the British Fertility Society said: "This is something women should be aware of.

He added: "It's not just about fertility. There is a long-term risk of oestrogen deficiency on bone health.

"The crucial thing is this notion of relatively subtle changes that can have effects that people weren't aware of."

But he disagreed with the analysis that menstruation could hide ovulation problems: "If a woman is having regular periods she should be reassured things will be OK."