Too thin to be fertile?

A chance chat at a conference on meat production led Rose Frisch to research the link between women's weight and fertility. Jon Marcus reports on her startling findings.

There is a distinct irony to new ground-breaking research by a Harvard University professor into the relationship between fertility and body fat in women.

In an era when thinness is in vogue, it was public demand for leaner pork in the UK that helped Rose Frisch reach a sudden moment of revelation: her discovery that young girls who are too thin reach puberty later, and that women with too little body fat cannot bear children.

Visiting a Nato conference on meat production, of all things, Frisch says, she got talking with a man from the Bristol Meat Institute who shared his frustration that pigs weren't fertile when you made them very lean. The rest reads like a detective story.

"Nature was telling us something," says 83-year-old Frisch, now an emerita associate professor of population sciences, in her office at the Harvard School of Public Health.

Twenty-five years later, Female Fertility and the Body Fat Connection , Frisch's new book on the subject, is challenging controversial cultural beliefs about body image encouraged by skinny supermodels and celebrities. "We would see these women who were living on low-fat milk, low-fat yoghurt, a little melon and a little pasta," Frisch says of her research. "And I would say, 'Do you exercise much?' And they'd say, 'No.' And I'd say, 'How much?' And they'd say, 'Oh, 30 miles a week.'" In fact, Frisch says, her work indicates that women need a body mass index - a formula based on height and weight, which she outlines in the book - of 20 to 25 for a successful pregnancy. If the index falls below 18, pregnancy is virtually impossible. The same is true if it rises above , though the connection between obesity and infertility is still not fully understood.

"We were amazed to find that women's bodies are 25 to 26 per cent fat. In essence, a quarter of the body weight was fat," Frisch says. "Men are half that. The average woman was storing 35 pounds of fat. I could hardly believe it. I looked up autopsy reports to be sure."

Simply put, she says, body fat provides the energy to reproduce. "Why store that much fat?" she asks rhetorically. "How many calories does it take to grow a baby - 50,000 to 80,000. And how many does it take to nurse - 500 to 1,000 calories a day." This connection, Frisch says, was probably understood by 19th-century British doctors. "They knew about the relationship, though they didn't know how it worked," she says. "When a woman came to them who couldn't get pregnant, they would fatten her up."

Frisch's work was more specifically prompted by research at Cambridge University in the 1960s, which found that the signal for puberty in rats was related to their intake of food. The resulting hypothesis: stored fat must be important for reproduction.

"That made so much sense to me," she says. "That's where I started from. I began estimating body fat of women when they first reached puberty. And one of the things I found was that what they had in common was the same relative fatness, which I could estimate from a fatness index."

Like the researchers at Cambridge, Frisch determined that a girl would not begin to menstruate until she had crossed a certain threshold of body fat. She traced this cut-off to a hormone called leptin, which served as a "switch" in the brain. "That provided this wonderful biochemical basis that the brain could know how fat you are," she says. "When you don't have enough fat to grow a baby properly, the switch turns off."

The effect can be dramatic. The average American girl reaches puberty just before turning 13. Studies showed that lean ballet dancers, like undernourished girls in third-world countries, did not begin to menstruate until after they were 15. Athletes, Frisch found, delayed the onset of menstruation by five months for every year they were in training.

Even after that, young teenagers who become pregnant are more likely to have low-birthweight infants or infants with neurological defects because they have not completed their own physical growth. "Even well-nourished US girls do not complete their growth in height, weight, and the reproductive organs, including the uterus, until ages 16 to 18," Frisch says.

Later in life, women who fall below a body mass index of 20 will be infertile, Frisch says, although they probably will not know the reason because their menstrual cycles will seem unaffected.

The margin between fertility and infertility based on body mass is "razor-thin," she says - often just a few pounds. Women who try to maintain a fashionable weight may continue to try in vain to get pregnant.

But the effect, Frisch says, is reversible. "I've had athletes who turned their cycles on and off."

She is not worried about the thinness fad causing a decline in fertility. In fact, Frisch says, her concern is the opposite: that as economic conditions improve in the third world, and poor women become better nourished, populations there may increase dangerously.

"In developing countries, when the economic situation rises, women who have been undernourished will become more fertile," says Frisch, who remains on the faculty of the Harvard Center for Population and Development.

"Those women will need more family planning, and we're at a time when we are not providing family planning to these women," she says.

Frisch has also been involved in research showing that moderate exercise may decrease the risk of breast cancer in women.

As early as the 1970s, Frisch showed a link between obesity and early maturation - a connection proven by a recent University of North Carolina study, which found that early maturing girls were heavier by more than 3.6kgs than those who matured later. Significantly higher proportions of overweight girls had their first periods before they had even turned 11.

All of these indicators illustrate the need for women to maintain what Frisch calls "critical fitness" - in a world that values thinness far too highly, and has done so for almost the entire three decades Frisch has been at work unravelling the body composition question.

"I still remember Twiggy," Frisch says. "I remember thinking that she's way too thin."

Female Fertility and the Body Fat Connection is published by University of Chicago Press, £13.00.