Ever wondered why you eat more before your period? The answer could provide a clue to preventing obesity, says Jerome Burne
Do you tend to eat more just before your period and rather less afterwards? A highly unscientific straw poll of my female friends showed that most of them had noticed a connection but hadn't thought much more about it. Or if they had, they put it down to just another of those annoying things that go with pre-menstrual tension. But this connection could provide a clue to one of the medical mysteries of our time. Why is it that women have more of a problem with overeating than men?

Of course there is no shortage of fat men, but women get it worse. Around 90 per cent of anorexics and bulimics are women, and women are four times more likely to be seriously overweight - 50 per cent over normal body weight - than men. Social and psychological factors obviously play a part - women still doing the bulk of domestic cooking, the extra emphasis on women's bodies - but according to research by Dr Nori Geary of Cornell University Medical College, the female hormone oestrogen plays a neglected but crucial role in controlling eating.

"We can see this very clearly in rats," she says. "While male rats eat much the same amount throughout a month, female rats' food intake varies dramatically during the ovarian cycle." When a rat is fertile and has higher levels of oestrogen, she eats less and tends to lose weight. If you then take out her ovaries, she will balloon because she starts eating more at each meal. Within five weeks she will be around 25 per cent heavier than she was before. If you then put her on a course of hormone replacement therapy, her weight will return to what it was.

Humans aren't rats, of course, but something similar seems to be going on. "Studies have found that women eat less when they are ovulating," says Geary. "But just before their period is when they are likely to have cravings for fat or carbohydrates." On the other hand loss of the ovaries doesn't seem to produce such a drastic change in humans. Women do tend to put on weight after the menopause - when oestrogen levels drop - but not so much as rats do. "However we have found that HRT can reduce post- menopausal weight gain," says Geary.

So how does this work? Why does an increase in oestrogen make you want to eat less? One possibility is that it somehow affects the "I'm full" messages that get sent from the stomach and small intestine to the brain during a meal. Pharmacological companies have been experimenting with synthetic versions of these chemical messengers for years in an attempt to create the dieter's magic bullet.

Several have their hopes pinned on a peptide known as CCK. Stop it from working, and rats eat more. "Several studies suggest that oestrogen may make CCK more effective," says Geary. "Interestingly, bulimic patients have a problem with producing CCK."

Oestrogen also has an effect in the brain. We know that a tiny region, called the hypothalamus, is involved in controlling appetites, such as hunger and sex. If you inject oestrogen into the PVN region of the hypothalamus in rats who have had their ovaries cut out, they eat less.

A brain chemical called neuropeptide Y, which also affects the hypothalamus, is well known to make both rats and humans very hungry and there's some evidence that oestrogen blocks it.

There is probably also a connection with the protein leptin, which is another candidate for the dieters magic bullet. A few years ago researchers found that rats without the gene that makes leptin got incredibly fat, so the hope was, control leptin and you control weight. It turned out not to be so simple, but there is an oestrogen connection - pre-menopausal women do have the most leptin and men have the least.

Finally there may be a flavour connection. Both humans and rats tend to eat more of sweet things and oestrogen makes rats less keen on sucrose. If it turns out that oestrogen does make women more vulnerable to eating disorders, this should take away some of the guilt involved in weight problems and it may hasten the arrival of the magic bullet. For those who can't wait that long, you might be interested to know that stress raises your oestrogen levels, but - there's always a but - too much oestrogen makes your memory worse.