Breastfeeding and having large numbers of children are the key to the developing world's low rates of breast cancer compared with Western countries such as Britain, according to new research published in The Lancet.

The study found that the longer women breastfeed, the more they are protected from the disease. This effect is over and above the protection gained from having children.

The researchers from Cancer Research UK believe their work is an important step forward in understanding how and why breast cancer develops. If scientists can understand the mechanism, it will help focus research toward developing drugs to replicate the protective effect.

They pooled data from 47 separate studies from 30 different countries, involving 50,000 women with breast cancer and nearly 100,000 women who did not have the disease.

They found that for every year that a woman breastfeeds, her risk of breast cancer goes down by 4.3 per cent, on top of the 7 per cent reduction for each child she has. If women breastfed each of their children for an extra six months, it could prevent over 1,000 cases of the disease in Britain each year.

Over the time period that women in the study would have been having children, women from the developing world breastfed for an average of about two years per child and averaged around six or seven children. By comparison, the average Western woman breastfed for just two or three months per child and had about two or three children.

Lead researcher Professor Valerie Beral, of the Cancer Research UK Epidemiology Unit in Oxford, says: "The results of this study are a major step forward in our understanding of why breast cancer incidence is so high in developed countries. It's long been known that breast cancer is common in situations where women have few children and breastfeed for short periods. We've shown that these factors alone account for much of the high rates of breast cancer in these settings."

If British reproductive habits mirrored those of developing countries, it is estimated that women's risk of breast cancer by the age of 70 would fall from 6.3 per 100 women to 2.7 per 100 women. Two thirds of the decrease would be a result of longer breastfeeding and one third because of the increase in number of children.

Fellow researcher Dr Gillian Reeves, also of the Cancer Research UK Epidemiology Unit, says: "To expect that substantial reductions in breast cancer incidence could be brought about by women returning to the pattern of childbearing and breastfeeding that typified most societies until a century or so ago is unrealistic. But even if women in the West were to breastfeed each of their children for an extra six months, this could prevent five per cent of breast cancers each year."

SOURCES:
The Lancet, July 20, 2002; 360(9328):187-195
Cancer Research UK (http://www.cancerresearchuk.org)