Breasts do not differ only in shape and size. It is much more complicated than that. The tissue of the female breast comprises a mixture of fatty and glandular tissues and also connective tissues, like collagen (support fibrous proteins), ligaments and blood vessels, but the proportion of every breast tissue is different from woman to woman. Women with much more glandular tissue have denser tissue on a mammogram and are about four times more likely to develop breast cancer than those with fatty breasts.

An Israeli research presented at the annual meeting of the Radiological Society of North America (RSNA) 2007 investigated the way Western lifestyle affects breast structure. A team led by Dr. Miriam Sklair-Levy from Hadassah Medical Center in Jerusalem made comparisons between the breast composition of Native or European-rooted Israeli women and Ethiopian Jewish women who came to Israel more recently.

The research discovered that the Ethiopian-born Jewish women who have immigrated to Israel presented much lower breast density than the women born in Israel. Moreover, older Ethiopian Jewish immigrants who had started to adopt the Western lifestyle (low number of children, western diet or use of hormonal pills) presented much higher breast densities than those who came to Israel more recently.

This research has the same results with those of another recent British study showing that women living in urban areas have unhealthy denser breasts than those inhabiting rural areas. Digital mammograms of the 972 British women, aged 45 to 54, revealed that 69 % of the rural women had dense breasts, while in suburban women the number went to 74 % and in urban-dwelling subjects to 78 %.

"I work all my time in the city of London and I read mammograms of very dense breasts. But every time I have gone outside to read mammograms elsewhere in more rural areas or parts of Europe that are rural, I am very impressed with the less dense breast tissue that is there," lead researcher Dr. Nicholas M. Perry, director of The London Breast Institute at The Princess Grace Hospital in London, U.K. told CTV.

The results of both studies have been related to lifestyle, like obesity, exercising and stress levels, but also to hormonal contraceptive pills. Also women in rural areas or developing countries have children earlier and in greater number. Some say this is due to the air pollution, specifically "polycyclic aromatic hydrocarbons", which imitate female sex hormones, unbalancing hormones in the body.