Lifelong vegetarianism is linked to a reduced risk of breast cancer, and this is most likely because of the large number of vegetables eaten, according to a new study.

Whether or not abstaining from eating meat has a role in this process, however, is unclear.

Researchers from the London School of Hygiene and Tropical Medicine studied women who had migrated to England from India, Pakistan, and Bangladesh. They reported their findings in the International Journal of Cancer (Vol. 99: 238-244).

Certain diets, such as those high in red meats and certain fats, seem to play a role in some cancers, such as those of the colon. But their influence on breast cancer remains unclear. Some studies, for example, have found that high fat diets appear to increase breast cancer risk, but others have not.

Regardless of its effect on breast cancer, a diet that is rich in vegetables and light on red meats and fatty foods is likely to have other positive health benefits, including lowering the risk of heart disease.

Vegetarianism is the practice of eating a diet consisting mainly or entirely of food that comes from plant sources such as fruits and vegetables. Vegetarian diets vary widely. Some include no animal products, while others include dairy products, eggs, and fish.

One of the problems in figuring out the effects of vegetarian diets on cancer risk is that most vegetarians in Western countries stop eating meat only in adult life, when the risk of cancer may have already been influenced.

In this study, researchers looked at women who were Hindus and were likely to be lifelong vegetarians, and they looked at Muslims who were mostly meat-eaters (except for pork).

The researchers studied 240 women who had developed breast cancer, and 477 women of similar age who had not. Each answered questions related to the foods eaten in previous years.

Women with breast cancer were more likely to be in a higher social bracket, were older when their first child was born, were less likely to have breastfed, and were more likely to have a strong family history of breast cancer — all known breast cancer risk factors.

Lifelong vegetarians had a slight reduction in the odds of developing breast cancer, even after adjusting for these other factors.

But vegetarians not only ate less meat; they also ate more vegetables than the meat-eaters. The researchers then attempted to determine which of these two factors was more important.

When other dietary factors were held constant, the difference between those who ate meat and those who did not seemed to disappear. But high intakes of vegetables continued to affect breast cancer risk even after adjusting for other factors.

The authors concluded, "Although it is not possible to exclude the possibility that meat abstention may also play a role, the findings provide evidence that a lifelong diet rich in vegetables, such as those typically found in South Asian diets, may be protective against this cancer."

Further studies will hopefully define the specific nutrients responsible for this effect, they added.