In 1700, the Italian physician Bernadino Ramazinni published the first book about occupational epidemiology. His method was simple: based on trends observed in his clinical practice, he studied his patients’ worksites and found that certain diseases were correlated with specific exposures, including heavy metals and dust with cancer in miners and industrial workers, and bad posture with pain in weavers. He even looked into how the dangers of an “overtaxed mind” could cause disease in learned men.
However, his most enduring legacy is the observation that nuns had more breast cancer and less cervical cancer when compared to the general population of females living in Padua (his hometown). It was a puzzling observation, and one that he did not expect (at the time, one theory about breast cancer was that it resulted from “vigorous sexual concourse”). His explanation followed a similar line of reasoning – the breast is a sexual organ, and without regular sexual activity it would decay. Cancer was the result. Interestingly, he did not apply this same logic to the low incidence of cervical cancer in nuns, but he did propose that it was somehow related to the celibate lifestyle.
As it turns out, Ramazzini was right – even if for the wrong reasons. Breast cancer is related to celibacy, but not in the way that you might think. When women become pregnant, their bodies undergo dramatic changes in hormone output, and levels of estrogen decrease markedly. Estrogen has been implicated in the development of breast cancer, and it’s thought that the decline in levels brought on by pregnancy has a protective effect. Indeed, women who first become pregnant before the age of 30 have a lower risk of breast cancer than women who first become pregnant after the age of 35.
With cervical cancer, the link is even more dramatic – almost all cases of cervical cancer (>99%) are caused by human papillomavirus (HPV), a sexually transmitted disease. Since the nuns were celibate, they never acquired the virus and therefore never developed cervical cancer. Taken together, these two observations were the first inkling that modern science into lifestyle factors as a possible cause of cancer. We still use this thinking today – smoking is a lifestyle factor, and its role in the development of lung cancer is widely accepted as fact. So although Ramazzini was right for the wrong reasons, his investigation of breast cancer in nuns is one of the great early epidemiologic studies and demonstrates how epidemiology can help us to understand ways to prevent disease and puzzle out risk factors even without a firm grasp of the biologic cause.