Pregnancy may lower a woman's risk of cancer but drinking milk could raise it, researchers reported this week. Both factors, as well as the use of hormone replacement therapy, affect levels of a hormone that may influence the development of some cancers, a team at Brigham and Women's Hospital and Harvard Medical School in Boston found.
The finding could explain why women who have had children have a lower risk of cancer - something doctors have noticed but been unable to explain, Dr Michelle Holmes, who led the study, said.
Pregnancy, HRT use and milk drinking all affect levels of insulin-like growth factor 1 or IGF-1, a hormone linked to an increased risk of cancer, Holmes and colleagues report in the journal Cancer Epidemiology Biomarkers & Prevention, which is published by the American Association for Cancer Research.
"This is the first study to report that the more pregnancies a women had, the lower was her blood level of IGF-1," Holmes said.
"Pregnancy is known to protect against several cancers such as breast and colon cancer. It is possible that the mechanism of this protection could be through lowering IGF-1 levels."
Women who had four or more pregnancies had IGF-1 levels that were on average 15 per cent lower than in women who had never been pregnant, the researchers found.
Using data from a large, long-term study of more than 1,000 nurses who record their diets carefully and who are then watched for changes in health, Holmes' team also found that those who drank the most milk had higher levels of IGF-1.
IGF-1 is important to the growth and function of many organs, but higher levels have been associated with an increased risk of prostate, colon, lung and breast cancer.
"We concluded that greater milk consumption was associated with higher levels of IGF-1," said Holmes. "This association raises the possibility that diet could increase cancer risk by increasing levels of IGF-1 in the blood stream. However, more research must be done to determine whether milk consumption itself is directly linked to cancer risk."
The role of HRT and cancer is less clear - it can raise the risk of breast cancer, for instance. But Holmes's team found that women who were taking oestrogen after menopause had the lowest levels of IGF-1, followed by women using oral oestrogen plus progesterone.
This may explain why HRT reduces the risk of colon cancer, Holmes continued. He stressed that now more research needs to be done to find out why this does not counteract the effect of HRT on breast cancer.