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Does Being Breastfed Affect Later Cancer Risk?

Study Shows Little Evidence of a Link
WebMD Health News
Reviewed by Louise Chang, MD

Oct. 4, 2005 - Being breastfed as an infant has little impact on cancer risk as an adult, according to a newly published study and a review of earlier research.

Researchers from the England's University of Bristol found no evidence that being breastfed increases cancer risk. A slight protective benefit against breast cancer prior to menopause was found for women who were breastfed as infants. But Richard M, Martin, PhD, who led the research team, tells WebMD that the finding is far from conclusive.

"These findings show that being breastfed doesn't increase cancer risk as an adult," he says. "There are still many unanswered questions, including whether breastfeeding is protective against heart disease and its role in brain development."

Early Studies

Animal studies from the 1930s and 1940s first led researchers to suggest that viruses transmitted in breast milk might cause cancer later in life. Although little evidence emerged to back the claim, as late as the 1970s new moms with a family history of breast cancer were often warned not to breastfeed their daughters.

High levels of a hormone associated with growth have also been linked to breast, prostate, and colorectal cancers. Breastfeeding is believed to increase circulating levels of the hormone, known as insulin-like growth factor-I (IGF-1) levels.

"Because breastfeeding is positively associated with height and IGF-1 and because both are, in turn, positively associated with breast, prostate, and colorectal cancers, we hypothesized that breastfeeding may be associated with an increase in the risk of these cancers," Martin and colleagues wrote.

Since breastfeeding is known to help prevent specific gastrointestinal infections, the researchers also suggested that it might help protect against GI cancers caused by these infections.

In an effort to clarify the role of early-life exposure to breast milk in later-life cancer risk, Martin and colleagues analyzed almost 65 years of data on about 4,000 people in Britain followed from the late 1930s. The study subjects were all younger than 20 at enrollment and were in their 60s, 70s, and 80s at follow-up.

The researchers also included 10 other studies that examined infant feeding and later cancer risk published between 1966 and 2005. The findings were published in the Oct. 5 issue of the Journal of the National Cancer Institute.

Clear Benefits of Breastfeeding

Neither analysis showed a conclusive link between breastfeeding history and cancer risk. But Martin says better studies are needed to answer this and other questions about breastfeeding once and for all.

The problem with the studies that have been done, says breastfeeding expert Ruth Lawrence, MD, is that they provide little information about how long babies were breastfed, whether they were breastfed exclusively, and other lifestyle factors that could influence disease risk.

Lawrence is a pediatrics professor at the University of Rochester School of Medicine in New York.

She says it is clear that breastfed babies have fewer middle ear infections and other infections, compared with babies that are bottle fed. Diarrhea, which is a major cause of death in developing countries, is also less of a problem for babies who are breastfed.

There is also a strong suggestion that breastfeeding speeds brain and visual development. The studies evaluating the protective effect of breastfeeding on allergy and asthma risk have been mixed, but Lawrence says it is clear that it can delay the onset of allergies in high-risk infants.

"We know that breastfeeding gives babies the best possible start in life," she says.

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