Babies who are breastfed have a lower risk of developing childhood leukemia, according to a new analysis of 14 studies by researchers at the University of California, Berkeley.
The paper, to be published November in the journal Public Health Reports, found that breastfeeding was linked to lower risks of both acute lymphoblastic leukemia (ALL), the most common of the childhood cancers, and acute myeloblastic leukemia (AML).
"Our paper is the first to systematically review the epidemiologic evidence of the link between maternal breastfeeding and the risk of childhood leukemia," said Marilyn Kwan, UC Berkeley post-doctoral researcher in epidemiology at the School of Public Health and lead author of the study. "We conducted this meta-analysis because the studies that had been conducted previously have been inconclusive and contradictory. Our review of the scientific literature shows that the evidence is definitely pointing towards the benefits of breastfeeding when it comes to the risk for two kinds of childhood leukemia, ALL and AML."
The 14 case-control studies, taken from around the world, were published between 1988 and 2003. They included 6,835 cases of ALL and 1,216 cases of AML.
While the causes of childhood leukemia are not completely understood, it is believed that the disease begins with a genetic change that occurs while the fetus is in the womb. This theory is supported by researchers, led by Mel Greaves of London's Institute of Cancer Research, who studied blood samples taken at birth and found the presence of an abnormal fusion of two genes, TEL and AML1. The cause of the gene fusion is not certain, said Kwan, but it has been shown to interfere with the normal formation and development of blood cells in animals and is found in 25 percent of children with leukemia.
The genetic abnormality does not guarantee that a child will go on to develop leukemia, said the researchers. Studies indicate that only 1 in 100 children with the gene fusion at birth go on to develop the disease.
"The gene fusion in and of itself doesn't cause leukemia," said Kwan. "There needs to be a second promoting step, a rare response in the child to early infections that can cause a secondary genetic change. That's where breastfeeding may come into play. It could be preventing that second event from occurring because the mother is passing along her antibodies to the child through her breast milk and strengthening the baby's immune system."
According to the National Cancer Institute, leukemia is the leading cause of cancer deaths in the United States among children younger than 15. From 1975 through 1995, ALL accounted for 78 percent of U.S. childhood leukemia cases, while AML accounted for 16 percent of cases. The bone marrow of both ALL and AML patients produces too many immature cells that fail to develop into mature white blood cells.
One of the 14 papers reviewed at UC Berkeley included a small case-control study authored by Kwan as part of the Northern California Childhood Leukemia Study. That study was funded by the National Institute of Environmental Health Sciences and led by Patricia Buffler, a UC Berkeley professor of epidemiology who also is co-author of the analysis.
Encouragingly, the analysis indicates that even short-term breastfeeding, for less than six months, was linked to a lower risk of ALL.
"Our data suggest that breastfeeding for even a short period of time is protective," said Buffler.
"That's actually not surprising. We know that much of the protection provided by maternal antibodies comes in the first couple of months of breastfeeding, so even breastfeeding for three months is beneficial."
The researchers found no significant association between short-term breastfeeding and AML.
While the classification of ALL in the analysis is straightforward, the researchers pointed out that the classification of AML is more variable. Four of the 14 studies specified "other leukemia" or "acute non-lymphoblastic leukemia," which the researchers categorized as AML since it represents the majority of non-ALL cases. However, the researchers acknowledged that the classification method somewhat limits the conclusions they can draw about the impact of breastfeeding on AML.
In addition, the researchers noted the inherent limitations of the case-control studies in the meta-analysis. The people in control groups in such studies tend to have a higher socio-economic status than those in case groups, the researchers said. They say that people of higher socio-economic status tend to be more educated about health issues and more willing to participate in epidemiological studies. They add that women with a higher socio-economic status also tend to report higher rates of breastfeeding.
The researchers said that larger cohort studies are needed to determine whether breastfeeding truly has a protective effect on childhood leukemia risk.
One such cohort study may be the U.S. National Children's Study, which examines the environmental influences on the health and development of more than 100,000 children across the United States. Participants in the study will be followed from birth until age 21. Preliminary results of the study are expected as early as 2008.
Nevertheless, authors of the meta-analysis said there is now enough evidence on the reduced risk of childhood leukemia to recommend breastfeeding. They also cited the other health benefits breastfeeding imparts to the child.
"The overall risk of a child developing leukemia is relatively small, so based upon this analysis, people shouldn't be made to feel guilty if they can't breastfeed," said Dr. Vincent Kiley, a pediatric oncologist with Kaiser Permanente and co-author of the paper. "But if you're on the fence about it, this study provides one more reason to encourage women to breastfeed, even if it is for just a couple of months."
Barbara Abrams, UC Berkeley professor of epidemiology, is another study co-author.
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