New mothers can significantly reduce their babies' risk of developing childhood leukemia by breast-feeding them, according to a new study.
Children and teens who were breast-fed for more than six months were 19% less likely to be diagnosed with leukemia compared with kids who nursed for six months or less, researchers reported Monday in the journal JAMA Pediatrics.
Cancer is the No. 2 cause of death among children under the age of 15, and leukemia is the most common type of childhood cancer. Acute myeloid leukemia (which causes a patient's bone marrow to make too many abnormal blood cells) and acute lymphocytic leukemia (which causes the bone marrow to churn out too many immature white blood cells) account for about 30% of childhood cancers.
The incidence of leukemia has been rising steadily for decades. In the United States, it has grown by an average of 0.7% per year between 1975 and 2011; in Europe, the incidence has risen by 0.6% per year between 1978 and 1997.
Scientists don't know what accounts for this, but one possibility is the so-called Greaves hypothesis. This idea posits that a prenatal genetic mutation makes children susceptible to leukemia, and then exposure to an "infective agent" after birth causes the cancer to start growing.
If this hypothesis is correct, breast milk could be a natural antidote to leukemia. Unlike formula, breast milk contains antibodies and other "immunologically active components" that help fight infections. They also seed the baby's gut microbiome, which boosts the immune system.
So the researchers, from the University of Haifa and the Israel Center for Disease Control, decided to test the Greaves hypothesis by examining the relationship between breast-feeding and leukemia.
They took a fresh look at data from 18 previously published reports that examined the relationship between breast-feeding and leukemia, the most common type of childhood cancer. Those studies included information about 10,292 children and teens who were diagnosed with leukemia and 17,517 matched controls who were not.
Overall, they found that kids who nursed for at least six months were 19% less likely to develop leukemia than kids who nursed for shorter periods of time or didn't nurse at all, according to the study.
The researchers went on to slice the data in various ways. Each time they did, they found an inverse relationship between breast-feeding and leukemia.
For instance, when they focused on studies conducted in developed countries, they found that children who nursed for more than six months were 16% less likely to get leukemia than kids who didn't. They got the same results when they limited their analysis to data from the four largest studies in their sample. And when they considered only the nine studies that were judged to have the highest-quality data, they found that more than six months of breast-feeding was associated with a 14% lower risk of leukemia.
They also ran the numbers excluding children who were diagnosed before the age of 1, since those malignancies were probably triggered by a prenatal event that had nothing to do with breast-feeding. With those cases out of the picture, the researchers found that children who nursed for more than six months were 17% less likely to get leukemia than kids who didn't.
Some of the studies differentiated between children who had acute lymphocytic leukemia (ALL) and those who had acute myeloid leukemia (AML). That allowed the researchers to calculate that children who nursed for more than six months were 18% less likely to get ALL. However, they found no link between breast-feeding duration and AML.
Fifteen of the 18 studies also kept track of whether children had been breast-fed for as little as one month. When the researchers made this the dividing line, they found that kids who nursed for at least a month were 11% less likely to develop leukemia compared with kids who didn't nurse at all or who nursed for only a few weeks.
The study authors concluded that "14% to 19% of all childhood leukemia cases may be prevented by breastfeeding for 6 months or more."
As cancer prevention efforts go, they added, breast-feeding is "highly accessible and low-cost." New mothers -- and the rest of the public -- should be made aware of this "so breastfeeding can be more socially accepted and facilitated."