By now, we all know how healthy breastfeeding is, but when exactly are mothers supposed to start their children on solid foods? A study that has not been published in a peer-reviewed journal yet but was presented at the American Association for Cancer Research’s Annual Meeting 2015 has found that infants who started eating solid food later than others have an increased risk for acute lymphocytic leukemia (ALL), the most common pediatric cancer in the United States.

"I think that infant feeding exposures are important because they're universal — all infants are going to be fed," Jeremy Schraw, lead researcher from the University of Texas, told LiveScience. "And they're modifiable, so we can change the way that infants are fed, according to our knowledge and best practices."

Schraw and his colleagues recruited 172 children from Texas diagnosed with ALL and 344 healthy children. Children included in the study were either fed via breast milk, infant formula, or a combination of the two prior to eating solid foods. Starting on solid foods around the age of 10 months or older was linked to a four times higher risk for being diagnosed with ALL compared to starting on solids food at around 8 months.

Although starting solid foods between the ages of 5 and 6 months did not have any effect on a child’s risk for ALL, children who started on solid foods between the ages of 7 and 9 months were three times more likely to develop ALL compared to children who started solid foods at around 4 months. Researchers from the study speculate that introducing infants to solid food past 6 months of age could have an adverse effect on their immune system.

Surprisingly, results showed that this link between starting solid foods later than most infants and risk for ALL was non-existent when the child had older siblings. A similar study conducted by the same research team determined that every one-month delay in the start of solid foods was associated with a 14 percent higher risk for ALL.

"Delaying introduction of solids might delay immune challenges to a child, and interfere with the normal development of the immune system, thereby increasing their risk" Schraw added. "These children with an older sibling are more likely to get their immune exposures elsewhere, and it seems like solid foods are less important."

According to the American Academy of Pediatrics, parents should start introducing their children to solid foods at around 6 months of age. After approximately 9 months, infants can start receiving two to three servings of fruits and vegetables per day.

Source: Schraw J, et al. Age at introduction to solids is associated with the odds ratio of pediatric acute lymphoblastic leukemia. American Association for Cancer Research Annual Meeting. 2015.