For some time now, most mothers have been advised by their physicians, as well as the American Academy of Pediatrics, that “breast is best” when it comes to feeding an infant during the first several months of life. This mandate has sometimes stirred its own fair share of debate. A new study will now add fuel to the the controversy, because it’s poked some holes in the theory that breastfeeding is beneficial.

According to TIME, the study was recently published in the journal Social Science & Medicine. The research encompassed three different sets of data populations: children (8,237), siblings (7,319), and sibling pairs (1,773) where one child was breastfed and the other not.

The study factored eleven patient outcomes previously shown to be linked to breastfeeding, including: BMI, obesity, behavioral compliance, hyperactivity, math skills, and other factors. The study concluded that when the sibling pairs were compared, there were no significant differences in the outcomes based on whether the child had been breastfed or not — with the exception of asthma.

According to researchers, looking within families rules out some of the issues previously linked to breastfeeding, including levels of parental education, socioeconomic levels, and race/ethnicity. Cynthia Colen, a lead author of the study and assistant sociology professor at Ohio State University, suggested in a statement that further study on factors such as childcare, maternity leave, housing, and employment should be undertaken for a more accurate picture as to how breastfeeding affects children later in life.

While many other studies indicate that breastfeeding is still beneficial for babies, and still recommended by most doctors, it does add fuel to the fire that breast may not always be best in every instance.

Source: TIME