People who order human breast milk online for their babies may end up receiving shipments contaminated with cow's milk, according to a new study.
Researchers purchased and tested 102 samples of human milk from the Internet.
There was enough evidence to suggest 10 samples were contaminated with cow's milk, which may affect kids with allergies or intolerance, researchers write in the journal Pediatrics.
"When you combine these results with our 2013 study that showed a high level of bacterial contamination in milk bought online, we conclude that milk purchased on the Internet is unsafe," said Sarah Keim, the study's lead author from The Research Institute at Nationwide Children's Hospital in Columbus, Ohio.
"You really just don't know what you're getting," Keim wrote in an email to Reuters Health.
In their previous study, the researchers found that about 75 per cent of the milk samples were contaminated with disease-causing bacteria like E. coli or harmful levels of bacteria such as Streptococci.
They say the US Food and Drug Administration recommends against feeding infants human milk from unscreened donors, but the practice of donating or selling human milk on the Internet is growing in popularity.
In their recent study, the researchers tested the DNA of 102 samples sold in 2012 as human milk, for $1 to $2 per ounce.
All of the samples contained human DNA, but 12 also tested positive for cow DNA. And 10 of those 12 contained what the researchers called a "significant amount" of cow milk products - enough to rule out minor or incidental contamination.
Human milk contaminated with cow milk may harm babies who are allergic to cow milk proteins, the researcher write. In past research, they found about a fifth of people buying human milk online were doing so for a child with a medical condition - some with formula intolerances.
Keim's group would like to see research into ways to increase breastfeeding support and improve access to nonprofit milk banks so women don't have to buy milk on the Internet.
Mothers who have difficulty making enough milk should work with their pediatrician to identify safe, healthy ways to feed their babies, she said.