Some human breast milk sold online has been contaminated with cow's milk, which could pose a problem to infants with milk allergy or intolerance, researchers say.

Health Canada and the U.S. Food and Drug Administration recommend against feeding infants human milk from unscreened sources because of infection and other risks. Despite the warnings, online sales, postings and advertisements for breast milk have grown in popularity.

Members of the Human Milk Banking Association of North America, which includes Canadian milk banks, don't compensate donors on the grounds that selling could increase the risk. The official banks screen and pasteurize donated milk to reduce the risk of infectious disease, but have struggled to keep up with demand from hospitals for the most premature and sickest babies.

The U.S. researchers bought 102 samples of human milk breast milk anonymously online and DNA tested for contamination with cow's milk.

"Eleven of 102 (11 per cent) purchased internet samples contained both human and bovine DNA. Ten had bovine DNA concentrations high enough to rule out minor, incidental contamination, suggesting some sellers unintentionally or intentionally added to human milk a significant amount of a cow's milk product," Sarah Keim of Nationwide Children's Hospital in Columbus, Ohio, and her co-authors concluded in Monday's issue of Pediatrics.

"Cow's milk can be problematic if ingested by an infant with cow's milk protein allergy or intolerance."

Since buyers or users aren't readily able to verify the composition of milk, "all should be aware of the possibility that it may be adulterated," they advised.

Previously, Keim's team reported that most of the breast milk they bought online grew pathogenic bacteria or had bacterial counts that made it unsuitable for infants to drink.