The study on the safety of human breast milk bought over the Internet found that 10 per cent of samples contained added cow's milk.
The discovery that purchased samples of human milk may be purposely "topped off" with cow's milk or infant formula confirms a danger for the large number of babies receiving the purchased milk due to medical conditions, researchers said.
These babies are also vulnerable to the risk of infectious disease from bacterial and viral contamination of such milk, which was identified in a prior study by the same research team led by Nationwide Children's Hospital in US.
"If a baby with cow's milk allergy were to drink this milk, it could be very harmful," said Keim, lead author on the study.
The study is the first to document that milk purchased online is frequently adulterated with intentionally added ingredients.
"Cow's milk and infant formula resemble human milk and could potentially be added to boost volumes without the recipient knowing.
Keim's team collaborated with researchers from The Ohio State University and Cincinnati Children's Hospital Medical Centre to purchase and test 102 samples of breast milk advertised on milk-sharing websites.
The team compared the purchased samples with their own preparations of human milk diluted with cow's milk to approximate the amount of contamination required in order to test positive for bovine DNA.
All purchased samples did contain human milk, but 11 also contained bovine DNA, 10 of which had results consistent with more than minor, accidental contamination with cow's milk.
The study was published in the journal Pediatrics.
