Human breast milk sold online is frequently adulterated intentionally, says a study led by Nationwide Children's Hospital, Ohio.
It found that 10% of the samples contained cow's milk.
This topping off with cow's milk or infant formula presents a danger for the large number of babies receiving the milk, as they can be vulnerable to diseases from bacterial and viral contamination of the milk, warns the study.
Cow's milk and infant formula which resemble human milk, can be added to the breast milk to boost volumes, without the recipient knowing.
"We found that one in every 10 samples of breast milk purchased over the Internet had significant amounts of cow's milk added, and this poses a risk to infants with an allergy or intolerance to cow's milk," said Dr Sarah A Keim of the Center for Biobehavioral Health, the lead author on the study. "If a baby with cow's milk allergy were to drink this milk, it could be very harmful."
10% of breast milk contaminated
Keim's team at the Research Institute at Nationwide Children's Hospital in Ohio collaborated with researchers from Ohio State University and Cincinnati Children's Hospital to test 102 samples of breast milk bought from milk-sharing websites.
Eleven of the samples contained bovine DNA, 10 of which had results pointing to more than an accidental contamination with cow's milk.
The team found that 21% of individuals seeking human milk online did so for a child with a pre-existing medical condition. Of this, 16% specifically sought human milk due to their baby's formula intolerance.
In a recent survey Keim conducted, out of 499 women, 25% of women considered milk sharing and almost 4% of women shared milk among friends or relatives who had a preterm infant.
She suggested that women with extra milk could help a baby in need by donating their milk to a non-profit milk bank, instead of selling it over the Internet.
Breast milk health benefits
The same team had made a discovery back in 2010 of bacterial or viral contamination in more than 75% of milk samples purchased online.
Feeding infants with breast milk brings many health benefits to a baby, include boosting their immune systems. Many recent programmes have been aimed at encouraging breastfeeding by new mothers.
A protein in breast milk was recently shown to have an antimicrobial action against HIV, which protected the child if the mother was infected.
Studies have also revealed how human breast milk protects babies against a fatal bowel condition called necrotising enterocolitis. A protein in human milk was seen to destroy cancer cells in a recent study.