Since the invention of the Internet, women have been buying and selling breast milk online. And for almost as long, experts have warned about the potential dangers of purchasing milk that may or may not be tainted with anything from toxins to bacteria to illegal drugs. Now those experts have a new warning: the breast milk may not be all human milk.

A new study released today in the journal Pediatrics found that 10 out of 102 breast milk samples purchased online were diluted with at least 10 percent cow's milk. It's not clear whether or not the milk came from baby formula or straight up cow's milk, but either way the situation is unnerving. Mothers who choose to purchase breast milk online are usually doing so because their babies cannot tolerate formula or because they have a specific ailment that could be aided by breast milk.

Breast milk sales have become so popular precisely because health experts "have done a really good job over the past 20 years of promoting the benefits of breastfeeding," said Sarah Keim, a researcher at Nationwide Children's Hospital in Columbus, Ohio, the agency that conducted the tests. "Unfortunately, some women are very focused on feeding their babies breast milk at all costs."

But breast milk that has been diluted with cow's milk could be unsafe for babies, particularly those with a compromised immune system. The American Academy of Pediatrics advises parents to avoid giving their babies cow's milk until they are at least 12 months of age.

"It could be very harmful to babies with allergies or intolerance to cow's milk," said Keim.

For the study, researchers used samples purchased from several websites, including one called Only The Breast that promotes breast milk sales as well as sites such as Eats on Feets and Human Milk 4 Human Babies, that facilitate the sharing of milk but discourage sales.

Keim and her team found that 10 percent of the breast-milk samples tested contained bovine DNA, meaning that the milk had been diluted with cow's milk products. The assumption is that sellers purposefully used cow's milk to increase their production and sales. 

In the vast majority of cases, women who are selling breast milk online are doing so because they genuinely care about babies and want to help those who for whatever reason cannot receive an adequate supply of breast milk from their mothers. (And the supplemental income at a time when budgets are tight is certainly an added bonus.) But experts say this latest study highlights the risks of purchasing breast milk online. A buyer can never know the motivations or ethics of a seller, and without the stringent milk screening processes used in hospital milk banks, it's simply too hazardous to leave to leave to chance.

In previous studies, Keim found that 75 percent of the breast milk samples analyzed were tainted with bacteria such as E. coli, Streptococci or even salmonella. The amounts detected in some of the samples were sufficient to make a baby — particularly a baby with a weakened immune system — very sick. 

Keim and her team hope that this latest study will convince moms to think twice before purchasing breast milk online. If breast milk production is the issue, it might be better to seek out advice from a lactation consultation than to look for supplemental sources. Sure, breast milk is best — but only if you know where it's coming from. 

Related on MNN:

Breast milk bought online may not be 100% human
New study reveals that as many as 10 percent of samples were diluted with cow's milk.