There’s a growing market online for nursing mothers who want to sell their breast milk-and weightlifters looking to build muscle are among the buyers. Just this week, one mom who gave birth in August made the claim that she’s pocketed $6,000 by selling her milk to bodybuilders.

The 24-year-old mom, Rafaela Lamprou, overproduces milk, according to the New York Post, so she sells the excess. It’s not clear why her body makes more milk than her infant son needs, but an oversupply can lead to uncomfortable side effects like breast engorgement and plugged ducts.

Lamprou said that she was initially giving away her excess milk to moms who had trouble breastfeeding. But then men started asking if they could buy it. “It started with men who were interested in bodybuilding…They say it is good for building muscle mass,” she said in an interview with a news wire service. She then set up a Facebook page to handle orders.

So how impressive nutritionally is breast milk, anyway? Well, it’s clearly impressive for a baby, as the American Academy of Pediatrics recommends exclusive breastfeeding for the first six months of an infant's life, and then breastfeeding while introducing other foods into the baby's diet until the child is one year old. But that hardly means breast milk is equally as good for an adult.

Breast milk is approximately 87% water, 7% lactose, 3.8% fat, and 1% protein. While breast milk nutrition changes depending on the baby’s age (and a mom's diet), the USDA says that one cup generally has 172 calories, 2.5 grams of protein, 10 grams of fat, and 16 grams of carbs.

“When you actually look at the nutrition facts, that’s not a lot of protein for a grown man,” says New York City fitness expert Chris Ryan, CSCS, CPT. Compare it to a cup of cow or soy milk, both of which have about 8 grams of protein per cup.

While breast milk supplies all the nutrition a newborn baby requires, an adult-especially a bodybuilder with high protein needs-should look elsewhere. “Technically, there hasn’t been research on grown-ups drinking breast milk, but Mother Nature has already done the study," says Ryan. "Children are eventually weaned off breast milk. You don’t see adult animals in the wild going back to their moms to suckle."

More importantly, “there are so many better options for an active adult,” he says. If someone is looking a good protein source, they should buy a basic grass-fed whey protein, which will have about 20 grams of protein per scoop. (A far better value, really.)

Proponents note that breast milk contains growth factors that can be helpful in muscle building. But Ryan says you don't need these to see results. Instead of pounding back breast milk, an adult should focus on things that will really make a difference in training, like adding extra weight to workouts, he advises.

Then there’s the fact that breast milk is expensive, going for around $1/ounce online. Other concerns include the cleanliness of the pumping parts, which have to be regularly sanitized. Buying breast milk from a stranger (especially off of Craigslist, which some people actually do), requires confidence that they cleaned the pump correctly.

Another risk: You won't necessarily know if the mom you're buying it from has been screened for infectious diseases-or if the breast milk itself is contaminated. It's a serious concern, since infections like HIV can pass through breast milk, reports the Centers for Disease Control and Prevention. A mother's diet also influences the nutritional profile of her breast milk, so buyers have to trust that the seller is eating well, adds Ryan.

Bottom line: If you’re an adult looking to bulk up, pick up the weights and feed your body a well-rounded diet. Leave the breast milk for the babies who need it.