A domestic service agency in Shenzhen, Guangdong province, is providing breast milk to adults, but police said it is hard to get evidence to define the service as prostitution, Southern Metropolis Daily reported on Tuesday.

Lin Jun, boss of the agency, said it offers wet nurses as well as domestic helpers. Mothers who are not able or don't want to breastfeed their babies can resort to this service. Adults who want breast milk to improve their health can also hire a wet nurse.

A wet nurse is paid 8,000 to 12,000 yuan ($1304 to $1,957) a month, and usually stays with the employer for six to eight months, Lin said.

Shenzhen police said it's difficult to prove that such wet nurses are involved in prostitution.

Zhang Maoxiang, a nutritionist, said that adults get enough nutrition in their daily diet, and there is no need to consume breast milk.

Mei Chunlai, a lawyer, said that though the national health authorities have deemed it illegal to trade breast milk, the stipulation is not authoritative enough thus is hardly enforceable.