Clenbuterol, known in China simply as ``lean meat powder,'' is a dangerous drug that's banned in China yet stubbornly continues to pop up in the food supply, laced into animal feed by farmers impatient to get their meat to market and turn a profit.

The drug accelerates fat burning and muscle growth, making it an attractive feed additive, sports performance enhancer and slimming drug, but overdoses can cause illness and, in rare cases, death.

Tour de France champion Alberto Contador is among the athletes who have tested positive for the drug, though he disputes the results, claiming he unknowingly ingested the drug by eating tainted steak.

How much of China's meat supply is tainted with clenbuterol is not clear. The government won't say how many cases of contaminated meat or related illness occur every year. But industry watchers say that, in the countryside at least, use of the drug is rampant.

In a country with an appetite-killing roster of food safety issues _ from deadly infant formula to honey laced with dangerous antimicrobials and eggs dyed with cancer-causing pigments _ the problem of clenbuterol-tainted pork is widely considered to be one of China's biggest food threats.

``It's really a big problem in China,'' said Pan Chenjun, a senior industry analyst with Rabobank in Beijing who focuses on the business of food in China. ``It's not reported frequently so people sometimes think it's not a big issue but actually it's quite widespread.''

Pan said improved food inspection in big cities like Beijing and Shanghai have made mass poisonings in urban areas unusual, and therefore newsworthy, but the problem is rampant in smaller cities and rural areas.

``I think a lot of people living in counties or towns may have a lot of exposure (to clenbuterol) if they eat street food,'' Pan said.

 Adding clenbuterol to feed can reduce a pig's body fat to a very thin layer and makes butchered skin pinker, giving the appearance of fresher meat for a longer time.

The appealing look is one reason Chinese meat suppliers sometimes demand clenbuterol-treated pork from pig farmers, said Wen Peng, editor of the Chinese-language version of The Pig Site, an online news aggregator for the global pork industry.

``When it comes to big large farms, there isn't much of a problem because they can't afford to be caught but there are a lot of small farms and they have a big market,'' Wen said. ``And slaughterhouses, they prefer their suppliers, the producers, to use clenbuterol because the meat looks better and more lean.''

The drug lingers in highest concentrations in organs such as liver and lung _ and poisonings appear more frequent in south China where organ meat is more popular.

In February 2009, 70 people were hospitalized in the southern city of Guangzhou with stomach pains and diarrhea after eating tainted pig organs sold in a local market. In 2006, more than 300 people in Shanghai were sickened by pig products. Last year, 13 people in the coastal city of Shenzhen near Hong Kong were hospitalized from eating clenbuterol-tainted snake. Local media said the snake had been fed frogs that were given clenbuterol _
apparently to make them grow faster.

Though Chinese regulations against ``lean meat powder'' are tough, including prison terms for those who produce or sell tainted food products, enforcement is spotty and offenders can often get off with a fine or a bribe, Wen said.

Government officials too have expressed frustration with the lingering problem. In a report to China's congress on Aug. 25, 2009, Wang Yunlong, the head of the legislative committee on agriculture and rural affairs told his fellow lawmakers that efforts to stop the use of ``lean meat powder'' had fallen short in many areas and called for a ``concentrated countrywide effort to bring it under control.''

China's Ministry of Agriculture did not respond to a faxed list of questions about the problem and measures being taken to deal with it.

The majority of poisoning cases in the news have involved pork, which is by far China's most popular meat. Nearly 50 million metric tons of pork are produced and consumed per year _ amounting to about half the global supply.

The world's second largest pork producer, the United States, harvested 118 million pigs in 2008. The same year, China produced 463 million and output is growing.

The sprawling industry is in the midst of a huge transformation, from mostly backyard farms a few years ago to increasingly large and medium sized facilities _ a shift which should make it easier to monitor for illicit feed additives like clenbuterol.

In 2001, 74 percent of China's pig farms were tiny, with 50 pigs or fewer, but today more than half are medium-sized facilities with up to 3,000 animals, a recent Rabobank report said.

But larger farms are no guarantee of quality. In Beijing's southern district of Daxing, an unlicensed farm with about 800 pigs lies hidden behind a metal gate. On a recent afternoon, a woman inside was stirring a large vat of what looked like mud, bones and garbage but said it wasn't for the pigs. ``It's for us,'' she said.

The man in charge, who would only give his surname, Xu, said he'd never heard of ``lean meat powder.'' He said the farm had no phone number, business name or card.

Farmers use clenbuterol because it boosts profits in two ways: It speeds up the growth of animals to get them to market quicker and creates meat for which consumers are willing to pay extra.

Chairman Mao Zedong's favorite dish is said to have been fatty slabs of belly pork braised in sugar and wine, a cholesterol wallop that was a welcome treat in frugal times. But today, relatively well-off and health conscious Chinese prefer lean pork and are willing to pay a premium for it.

``I always buy lean meat, and make dumplings or meatballs and I don't mind paying a bit more,'' said Wang Xiaoyan, 48, as she bought two pounds of freshly ground lean pork for about $3 in a Beijing market. ``Young people now are afraid of getting fat, like my son, so we buy lean meat and use vegetable oil for cooking. It's better.''

It's not clear exactly where the clenbuterol comes from _ two Chinese chemical factories that advertise clenbuterol on their websites denied ever selling it when reached by telephone.

 Whistle-blowing on food safety can be dangerous in China. The government is wary of any kind of grass roots organizing, particularly when it deals with sensitive social issues. Zhao Lianhai, whose son was sickened from drinking milk contaminated with an industrial chemical, was sentenced to 2.5 years in jail for protesting outside a milk factory and a court, though he was later released on medical parole.

Instead of protesting, some Chinese are working around the dangers. Pan, the Rabobank analyst, said she mainly eats vegetables and when she does eat meat, will avoid pork in favor of lamb or beef.

A few small farms committed to organic farming or limiting their use of chemical additives have also popped up in China, mainly close to Beijing and other cities with relatively high average incomes. On the outskirts of the capital is Little Donkey, a produce and livestock farm set up in 2008 that uses no chemical additives or antibiotics to raise its few dozen pigs.

Chinese angelica herb is used to treat pig illness and garlic juice is spread around the pens as disinfectant. But the meat sold there is three times the cost of pork in most supermarkets, and out of reach of the average consumer.