CHENGDU - Suspected food poisoning landed 57 people in hospital on Tuesday in Southwest China's Sichuan province, local authorities said Wednesday.

The patients suffered from vomiting after eating at a restaurant in Xiping township of Santai county in Mianyang late Tuesday afternoon, the press office of the county committee of the Communist Party of China said.

Of the 57 people who were hospitalized Tuesday night, one was still under observation and the other 56 had been released by Wednesday morning, it said.

Specialists with the local disease control and prevention center said they were poisoned by nitrite, as lab tests found the chemical in the restaurant's kitchen and in samples of the patients' vomit.

The restaurant has been closed for further investigation.

While industrial nitrates are poisonous, sodium nitrate and nitrite approved for human consumption are commonly added to food as preservatives and color-enhancers.