More than 30 percent of nostoc, or fat choy, a bacterium sold in markets in Hong Kong are counterfeit, made with starches and dyes, according to a study by the Chinese University of Hong Kong.

But health experts say eating real nostoc is no good, either, because it contains MBAA, an amino acid that damages nerve cells and is related to Alzheimer's disease and Parkinson's disease.

Harvesting nostoc is illegal on the mainland since the activity accelerates desertification. But its sale is still permitted in Hong Kong, where people often use it in dishes for Chinese New Year because its name sounds like "making a fortune" in Cantonese.

(Wen Wei Po)