HCA or Hydroxycitric acid is a derivative of citric acid that is found in a variety of tropical plants including Garcinia cambogia & Hibiscus subdariffa. Laboratory and animal studies of HCA have produced results that indicate a potential for modulation of lipid metabolism; HCA is an ingredient in some weight loss products & dietary supplements.

However, there is no clinical evidence that HCA has any benefit to humans in either of these applications. Garcinia cambogia extract containing HCA showed that high doses led to significant suppression of epididymal fat accumulation, but also caused potent testicular atrophy & toxicity. [...]

http://en.wikipedia.org/wiki/Hydroxycitric_acid