Hydroxycitric acid (HCA) is a compound found in certain species of plant. Commercially hydroxycitric acid is most commonly extracted from the fruits of the Garcinia cambogia, Garcinia indica and Garcinia atroviridis plants. Hydroxycitric acid is interesting nutritionally because it may be an effective weight loss aid. Both animal and human studies have been performed using hydroxycitric acid, with evidence supporting a role for its weight loss effects. Hydroxycitric acid is different to many weight loss supplements because it does not stimulate the central nervous system through agonism of beta adrenergic receptors. Studies investigating the biochemistry of hydroxycitric acid show that it may cause weight loss through its ability to alter the metabolism of carbohydrates. In particular, hydroxycitric acid is able to inhibit the mitochondrial enzyme ATP citrate lyase, an important enzyme in fatty acid biosynthesis. By inhibiting this enzyme, hydroxycitric acid decreases the synthesis rate of acetyl coenzyme A (acetyl-CoA), which is used as a building block for the synthesis of fatty acids (figure1).
|Figure 1. The possible weight loss effects of hydroxycitric acid.
Hydroxycitric acid is therefore an inhibitor of the de novo lipogenesis pathway, and in this way may be able to reduce the conversion of carbohydrates to fatty acids in the liver. Although it is often assumed that this is beneficial because it causes a reduction in the accumulation of fatty acids, which in turn decreases the availability of triglycerides for deposition in adipose tissue, this theory may be too simplistic. Activation of the de novo lipogenesis pathway by high intakes of fructose are thought to increase the risk of developing insulin resistance. The resultant fatty acids accumulate in liver and skeletal muscle and are thought to interfere with the insulin signal cascade. By reducing flux through the de novo lipogenesis pathway, hydroxycitric acid may limit the damage caused by the accumulation of ectopic lipids. By inhibiting ATP citrate lyase, hydroxycitric acid may also increase the pathway that leads to the synthesis of glycogen, the storage form of carbohydrate in the liver. As full glycogen stores feedback to the central nervous system to induce satiety, hydroxycitric acid may also be an effective appetite suppressant.