Chillis Affect Appetite and MetabolismThe chemical in chillis that give them their heat is capsaicin, a waxy compound that has very little taste or odour in its own right but interacts with a family of ion channels known as the vanilloid receptors. The major channel, known as TRPV1, is located on many sensory nerve endings and plays an important role in sensing noxious stimuli such as heat, and once activated the receptor causes the release of several neurotransmitters which signal a pain response. Interestingly the same neurotransmitters have been found to increase metabolic rate and reduce appetite, according to a study by Motter and Aherne in FEBS letters (2008). Interestingly, mice given capsaicin or other chemically similar compounds (including vanillin, a substance found in vanilla pods) eat less food and gain less weight than controls when given a high fat diet.
Paradoxically however knock-out mice that have the TRPV1 receptor deleted from their DNA also have reduced food intake and body weight according to the same paper, and the authors admit that the exact mechanism of action of the receptor is unclear. One thing they are sure of though is that chronic treatment with capsaicin and other similar compounds prevent dietary weight gain, confirmed in another recent study by Reinbach et al. in Clinical Nutrition (2009). This experiment took healthy human volunteers and found that their appetites were reduced and metabolic rates increased when given capsaicin.
A Potential Treatment for Obesity and DiabetesAmong the proposed mechanisms of action one of the most interesting appears to be the control of a substance called calcitonin gene-related peptide (CGRP), a neurotransmitter released from sensory nerves. Motter and Aherne's study showed that precursor fat cells, preadipocytes, were sensitive to CGRP and could transform into mature fat cells when exposed to this substance, a process that appears to be inhibited by treatment of capsaicin. The authors postulated that capsaicin could help prevent age-onset obesity and insulin resistance (a pre-diabetic state), two of the biggest risk factors for heart disease, through this mechanism.
Judging by the amount of interest being generated around capsaicin and the TRPV1 receptor it would appear that drug companies are paying close attention to this field of study. Certainly the limited evidence gathered so far would suggest that capsaicin and other compounds from the vanilloid family may have a role to play in obesity and diabetes. A capsaicin-like compound that could reduce body weight without the heat of a chilli pepper would possibly be an excellent aid for people trying to lose weight. However, for those that like their food spicy, there is all the more reason to put a little extra chilli in your dinner. If you're brave enough, you could always risk the Dorset Naga.
Author's Note: There is no direct evidence of weight loss caused by a high chilli intake. Diet and exercise are still the best way to lose weight, but the ingestion of chilli must be exercised with caution, with no expectancy of weight loss. Chillis and high concentrations of capsaicin are certainly an irritant and excess consumption may even be harmful.