Background information
Capsaicin is the active component of cayenne pepper or commonly known as chili or red hot pepper. It is the major substance that contributes to the pungent properties of chili. Because of its importance in spices, food additives and drugs, there has been an increasing interest and studies towards this component especially in Japan. Recently, the effects of capsaicin on boosting fat metabolism have received much attention and proven.
 
History and folk use
Traditionally, chili has added spice to many cuisines around the world, including those of Southeast Asia, China, South America, southern Italy, and Mexico. Chili originated from South America and the West Indies, but is now cultivated in tropical locations throughout the world. The folk use of chili is quite extensive, including asthma, fever, sore throats and other respiratory tract infections, digestive problems, toothaches, and even certain cancers.
 
Key uses
Fat reduction Aid digestion Circulatory system health (traditional) Pain relief (topical application) 
 
Active constituents
Major active constituents found in chili include capsaicin, vitamin A, C, and E.
 
Recommended use
For internal use, it can be taken up to 1200 to 1500 mg per day. 
 
How it works
Before getting into the details on how capsaicin helps in burning fat, let's starts with the basics. Metabolism is a key term here. It is defined as the sum of all chemical processes that takes place in the body as they relate to the movement of nutrients in the blood after digestion, resulting in growth, energy, release of wastes, and other bodily functions. Exercise, body temperature, hormone activity, and digestion of certain food can increase the rate of metabolism.

Wanatabe et al. (1987, 1988) have investigated neurophysiological functions of capsaicin and have demonstrated that capsaicin increases energy metabolism by activating sympathetic nervous system. This leads to the release of chemical messengers like noradrenaline and adrenaline that increases the cellular metabolic rate to generate energy from glycogen and fat store. Such effect of capsaicin was again demonstrated in a human study by Yoshioka et al. (1995).

Yoshika et al. (1998) further investigated the effect of red pepper added to high-fat diet and high-carbohydrate meals on energy metabolism and nutrient utilization in Japanese women. Results of the study indicate that red pepper increases diet-induced thermogenesis (i.e. energy generation) and fat oxidation or so-called fat-burning.

Kawada et al. (1986) reported that dietary supplementation of capsaicin in high fat diets lowered fat tissue and blood tryglyceride level in rat due to enhancement of energy metabolism. For instance, it was shown that the activity of liver enzymes involved in breaking down fat increases when capsaicin was added to the diet.

As mentioned before, exercise is another factor that can increase metabolic rate. Study done by Yoshioka et al. (2000) investigated whether additional effect of capsaicin could be produced when one is already having regular exercise. All rats subjects were submitted to a treadmill exercise for a period of time, but some were fed with capsaicin diet and some were not (i.e. so-called control group). The fat deposit weight and blood free-fatty acids level were significantly lower as compared with the control group which were not fed with capsaicin.
 
Health applications of capsaicin
Another popular clinical use of capsaicin is the topical or external application of capsaicin cream for pain relief. For example, it has been shown to relieve pain associated with cluster headache, arthritis, diabetic neuropathy and postmastectomy (after breast reconstruction).

So how does it work in reducing pain when applied topically? It works by depleting a neurotransmitter called substance P, which has been linked closely with many pain disorders such as those mentioned above. Research also show that it increase the collegenase and certain prostaglandins that reduces both pain and inflammation.
 
Safety
Capsaicin is generally recognized as safe (GRAS). As a matter of fact, chili can be added liberally in our diet. Externally applied capsacin may produce a burning sensation, however, such effect fades gradually with time and is rarely a major concern for most users who get accustomed to the sensation!

Note: Information provided are based on thorough research done on scientific and reliable sources, however, is not meant to substitute for the advice provided by your own physician or other medical professional.
 
REFERENCES
1. Kawada et al. Effects of capsaicin on lipid metabolism in rats fed a high fat diet. J. Nutr. 1986;116:1272-1278.

2. Wanatabe et al. Enhancement by capsaicin of energy metabolism in rats through secretion of catecholamine from adrenal medulla. Agric. Biol. Chem. 1987;51:75-79.

3. Wanatabe et al. Adrenal sympathetic efferent nerve and catecholamine secretion excitation caused by capsaicin in rats. Am. J. Physiol. 1988;255:E23-E27.

4. 4. Yoshioka et al. Effects of red-pepper diet on the energy metabolism in men. J. Nutr. Sci. Vitaminol. 1995;41:647-656.

5. Yoshika et al. Effect of red pepper added to high-fat and high-carbohydrate meals on energy metabolism and substrate utilization in Japanese women. Br. J. Nutr. 1998;80:503-510.

6. Yoshioka et al. Effects of capsaicin on abdominal fat and serum fatty-acids in exercise-trained rats. Nutr Res. 2000;20(7):1041-1045.