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Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist, such as the Eastern skunk cabbage, the Voodoo lily, and the giant water lilies of the genus Victoria.
Depending on whether they are initiated through locomotion and intentional movement of the muscles, thermogenic methods can be classified as one of the following:
- Exercise-associated thermogenesis (EAT)
- Non-exercise activity thermogenesis (NEAT)
- Diet-induced thermogenesis (DIT)
One method to raise temperature is through shivering. It produces heat because the conversion of the chemical energy of ATP into kinetic energy causing some of the energy to show up as heat. It is not 100% efficient, meaning while some of the energy becomes heat, a portion is transferred to the kinetic energy that produces its characteristic muscular twitches. No productive movement is produced in shivering because antagonistic muscle pairs are simultaneously activated. Shivering is the process by which the body temperature of hibernating mammals (such as some bats and ground squirrels) is raised as these animals emerge from hibernation.
Non-shivering thermogenesis occurs in brown adipose tissue (brown fat) that is present in all mammals (porcine being the only exception currently known). Brown adipose tissue has a unique protein (uncoupling protein-1) that allows the uncoupling of protons moving down their mitochondrial gradient from the synthesis of ATP, thus allowing the energy to be dissipated as heat.
In this process, substances such as free fatty acids (derived from triacylglycerols) remove purine (ADP, GDP and others) inhibition of thermogenin (uncoupling protein-1), which causes an influx of H+ into the matrix of the mitochondrion and bypasses the ATP synthase channel. This uncouples oxidative phosphorylation, and the energy from the proton motive force is dissipated as heat rather than producing ATP from ADP, which would store chemical energy for the body's use. Thermogenesis can also be produced by leakage of the sodium-potassium pump and the Ca2+ pump. Thermogenesis is contributed to by futile cycles, such as the simultaneous occurrence of lipogenesis and lipolysis or glycolysis and gluconeogenesis.
The low demands of thermogenesis mean that free fatty acids draw, for the most part, on lipolysis as the method of energy production.
Non-shivering thermogenesis is regulated mainly by thyroid hormone and the sympathetic nervous system. Some hormones, such as norepinephrine and leptin, may stimulate thermogenesis by activating the sympathetic nervous system. Rising insulin levels after eating may be responsible for diet-induced thermogenesis (thermic effect of food).
Potential use in fighting obesity
As a significant component of the metabolic rate, thermogenesis can potentially be stimulated to increase energy expenditure and fat oxidation (see Blair and Ellison's study, "The Seven Laws of Thermogenesis" (1998)). Thermogenics are commonly made up of ephedra, bitter orange, capsicum, ginger, guar gum, and pyruvate. Caffeine and EGCG, both found in green tea, may increase thermogenesis regulated by catecholamines such as norepinephrine. Researchers are also investigating the possibility of increasing the amount of brown adipose tissue in the body, a site of thermogenesis. Leptin infusions, being studied due to the leptin resistance seen in obesity, may boost metabolism by stimulating thermogenesis and also reduce hunger and appetite.
- Cannon, B.; Nedergaard, J. (2004). "Brown Adipose Tissue: Function and Physiological Significance". Physiol. Rev. (84): 277–359. PMID 14715917.
- Dulloo, Abdul (March 31, 1999). "Effects of green tea extracts on non-shivering thermogenesis during mild cold exposure in young men.". The American Journal for Clinical Nutrition 70 (6): 1040–1045. PMID 23237788. Retrieved 15 May 2013.