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The human body has two methods of thermogenesis, which produces heat to raise the core body temperature. The first is shivering, which occurs in an unclothed person when the ambient air temperature is under 25 °C (77 °F) [dubious – discuss]. [18] It is limited by the amount of glycogen available in the body. [5]
As in other mammals, human thermoregulation is an important aspect of homeostasis. In thermoregulation, body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles. [1] Humans have been able to adapt to a great diversity of climates, including hot humid and hot arid.
The low demands of thermogenesis mean that free fatty acids draw, for the most part, on lipolysis as the method of energy production. A comprehensive list of human and mouse genes regulating cold-induced thermogenesis (CIT) in living animals or tissue samples has been assembled [15] and is available in CITGeneDB. [16]
The human outer or peripheral shell (skin, subcutaneous fat etc.) acts as an adjustable insulator/radiator with the main mechanism of adjustment being blood flow to this compartment. If the surroundings are warm then heat loss is less, so the body directs more blood to the periphery to maintain the gradient for energy flow.
Starvation response in animals (including humans) is a set of adaptive biochemical and physiological changes, triggered by lack of food or extreme weight loss, in which the body seeks to conserve energy by reducing metabolic rate and/or non-resting energy expenditure to prolong survival and preserve body fat and lean mass.
Thermogenesis (shivering) is when the body comes in contact with cold temperatures in the environment and the body feels as though it needs to raise its core body to protect itself. This is a primary trigger for the mechanism of heat production in mammalian bodies when exposed to cold.
The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain. The brain generates about 16% of the total heat produced by the body. [8] Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume.
Approximately all enzymatic reaction in the human body is thermogenic, which gives rise to the basal metabolic rate. [2] In bodybuilding, athletes wishing to reduce body fat percentage use thermogenics in order to attempt to increase their basal metabolic rate, thereby increasing overall energy expenditure.