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Humans are endotherms, animals that keep their body temperature within a stable range using heat production and heat dissipation. The ability to produce heat from calories is an essential mechanism required for life-sustaining cellular reactions that need a sufficient intake of calories. The molecules in food contain energy, or calories, stored in chemical bonds. Metabolic reactions can extract energy from these chemical bonds and use them to power various metabolic reactions that maintain the body's homeostasis. Energy metabolism is a highly regulated process to meet the energy demands of our body under variable conditions at rest and during work or exercise. [1]

Body temperature is tightly regulated by a process called thermoregulation, controlled by a master regulator, the hypothalamus that modulates the heat gain or loss by the body. Up to 60% of the heat generated during metabolic processes is used to maintain body temperature. Accordingly, dysregulation of thermoregulatory mechanisms can result in hypothermia or hyperthermia [8]. Hypothermia: If the heat produced by these reactions is exceeded by total body heat loss, the body is in a state of hypothermia. Hypothermia is defined as a core body temperature below 35 degrees Celsius (C) or 95 degrees Fahrenheit (F). Hypothermia can be classified as mild, moderate, or severe. Mild hypothermia is characterized by substantial shivering and behavior changes. It is defined as a core temperature between 32-35 degrees C (89.6-95 degrees F). A core body temperature between 28-32 degrees C (82.4-89.6 degrees F) is defined as moderate hypothermia. It is characterized by dilation of pupils, cardiac arrhythmias, confusion, possible loss of consciousness, and lack of shivering. Severe hypothermia is defined as a core temperature below 28 degrees C (82.4 degrees F). Severe bradycardia and ventricular fibrillation can occur at this stage. Cardiac arrest becomes more likely as the body becomes colder. To counteract hypothermia, the hypothalamus can increase the body’s overall metabolic rate generating more heat. Shivering is an involuntary response to cold temperatures that uses muscle contractions to generate heat. It can increase the basal metabolic rate by as much as 5 to 6 times. The intensity of shivering depends on the core temperature and a person’s BMI. Studies show that decreased shivering is correlated with increased body fat.  Additionally, peripheral vessels can undergo vasoconstriction keeping blood centrally and minimizing heat loss to the environment. The outer body then acts as a barrier between the body’s core and the environment. Frostbite risk increases as peripheral vasoconstriction increases. The treatment of mild to moderate hypothermia is the removal of cold, wet clothing and rapid-rewarming with a hot bath at 37-39 degrees C. If the hypothermia is severe, active internal rewarming is indicated. The treatment process is often painful and may require pain medication [9][10].

Shivering is an involuntary response to cold temperatures that uses muscle contractions to generate heat. It can increase the basal metabolic rate by as much as 5 to 6 times. The intensity of shivering depends on the core temperature and a person’s BMI. Studies show that decreased shivering is correlated with increased body fat.  Additionally, peripheral vessels can undergo vasoconstriction keeping blood centrally and minimizing heat loss to the environment. The outer body then acts as a barrier between the body’s core and the environment. Frostbite risk increases as peripheral vasoconstriction increases. The treatment of mild to moderate hypothermia is the removal of cold, wet clothing and rapid-rewarming with a hot bath at 37-39 degrees C. If the hypothermia is severe, active internal rewarming is indicated. The treatment process is often painful and may require pain medication [9][10]. Hyperthermia: Hyperthermia is the state of increased core body temperature resulting from the body creating more heat than it can dissipate. A temperature higher than 37.5–38.3 °C (99.5–100.9 °F) classifies as hyperthermia in humans. Causes of hyperthermia encompass a wide range of factors such as drugs, toxins, as a compensatory immune reaction to certain infections, and malignant hyperthermia. Normally, the core body temperature is maintained by thermoregulatory mechanisms such as sweating, shivering, and vasoconstriction that are controlled by the hypothalamus. The breakdown of these thermoregulatory mechanisms in the body can cause elevation of temperature, ranging from mild to dangerously high levels, and can be life life-threatening.