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Metabolism describes the body's ability to regulate it's use of food energy. It is a complicated system of relationships between nutrition and hormone feedback systems. In a healthy person, energy use can be largely regulated on demand. However, there are several illnesses that can result in the inability of the body to properly utilize energy even if a person is well fed, is digesting food properly, and has sufficient oxygen. The branch of medicine which deals with metabolism and the effect of hormones is endocrinology.
All the living tissues of the body have a constant need for energy - this is referred to as the base metabolic rate (BMR) and is for the most part the body's use of energy when it is asleep. The energy need under the BMR, as well as all other energy use, is largely a product of the person's weight. The BMR and averages about 50 calories per hour. Of that, the organ with the highest BMR is the liver, which uses between 25 and 30 percent of the body's energy at rest. However, the brain has the highest BMR per unit of weight - the three pound brain uses about 20% of the body's BMR, and even that rises during intense activity, such as dreaming.
When a body is active, the body's hormones regulate the uptake of glucose from the blood to the active cells, primarily the muscles. Insulin from the pancreas allows the cells to easily absorb glucose, which is then converted to energy within the cell. When the body is at rest, glucagon resists the effect of insulin and prevents cells from absorbing glucose, particularly when blood sugar starts to drop. Adrenal hormones, such as cortisol and adrenaline, regulate the production of the pancreatic hormones. These in turn are regulated by thyroid hormones, which are in turn regulated by hormones from the pituitary gland. Other hormones regulate the conversion of fat and protein to usable sugars, and can also regulate the conversion of sugar to fat.
As such, several links in this complex chain can be broken, disastrously affecting the body's ability to use available glucose on demand:
- Of these, diabetes mellitus is clearly the worst. It largely prevents sugar from entering cells, forcing the body to rely solely on the energy producing by-products of proteins instead. The body is largely starved of energy and the excess sugar can only be expelled through urination, requiring large amounts of water.
- Addison's disease, the result of a lack of cortisol, interferes with the production of insulin on demand, leading to fatigue and muscle weakness.
- Cushing's syndrome, the result of too much cortisol, forces the body to store fat, building it up no matter what the patient's diet or exercise patterns.
- Hypothyroidism and hyperthyroidism both interfere with all the metabolic processes, resulting in fatigue in the prior case and rapid weight loss in the latter.
Metabolic diseases are often difficult to diagnose as they disguise themselves as lifestyle diseases such as obesity. Doctors often feel their patients are lying to them about their diet and exercise patterns.