As with many sport science terminology, the term diuretics has various meanings, each determined by the circumstances of its use. In its strict and most technical sense, a diuretic is any agent that promotes the production of urine from the kidneys, and a corresponding excretion of urine from the body; diuretic is from the Greek words meaning "through urine." In popular language, diuretics are taken to be anything that acts to dehydrate or reduce the fluid levels within the body.
An understanding of diuretic function begins with the manner in which water operates within the human body. Water is essential to the function of numerous human systems, including the operation of the cardiovascular system. Blood, through its fluid component plasma, is 90% water. Water is ingested through both fluids and foods, and it enters the bloodstream through the digestive process, centered at the small intestine.
The kidneys are the organs whose primary function is the regulation of both the amount of fluid within the body as well as the chemical and electrical balances of those fluids. The kidneys comprise one of four components of the excretory system, where waste products are processed and removed, although the excretion of waste is a secondary function of the kidneys. The kidneys, the ureter (a muscular connective tube), the bladder (which stores urine), and the urethra are the mechanisms by which urine is generated and transported.
When the cardiovascular system directs blood into the kidneys, the pressure conveyed along the renal artery (the main blood conduit to the kidney) forces water and some water-soluble products from the blood, where the kidney filtration structure collects them and redirects the product to the bladder as urine. The reabsorption of water by the body is triggered by the antidiuretic hormone, ADH. ADH is produced by the pituitary gland when signaled by the hypothalamus, a regulatory center in the brain, that the fluid levels within the body have dropped. ADH will signal the kidneys to absorb more water into the blood to increase fluid level; the urine produced is therefore more concentrated.
When the body has too much fluid in its system, the hypothalamus signals a reduction in ADH, which increases the amount of water absorbed by the kidneys, with a resultant larger, and more dilute, urine production.
Interrelated with the operation of ADH is the presence of sodium in the blood and kidneys. Sodium is the mineral essential to the regulation of water retention and excretion in the body. It is regulated by a kidney hormone, aldosterone. When fluid levels are low, aldosterone precipitates a release of sodium from the kidneys into the bloodstream; through the process of osmosis, water enters the blood.
Diuretics function through the disturbance of the fluid level balance achieved by the kidneys, and through the hormones that act as the signals to increase or decrease fluid levels. Diuretics are also specifically chemically formulated to combat various types of illnesses. Diuretics are present in many foods. Herbal mixtures containing dandelion root and parsley have a proven diuretic effect. The best-known food products possessing diuretic properties are those that contain caffeine or alcohol.
Caffeine—the most consumed stimulant in the world—is prized by athletes for both the additional "spark" given to powers of concentration as well as its fatigue-fighting qualities. The presence of caffeine in the bloodstream as a byproduct of digestion will induce the production of ADH as well as a greater output of urine. Caffeine will therefore tend to reduce blood volumes and presents a mildly negative impact on the function of the cardiovascular system unless countered with increased fluid intake.
Alcohol, as found in varying quantities in beers, wines, and spirits, is readily soluble in water and it is equally readily absorbed into the bloodstream through digestion. Alcohol will also trigger a milder increase in urine production through the kidneys; the impact of alcohol as a diuretic for an athlete is usually more pronounced when the athlete has indulged in the consumption of alcohol the day before training or competition, as alcohol has few performance-enhancing qualities.
There are a number of diuretics that have been created chemically to assist in the management of various conditions. These products may be classified as acetazolamide (or similar products), thiazides, or "loop" diuretics. Acetazolamide compound is used in the treatment of glaucoma and various types of seizures, when the reduction of fluid pressure in the eye or other organs is critical. Thiazides are a class of substances that are used to treat hypertension (high blood pressure) through interfering in the usual kidney processes that permit sodium to be reabsorbed, leading to the osmosis of water into the bloodstream. So-called loop diuretics, which are designed to reduce edema, or swelling, in the organs or tissues.
An unforeseen, yet highly effective, diuretic application arose regarding athletes who were subject to various types of doping tests that involved a urine sample. Diuretics, given their tendency to impel the body to produce greater amounts of urine, create a flushing effect on the entire renal system, resulting in the discharge of evidence of anabolic steroids, erythropoietin (EPO, the blood-doping hormone), or other metabolized particles of evidence. Both the World Anti-Doping Agency (WADA) as well as the International Olympic Committee (IOC) now penalize athletes for the presence of diuretics in a doping test in the same fashion as a positive text for the performance-enhancing substances themselves.