Glycerol, also known as glycerin, or glycerine, is a hygroscopic (water-absorbent) fluid. In its pure form, it is a sweet-tasting, colorless, odorless, and viscous liquid that is 100% soluble in water. Simply put, glycerol is a combination of sugar and alcohol; glycerol is the term used whenever the biological qualities of this compound are discussed.

Glycerol, either in a freestanding state or as a derivative compound, is an important feature in a number of important chemical functions within the body. The first of these is glycolysis, the process by which the carbohydrate product glucose is converted to a usable energy form. Glycolysis is a Greek word, a combined meaning "sweet" and "splitting," which neatly summarizes how the glucose molecule is reduced for the purpose of fuel production of adenosine triphosphate (ATP). Glycerol, in the form of glycerates, is created in this process.

Glycerol is also an important component of triglycerides, the storage form of fats that are ingested into the body as food. These compounds take their name from the fact the glycerol binds three fatty acids together into one compound for storage. Triglycerides are stored in adipose tissues, chiefly located at the abdomen and the buttocks. Glycerol is commonly referred to as the backbone provided to the fatty acids that are the largest component of the fats when they are stored in their triglyceride form. Glycerol and fatty acids are released through the breakdown of fats through the process of lipolysis. Adipose tissues are those specially constructed in the body to accommodate stores of fat for future energy use. When the adipose tissues are triggered to release their fat stores, the triglycerides are reduced to their two constituent parts, fatty acids and glycerol. The fatty acids are directed into the bloodstream for transport to a desired muscle for conversion into energy; the glycerol is transported to the liver where it is reformulated into glucose, and ultimately redirected for energy creation.

In its state as found outside of the body, glycerol has many uses. It is approximately 60% as sweet as granular table sugar and is consequently used as a food and beverage sweetener. Glycerol and its derivatives are also used as humectants, substances that will keep materials moist (as glycerol is a powerful water-absorbing agent). Glycerol also has a number of uses as a lubricant.

Glycerol has been the subject of considerable sports science review. The main research focus has centered on the question of whether the ability of glycerol, if added to the water or other fluids consumed by endurance athletes as part of their competition hydration strategy, would assist in the creation of a hyper-hydration state, in which the effect of a particular quantity of fluid consumed in hot or humid conditions would be increased. In theory, the consumption of glycerol with fluid will increase the effective concentration of the water, known as the osmolarity, in the blood and the tissue of the athlete. This increased osmolarity would remain until the glycerol was removed from the bloodstream by the kidneys and broken down by the body. The simple ingestion of large volumes of water will also increase osmolarity, but only in the short term; the body will quickly begin to produce greater corresponding amounts of urine to offset the osmolarity effect. Numerous scientific studies had been conducted employing varying amounts of glycerol in relation to fluids, with inconclusive results. The bulk of these studies were conducted using exercise samples that were of moderate intensity, approximately 50% of the subject's VO2 max.

At the 2004 Athens Olympics, American marathoner Deena Kastor won a bronze medal in what was widely viewed as a very surprising result. In the hot weather conditions of that intense championship race, Kastor had hydrated herself with a glycerol solution. Further research has tended to confirm that in high-intensity endurance exercise scenarios, glycerol will promote greater hydration than water alone. Glycerol is broadly available as a supplement for the purpose of producing a hyper-hydration state; until 1997, glycerol was a prohibited substance in international athletics. Glycerol is now permitted for use by athletes by the World Anti-Doping Agency (WADA).

Glycerol, in the form of iodinated glycerol, is sometimes prescribed as an expectorant, a medication designed to loosen the phlegm and mucous of the lungs and the breathing passages in asthma or bronchitis sufferers. It has a number of side effects, particularly nausea, headaches, and stomach upset.

SEE ALSO Fat intake; Liver function.