Glycerol is produced from glucose, proteins, pyruvate, triacylglycerols and other glycerolipid metabolic pathways and it is a junctional metabolite in numerous pathways. In particular, the metabolic importance of glycerol is based on the deprivation of glucose under aerobic and anaerobic conditions. In humans, gluconeogenesis, glucose biosynthesis from non-carbohydrate precursors, mainly occurs in the liver and kidneys. While under normal health and dietary conditions, gluconeogenesis from glycerol accounts for less than 5% of glucose production; however, it appears that, after 62–86 hours of starvation, more than 20% of such production is derived from glycerol metabolism. During prolonged fasting, glycerol is the only source for gluconeogenesis, since glycogen reserves are depleted within two fasting days. This capacity to divert glycerol turnover into glucose production is an important evolutionary adaptation and allows for survival during undesirable conditions.
Glycerol is a safe agent that does not approach toxic levels when administered orally in doses of <5 g/kg body weight. Glycerol, which can be ingested in comparatively large amounts, accumulates in body fluids, except for those of the brain and eyes, increasing osmotic pressure and the total volume of water in the body. Glycerol could be used as an energy substrate in nutrition, and could significantly contribute to the energy yield during exercise. Considering its energy substrate function, glycerol could efficiently improve athletic performance. Glycerol’s osmoprotective solute quality can be used to improve physical endurance. The reduction in serum blood osmolality and the effects on ionic gradients caused by glycerol ingestion delay fatigue; therefore, endurance and athletic performance are improved. For a long time, the combined ingestion of glycerol and liquid has been used to increase body water volume, thus maintaining hydration by reducing the kidney’s’ water elimination rate. Glycerol could, therefore, play a very important role in thermoregulation, resistance to high temperatures and endurance in physical activities.
When consumed orally, glycerol is rapidly absorbed and distributed between body fluid compartments before being slowly metabolized via the liver and kidneys. When consumed in combination with a substantial fluid intake, the osmotic pressure enhances the retention of this fluid and the expansion of various body fluid spaces. Typically, this allows fluid expansion or retention by reducing the urinary volume. It’s reported that this fluid retention volume is in the range of 300–730 ml. It’s also reported that the use of glycerol increases blood osmolality and, when accompanied by large amounts of water (1500–2000 ml, or 26 ml/kg body weight), provides an osmotic drive that augments the retention of large quantities of water, which would otherwise be eliminated by the kidneys.
Glycerol, a naturally occurring metabolite, has been shown to be a safe and effective hyperhydrating agent. Glycerol combined with water hyperhydration increases total body water when compared with water hyperhydration alone. Different authors have shown conflicting results when assessing the effect of pre-exercise glycerol administration on subsequent performance functions. Several researchers have shown positive effects on performance after glycerol ingestion. For example, it has been suggested that glycerol-induced hyperhydration increases exercise performance. Similarly, it has been demonstrated that glycerol ingestion increases, exercise tolerance in terms of time by approximately 24%. Glycerol ingestion increases the length of time that can be spent exercising because of the improvement in physical endurance. In addition, heart rate during exercise appears to be significantly lower after glycerol intake. Despite these findings, others have shown no benefits from pre-exercise hyperhydration with glycerol compared with hyperhydration with water alone. Total body glycerol disposal can be divided into oxidation and gluconeogenesis. Most of the glycerol is turned into glucose in the liver by gluconeogenesis and the remainder is oxidized. The glucose produced is circulated in the blood stream and what is not required is converted into glycogen in the liver and muscles. Muscle glucose decomposes into pyruvic acid which supplies energy for exercise.