I'd rather not try to describe the process it undergoes in the body to create its mechanism of action, so here is some information for you that you may find helpful
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Glycerol is a liquid alcohol most commonly found in the diet as a component of fat or triglycerides. The glycerol serves as the backbone onto which fatty acid molecules are attached. Commercial preparation of glycerol can be obtained by hydrolysis (removal) of the fatty acids from the glycerol molecule.
- Claims Increase blood volume
- Enhanced temperature regulation
- Improved exercise performance in the heat
Theory:
Glycerol is proposed to help "hyperhydrate" the body by increasing blood volume levels and helping to delay dehydration.
Scientific Support:
At least a few studies support the theory that glycerol added to fluids will increase the hydration compared to drinking fluid alone (without glycerol added). Following glycerol consumption, heart rate and body core temperature are lower during exercise in the heat, suggesting an ergogenic (performance enhancing) effect. In long duration activities, a larger supply of stored water may lead to a delay in dehydration and exhaustion.
Safety:
Straight (undiluted) glycerin is not recommended for internal consumption, but no significant adverse side effects are associated with glycerin diluted with fluids (as noted above). In some subjects, glycerol consumption may lead to headaches or nausea. Individuals in whom increased blood volume may be undesirable, including conditions such as pregnancy, high blood pressure, diabetes and kidney disease, should avoid glycerol supplementation.
Value:
For endurance athletes engaged in strenuous training or competition in hot environments, consumption of glycerol containing beverages may help hydrate tissues, increase blood volume and delay fatigue and exhaustion associated with dehydration.
Dosage:
Glycerol dosage relates to the amount of total body water � so bigger people have more body water and require more glycerol to obtain an effect. Approximately 1 gram of glycerin per kilogram (2.2 lbs) of body weight is diluted in 20-25 milliliters of liquid. A 70 kg man (154 lbs), therefore, would need 70 grams of glycerin diluted in 1400-1750 ml of fluid (about 1-2 liters of fluid). The mixture should be consumed slowly over the course of 1-2 hours prior to exercise in the heat.
References 1. Arnall DA, Goforth HW Jr. Failure to reduce body water loss in cold-water immersion by glycerol ingestion. Undersea Hyperb Med. 1993 Dec;20(4):309-20.
2. Boulay MR, Song TM, Serresse O, Theriault G, Simoneau JA, Bouchard C. Changes in plasma electrolytes and muscle substrates during short-term maximal exercise in humans. Can J Appl Physiol. 1995 Mar;20(1):89-101.
3. Inder WJ, Swanney MP, Donald RA, Prickett TC, Hellemans J. The effect of glycerol and desmopressin on exercise performance and hydration in triathletes. Med Sci Sports Exerc. 1998 Aug;30(8):1263-9.
4. Jimenez C, Melin B, Koulmann N, Allevard AM, Launay JC, Savourey G. Plasma volume changes during and after acute variations of body hydration level in humans. Eur J Appl Physiol Occup Physiol. 1999 Jun;80(1):1-8.
5. Meyer LG, Horrigan DJ Jr, Lotz WG. Effects of three hydration beverages on exercise performance during 60 hours of heat exposure. Aviat Space Environ Med. 1995 Nov;66(11):1052-7.
6. Mitchell JB, Braun WA, Pizza FX, Forrest M. Pre-exercise carbohydrate and fluid ingestion: influence of glycemic response on 10-km treadmill running performance in the heat. J Sports Med Phys Fitness. 2000 Mar;40(1):41-50.
7. Montner P, Stark DM, Riedesel ML, Murata G, Robergs R, Timms M, Chick TW. Pre-exercise glycerol hydration improves cycling endurance time. Int J Sports Med. 1996 Jan;17(1):27-33.
8. Wagner DR. Hyperhydrating with glycerol: implications for athletic performance. J Am Diet Assoc. 1999
glycerol is one of the most common alcohols found in human metabolism. It is a 3-carbon molecule containing three hydroxyl groups. Its molecular formula is C3H8O3, and its molecular weight is 92.09 daltons. A syrupy liquid with a sweet taste, glycerol is about 0.6 times as sweet as cane sugar. Glycerol is also known as glycerin, glycerine, 1,2,3-propanetriol and trihydroxypropane. The structural formula for glycerol is:
Glycerol is the backbone of triacylglycerols (triglycerides or neutral fats) and phospholipids. These substances are present in most life forms, and dietary intake of glycerol comes mainly from these molecules in animal and plant products. Glycerol is also used as a sweetner in syrup, liquor and some foods.
Glycerol has had diverse uses in medicine. It has moisturizing and lubricating properties and can increase serum osmolality. It is given orally to reduce intraocular pressure and vitreous volume in eye surgery and is used as an adjunct in the management of acute glaucoma. Glycerol may also be used topically to reduce corneal edema, which may be of use in eye examinations. Glycerol has also been used (intravenously or orally) for the management of cerebral edema secondary to acute stroke, to lower intracranial pressure and to improve rehydration during acute gastrointestinal disease. It is a hyperosmotic laxative and may be used rectally in suppositories. It is used as a cerumenolytic and as a demulcent in cough preparations.
Oral glycerol by itself may have dehydrating activity. However, if ingested with added fluid, it may increase total body water. This is known as glycerol hyperhydration, and glycerol is used by some athletes to improve thermoregulation and endurance during exercise or exposure to hot environments.
ACTIONS AND PHARMACOLOGY
ACTIONS
Supplemental glycerol has putative hyperhydration and athletic performance-enhancing activities.
MECHANISM OF ACTION
The main effect of glycerol itself results from its dehydrating activity. For example, oral glycerol can increase serum osmalility, thus drawing fluid from other parts of the body. This is the mechanism, at least in part, for its ocular hypotensive effect in the treatment of acute glaucoma. Therefore, it seems paradoxical to talk about glycerol's hyperhydration effect. However, in combination with ample water, it may be more hydrating than water alone, at least for some. The mechanism of this effect and how it may relate to enhanced athletic performance is still highly unclear.
PHARMACOKINETICS
Following ingestion, glycerol is efficiently absorbed and rapidly metabolized. It is absorbed from the lumen of the small intestine into the enterocytes and from those cells into the portal circulation, from whence it is transported to the liver. In the hepatocytes, much of glycerol is phosphorylated via the enzyme glycerol kinase to produce L-glycerol 3-phophate. ATP is necessary for this reaction.
Some L-glycerol 3-phosphate reacts with fatty acyl CoA molecules to ultimately form triglycerides (triacylglycerols) and phospholipids. Most L-glycerol 3-phosphate undergoes oxidation to dihydroxyacetone phosphate, catalyzed either by cytosolic glycerol 3-phosphate dehydrogenase, an enzyme that requires NAD+ as electron receptor, or by mitochondrial glycerol 3-phosphate dehydrogenase. The coupling of the two glycerol 3-phosphate dehydrogenases leads to the passage of electrons from NADH to enter the mitochondrial electron transfer chain for the production of ATP. This transfer of electrons from cytosolic NADH to the mitochondrial electron transport chain is known as the glycerol phosphate shuttle. Dihydroxyacetone phosphate can either move in the direction of further oxidation to finally yield carbon dioxide, water and ATP or be converted to glyceraldehyde 3-phosphate and move in the direction of glucose and glycogen synthesis.
Glycerol not metabolized in the liver is transported to various tissues and undergoes metabolism. Similar reactions to those discussed above occur in the kidney.
Glycerol is eliminated in the kidney by filtration and, in concentrations up to 0.15 mg/ml, undergoes complete tubular reabsorption. In higher concentration, glycerol begins to appear in the urine and induces an osmotic diuresis.
The elimination half-life of glycerol is from 30 to 45 minutes.
INDICATIONS AND USAGE
Glycerol may be useful for improving hydration and, thus, exercise endurance in some, but research results related to this putative benefit are mixed. There is no evidence that glycerol helps with weight loss. Intravenous glycerol is helpful in some with acute ischemic cerebral infarct. Oral glycerol has been useful in preventing some of the neurologic and audiologic sequellae of childhood bacterial meningitis.
RESEARCH SUMMARY
Pre-exercise administration of glycerol significantly improved cycling endurance time in two double-blind, randomized, cross-over trials. Mean heart rate was also reduced in glycerol-supplemented subjects.
Pre-exercise administration of glycerol in another study, however, failed to affect exercise performance in a group of triathletes. Results of animal work have been similarly mixed, and more research is needed.
Glycerol did not increase weight loss in a placebo-controlled trial.
Intravenous administration of glycerol in subjects with acute ischemic cerebral infarct has resulted in significantly fewer neurological deficits in one study. Several other trials demonstrating similar benefits have been double-blind, randomized trials, but some of these benefits have been transient.
Glycerol-treated infants and children with bacterial meningitis had less severe hearing impairment and fewer neurologic deficits than did controls. Oral administration of glycerol was utilized.
CONTRAINDICATIONS, PRECAUTIONS, ADVERSE REACTIONS
CONTRAINDICATIONS
Supplemental glycerol is contraindicated in those with severe dehydration, anuria, congestive heart failure and pulmonary edema. It is also contraindicated in those who are hypersensitive to glycerol.
PRECAUTIONS
Glycerol supplementation should be avoided by pregnant women and nursing mothers.
Those with cardiac, renal or hepatic problems should avoid supplemental glycerol, as should those with diabetes and those with hemolytic anemia.
Anyone using oral glycerol for supplementation must drink plenty of fluid concomitantly. See Dosing and Administration.
Those using glycerol need to be aware that contact of glycerol with strong oxidizing agents, such as potassium permanganate, potassium chlorate or chromium trioxide, may produce an explosion.
ADVERSE REACTIONS
There are rare reports of cardiac dysrhythmias occurring with oral glycerol use and one report of hypertension occurring. Other adverse reactions include headache, dizziness, confusion and amnesia (in elderly subjects) and hyperglycemia. Hyperosmolarity, which occurs with oral glycerol, is usually clinically significant only in those with type 2 diabetes. Those with type 2 diabetes may develop nonketotic hyperosmolar hyperglycemia.
The most frequent adverse reactions are gastrointestinal and include nausea and vomiting, bloating and diarrhea.
DOSAGE AND ADMINISTRATION
The doses are variable in those who use glycerol for hydration purposes and for possible exercise performance enhancement. Some use 2 to 4 tablespoons of glycerol in water, orange juice or a sports drink. The ratio of fluids to glycerol is about 20 to 1. This is taken approximately 2.5 hours prior to exercise. The volume of a tablespoon is 15 ml, and one ml of glycerol weighs 1.25 grams. The energy value of glycerol is about 4 kcal or 4 Cal per gram. Pharmaceutical grade glycerol is used.
HOW SUPPLIED
Liquid
LITERATURE
Inder WJ, Swanney MP, Donald RA, et al. The effect of glycerol and desmopressin on exercise performance and hydration in triathletes. Med Sci Sports Exerc. 1998; 30:1263-1269.
Montner P, Stark DM, Riedesel ML, et al. Pre-exercise glycerol hydration improves cycling endurance time. Int J Sports Med. 1996; 17:27-33.
Robergs RA, Griffin SE. Glycerol. Biochemistry, pharmacokinetics and clinical and practical applications. Sports Med. 1998; 26:145-167.
Wagner DR. Hyperhydrating with glycerol: implications for athletic performance. J Am Diet Assoc. 1999; 99:207-212.
