<body><h1>Glycerol</h1><p><br></p><p> </p><p> syn. glycerin/glycerine</p><p> syn. 1,2,3-propanetriol </p><p> </p><p><div id=’imgblock’><img data-pointer=’9109’/></div></p><h2> Description:</h2><p> Glycerol is an organic compound of the alcohol family (it is specifically classified as a trihydric 3-carbon alcohol). It occurs naturally in the human body, and serves as a backbone component of triglycerides and cell phospholipids. Glycerol is abundant in nature, and can be found (in the form of glycerol esters) in all animal fats and vegetable oils. Glycerol is a sweet odorless clear viscous fluid, with a density of approximately 1.25 grams per milliliter. It is considered a caloric macronutrient, supplying approximately 4 calories per gram. It is commonly used as an ingredient in prepared foods, and also in the manufacture of cosmetic and drug products. It is also used as a sports nutrition product to improve endurance and slow dehydration.</p><div id=’imgblock’><img data-pointer=’7464’/></div><p> Glycerol is classified as a hyperosmotic and osmotic diuretic agent. When administered in a sufficient dosage, glycerol induces an osmotic gradient. It can draw fluids into plasma, and may reduce the excretion of urine by enhancing the reabsorption of water in the nephrons.394 As an osmotic, glycerol has some direct medical value in the treatment of many health ailments. This includes cerebral edema due to stroke, elevated intraocular pressure/glaucoma, intracranial hypertension, and postural hypotension (sudden drop in blood pressure when changing position, usually from sitting to standing). It is also applied to improve the hydration status of patients during certain types of acute gastrointestinal disease. </p><p> The ability of glycerol to act as an osmotic agent also lends it some tangible value as a sports ergogen. In this context, it is widely used by endurance athletes to increase the retention of water before an endurance event. This enhanced-hydration effect is referred to as “glycerol hyperhydration”, and requires the co-consumption of a high volume of water. When properly applied, glycerol hyperhydration will result in a significant but temporary water increase and expansion of the plasma volume. It is important to note that the consumption of glycerol without water loading can actually have a mild diuretic effect, increasing urine outflow by decreasing the reabsorption of water by the kidneys. This could be counterproductive for fluid balance and endurance performance. </p><p> </p><p> Clinical studies strongly support the effectiveness of common glycerol hyperhydration protocols for increasing water retention. This practice typically involves the loading of 1 g/kg of glycerol and approximately 1.5 L of water 1-2 hours prior to an endurance event.395 This has been shown to result in a net increase of up to 900 mL of total body water compared to hydrating with water alone.396 While the exercise and glycerol loading studies are not all in agreement about how this affects performance, there have been clear instances in which this additional fluid has slowed dehydration, reduced heat stress, and improved endurance. Taken as a whole, the studies do suggest that glycerol is effective for pre-event hydration, and may be especially useful at reducing dehydration and improving performance when access to water is limited during competition.</p><p> As a macronutrient, glycerol is classified as a carbohydrate. While this designation is sometimes contested based on the metabolic characteristics of this nutrient, it is appropriate given its structure. Glycerol can be utilized for energy via metabolism to glucose in the liver, although this is not an efficient path of metabolism. Because of this, it has only a minor effect on blood sugar levels compared to traditional carbohydrates.397 It also does not serve as a significant energy source during intense exercise.398 Given these properties, glycerol is widely regarded as a “low impact” carbohydrate, possessing reduced net caloric value. As such, it is not a suitable substitute for traditional carbohydrates with regard to glycogen replenishment or energy provision.</p><p> </p><p> Glycerol syrup has a very sweet taste. Given that it is also regarded as a low impact carbohydrate source, it is widely used as a sweetener and sugar substitute in reduced calorie food products and meal replacement supplements. Since FDA regulations do stipulate that the glycerol content must be included in the total carbohydrate count on product labeling, many manufacturers include a second “net impact carbohydrates” listing on their products. This typically does not include low impact carbohydrates such as glycerol and sugar alcohols. The exact energy impact of glycerol is the subject of much debate, however, and likely varies depending on the metabolic conditions of the user. It cannot be assumed to be completely devoid of caloric influence.</p><p> </p><p> In addition to free glycerol, some manufacturers sell glycerol monostearate (GMS), which is glycerol attached to stearic acid. This is a powdered form of glycerol, better suited for tabletted, encapsulated, or powdered products. Glycerol monostearate contains approximately 25% glycerol by weight. It has not been studied for its potential performance-enhancing properties, or as a hyperhydrating agent prior to endurance competition. A large dose would be necessary to reach the equivalents of common glycerol hyperhydration protocols. It is reasonable to conclude, however, that the inclusion of GMS may still offer some osmotic/plasma-expanding effect in a sports nutrition formula depending on the dosage and additional ingredients used. </p><p> </p><h2> Promoted Benefits:</h2><p> </p><p> Glycerol is promoted to increase hydration prior to endurance exercise, slowing dehydration, reducing heat stress, and improving overall performance.</p><p> </p><h2> Clinical Studies: </h2><p> </p><p> Glycerol has been the subject of extensive clinical study. Glycerol hyperhydration has been shown to improve performance in placebo-controlled studies with trained adults. Its Clinical Support Rating is 5 (5/5).</p><p> </p><p> One placebo-controlled study examined the effects of glycerol hyperhydration on exercise performance in a group of Olympic triathletes.399 The subjects consumed glycerol (1.2 g/kg of bodyweight), along with 25 mL/kg of diluted carbohydrate-electrolyte drink (Gatorade .75 g/kg) during a 60-minute period two hours prior to competition. The subjects competed in two separate distance triathlons 2 weeks apart, each consisting of a 1.5-km swim, a 40-km bicycle ride, and a 10-km run. One triathlon was under warm temperature conditions (77 degrees Fahrenheit), and the other hot conditions (87 degrees). Given the increased temperature, subjects were under greater heat strain during the second triathlon, and noticed lower performance as a result. The subjects taking glycerol had significantly greater retention of water compared to those consuming the placebo. This was associated with better performance during the second triathlon, as noted by a significantly lower increase in total race time (+1:47 min glycerol vs. +11:40 min placebo). The greatest improvements of performance were noted during the final 10-km run, when involuntary dehydration should have reached their highest levels.</p><p> Another placebo-controlled crossover study examined the effects of glycerol hyperhydration during exercise in a hot environment.400 Each subject pre-hydrated with water (24.1 ml/kg bodyweight) 2.5 hours prior to prolonged exercise at 60% Vo2max (equivalent in comfortable environment). When glycerol was added to the hydration regimen, there was an elevated sweat rate and reduced rectal temperature during exercise, suggesting that glycerol improved thermoregulation and reduced the impact of heat stress during exertion in a hot climate.</p><p> </p><p> In another placebo-controlled study, the effect of glycerol hyperhydration on performance was examined in a group of competitive cyclists.401 Subjects consumed 1 g/kg of glycerol along with 22 mL/kg of diluted Gatorade prior to exercise evaluations, which consisted of two 60-minute cycle ergometer time trials at constant and variable workloads. Glycerol hyperhydration resulted in a 600 mL increase in total body water over hydration with water alone. The glycerol group also noticed a 5% improvement in performance during the variable workload phase compared to placebo, though the researchers could not attribute this to a reduction in core temperature or lower cardiac output.</p><p> The clinical studies are in strong agreement that glycerol hyperhydration increases the retention of water over hydration with water alone. Many studies, however, conflict (in whole or in part) with studies such as the above, showing no improvements in temperature regulation or athletic performance. Why some studies show these improvements while others do not remains unclear. This may be due methodological difficulties, the rate or timing of hydration, or varying environmental or physiological circumstances in which glycerol hyperhydration can significantly improve performance. Some studies do suggest that the advantages are most apparent when access to water during competition is limited, and may be negligible when fluids are readily available and consumed during the activity.402</p><p> Overall, clinical studies do suggest that glycerol hyperhydration prior to prolonged endurance competition can delay dehydration and potentially even improve performance, especially when access to water may be limited during the activity. Further research is needed to better understand the optimal conditions for applying glycerol hyperhydration protocols. </p><h2> Empirical Evidence: </h2><p> Glycerol hyperhydration has been widely applied prior to prolonged endurance activities. The feedback on its use has been strongly positive. A majority of users report noticeable improvements with regard to involuntary dehydration, perceived sense of thirst, thermal stress, endurance, and overall athletic performance. A minority of users do not find glycerol hyperhydration useful, often because the practice induces strong gastrointestinal side effects that hinder comfort and performance. Since the majority of users experience minor or no side effects, as well as improved performance, glycerol hyperhydration has become an established practice in many areas of athletic competition (governing athletic rules permitting). Glycerol has an Empirical Evidence Rating of 5 (5/5).</p><h2> Effective Dosage: </h2><p> Based on clinical studies, a dosage of 1 g/kg (bodyweight) of glycerol along with 21-26 mL/kg (1-2 liters total) of water or a diluted carbohydrate-electrolyte drink (such as Gatorade with added water) over 2 hours prior to an endurance competition is recommended. An equivalent/effective dosage of glycerol monostearate has not been established.</p><h2> Side Effects / Safety: </h2><p> Glycerol is considered safe in doses below 5 g/kg of bodyweight.403 Although glycerol hyperhydration is generally well tolerated, some individuals do report gastrointestinal distress (bloating, nausea, cramping, diarrhea), or other non-GI related side effects such as headache and blurred vision.404,405,406 </p><p> </p></body>
