Glycerin, best known in sports as glycerol, is used as a dietary supplement aimed at boosting hydration.
What is Glycerol?
Glycerol is an alcohol containing 3 hydroxyl groups (1,2,3-propanetriol), our body produces and recycles glycerol continuously as it participates in numerous processes of metabolism of carbohydrates and fats.
Figure I. Molecular structure of glycerol.
What Glycerol does in our body
The glycerol that our own body produces and consumes is part of a kind of “closed loop”: at normal glycerol concentrations (i.e. without specifically administering it through supplements or drugs), its elimination is minimal, almost imperceptible.
This is because the glycerol that is “released” travels in very low amounts through the blood and goes where it is required to convert it into something else or spend it; that is, «the hens that enter through where they leave.»
Glycerol is metabolised in the liver and kidneys to glycerol-3-phosphate, which is the “key” metabolite through which glycerol works in the body:
- Creating new triglycerides by binding 3 fatty acids to their carbonated skeleton.
Figure II. Chemical structure of glycerol and triacylglyceride (glycerol 3 fatty acids).
- Transforming into dihydroxyketone, which:
- Will be transformed into pyruvate and:
- Will be transformed into Acetyl-CoA to be used as energy in the ACT (during aerobic exercise)
- It will be integrated into the glycolysis process to obtain energy from it (during anaerobic exercise), converted to lactate and reintegrated into glucose through the Cori cycle to be used again in glycolysis.
- Will be transformed into pyruvate and:
Figure III. Endogenous glycerol metabolism.
You may see it clearer in the image (starts from bottom right):
Figure IV. Exogenous metabolism of glycerol.
This has led many researchers to think of glycerol as a source of energy.
Why do we supplement with Glycerol
Well, we have seen the role of glycerol in the body, but then why do we see athletes taking glycerol before competitions?
It has already become clear to us that it’s not for energy, as it’s not efficient. Then…
They use it because glycerol is a hyper-hydrating agent.
That is, they use glycerol to prevent, treat, or replace body fluids and prevent dehydration.
Figure V. Boxer rehydrating.
How does a glycerol supplement work?
We already know the metabolism of glycerol at physiological concentrations, but when administered from outside, it changes, it is no longer at “normal concentrations”, and its metabolism is completely altered. How? Pay attention!
The glycerol we consume freely is transported quickly in the cells of the intestine, presumably sharing transporters with glucose, so absorption is fast and requires no digestion of any kind.
Figure VI. A mechanism for transporting glucose (shared with glycerol) in the epithelial cells of the human intestine.
Once the basolateral membrane passes into the blood where it acts as an osmolite, i.e. increases the concentration of solutes by increasing the pressure in the compartment, and because the body has to be balanced, we co-administer it with water, increasing the total amount of water in the body.
The body always has to maintain a concentration of “little things” in the middle (“water”), this concentration is 275-295mmol/kg; but that doesn’t interest us, let’s imagine:
We have a 2 litre bottle where we throw in a litre of water and 100g of whatever, sand for example. The concentration of sand is 10% (1000/100=10).
Imagine that now in the same bottle you add another 100g of sand, the concentration has changed, and has become 20%; as you always want to maintain the 10% concentration, you add one litre of water and it rebalances (2000/200=10).
With glycerol it passes in the same way through the body: if we consume a large amount of glycerol without water instead of hydrating, it would dehydrate us, but as we consume it with water, it is like adding more water and sand to the bottle to maintain balance.
We are balanced!
Figure VII. Osmotic exchange representation from the compartment with less solutes to the compartment with more solutes to balance concentrations.
When concentrations increase in the blood, renal elimination also increases dramatically, and it is no longer a closed circuit.
Figure VIII. Renal metabolism of glycerol where the great reabsorption to the plasma is observed.
Why use glycerol?
Dehydration is an extremely common phenomenon among athletes of virtually any sport discipline.
Dehydration of even less than 2% can seriously affect the performance and health of the sportsman. Amongst other phenomena:
- Reducing blood supply to muscle tissue, because as there is less plasma (water), central pressure needs to be maintained.
- Body temperature increases, we lose the ability to thermoregulate and dramatically increase the risk of potentially lethal cardiac arrest.
- Athletes who use glycerol pre-training use it to become hyperhydrated.
- Those who use it in-training do so to not dehydrate.
- Those who use it post-training do it to rehydrate.
How to use Glycerol
The most widespread protocol for glycerol use is based on its pre-training use.
What it’s used for
It is used as an osmolite together with large volumes of water to prevent dehydration during the race.
When to use it
In workouts where we expect water losses from sweat to be around or exceed 2% of body weight.
How much to use
Here there’s a bit of a debate… I’ll explain: The amounts of glycerol circulating in the blood are around 0.1 mmol/kg, and can reach 3 mmol/kg in extreme cases of high lipolysis.
To “hyperhydrate” you need to reach 15mmol/kg.
The problem is that we do not know why (it is for some aspect related to intestinal absorption), glycerol levels in the blood are not the same in all people, and certain people respond much more than others.
The proposed dose response curve had been postulated to be linear, i.e. at more doses, more effect.
Figure IX. Response of plasma glycerol concentrations based on the dose consumed, without liquid (continuous line) or with liquid (dashed line).
However, the study graph is from a study published in 98, and a 2010 review found that at 1.5g/kg the amount of glycerol removed by urine was 2 to 3 times greater than that eliminated if we took 1g/kg.
They found that the optimal dose was about 1-1.2g/kg; 1.2g/kg was more than double the effective amount of water retaining than the dose of 1g/kg.
How much water to use with it
Let’s get straight to it, the ideal amount of water to maximise fluid retention is 26mL/kg body weight.
How to take it
Ideally in a oner, that is, take all the glycerol, take it dissolved in a little water and drink the amount of water you need after glycerol. You don’t have to be a genius to see that…
26mL/kg * 70kg = 1820mL = 1.82L. of water…
In about half an hour or so (since our water absorption limit is about 400mL every 10 minutes).
Although this, in my opinion, if the race is not very long, may be less effective.
How long before I have to take it
The plasma glycerol peak is reached between approximately 60 and 150 minutes, so it is also the ideal time to use the hyperhydration protocol.
Figure X. Response of glycerol plasma concentrations in relation to the time from consumption of a dose of 1.2g/kg in bolus.
The possibility that co-administration of a sports drink (with carbohydrates and sodium) may be superior to water is assessed, and this has been seen in in vitro studies with mouse epithelial cells.
However, in my opinion, co-administering glucose is a mistake, as it can saturate intestinal transporters with large amounts of glycerol and cause us gastrointestinal discomfort.
i.e. consume 1g of sodium in the volume of water proposed for a man of 70kg (1820mL).
How long should it last
Until plasma glycerol amounts are suboptimal, starting from the fact that we have reached approximately 20mmol/L (standard response before 1.2g/kg glycerol) -> 4 hours.
Another protocol of fairly widespread use, but less frequent, less studied, and a priori, less effective than pre-training.
What is it used for
To rehydrate, to maintain water concentrations in the body during exercise despite sweating.
When to use it
During exercise that can cause the athlete to lose close to (or more than) 2% of his/her body weight through sweat.
When not to use it
Given that the average sweating rate is 0,5-2L/hour, if exercise lasts less than 75 minutes, intra-training glycerol should not be used if pre-training has already been used.
And if it lasts longer than 75 min?
If it lasts more than 75 min and I have not hyperhydrated before physical exercise?
Consume 0.4g/kg body weight + water (at the concentration you prefer) within the first 4 hours of physical exercise (you should consume between 1.6 and 3.2l. of water approximately in those 4 hours).
The most unknown protocol for the use of glycerol.
What is it used for
To rehydrate after physical exercise, especially when weight loss is big
What to take into account
- If you don’t get exercise again until the next day, or more, rehydrate with water and food, don’t worry about hyper-re-hydration.
- If you exercise again in a few hours use 1.5L of liquid for every 1kg of mass you have lost (you must weigh before and after); and accompany every 1.5L of water with 1g of glycerol/kg body weight.
What can I expect from glycerol supplementation?
At the above doses, an increase in body fluid retention compared to water consumption, or sports drinks.
Figure XI. Water retention in various trials with different doses of glycerol, water, and sports beverage simulations.
As we have seen, the effects are dose-dependent up to 1-1.5g/kg where they stabilise.
Figure XII. Water retention over the physical exercise time based on the dose of glycerol + NaCl or only NaCl.
And this increased water retention will help you to keep you performance free from dehydration!
Glycerol is a hyper-hydrating agent, which acts as an osmolyte and is effective at improving hydration status if appropriate protocols are used.
The problem is that the most widely marketed format is glycerol stearate (mono-, di-, or tri-stearate), which is glycerol bound to 1, 2, or 3 fatty acid chains, respectively. That must be split, we no longer know at what rate it is absorbed, it no longer feels as good on a gastrointestinal level as free glycerol… a problem…
In addition, the recommended doses are usually around 5-10g. Which, as we have seen, between that and not taking anything, is the same.
Taking into account the price of glycerol, which is expensive to produce, and expensive to buy; and the amounts necessary to have an ergogenic effect:
- Montner, P., Stark, D. M., Riedesel, M. L., Murata, G., Robergs, R., Timms, M., & Chick, T. W. (1996). Pre-exercise glycerol hydration improves cycling endurance time. International Journal of Sports Medicine, 17(1), 27–33.
- Nelson, J. L., & Robergs, R. A. (2007). Exploring the potential ergogenic effects of glycerol hyperhydration. Sports Medicine, 37(11), 981–1000.
- Robergs, R. A., & Griffin, S. E. (1998). Glycerol: Biochemistry, pharmacokinetics and clinical and practical applications. Sports Medicine, 26(3), 145–167.
- Van Rosendal, S. P., Osborne, M. A., Fassett, R. G., & Coombes, J. S. (2010). Guidelines for glycerol use in hyperhydration and rehydration associated with exercise. Sports Medicine, 40(2), 113–139.
- Wagner, D. R. (1999). Hyperhydrating with glycerol: Implications for athletic performance. Journal of the American Dietetic Association, 99(2), 207–212.
- Wingo, J. E., Casa, D. J., Berger, E. M., Dellis, W. O., Knight, J. C., & McClung, J. M. (2004). Influence of a pre-exercise glycerol hydration beverage on performance and physiologic function during mountain-bike races in heat. Journal of Athletic Training, 39(2), 169–175.
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