Hydration is a hot topic. Everyone seems to want to know how to hydrate right. It gets confusing with so many products on the market. There are hydration powders, sports drinks, electrolyte tablets, salt tablets, coconut water, plain old water, and more.
Beyond which product you should use, there’s something else to consider. Should you pre-hydrate? What is that? How do you do it?
If you have a big endurance event, such as an all-day tournament, marathon, or triathlon, you want to start out hydrated. It has great uses for firefighters and military personnel on long missions as well. These people depend on staying hydrated because their life depends on it (and yours might too!).
If you read any of the scientific literature, you’ll see the term “euhydrated.” Starting out euhydrated means you are adequately hydrated. You’ll set yourself up for better success. Having a smart hydration plan for the day will help you maintain performance. Research shows that becoming dehydrated can lead to decreased concentration, increased perceived fatigue, and decreased overall performance. Clinical dehydration can even lead to death.
On the flip side, becoming over-hydrated can be serious as well. This condition, known as hyponatremia, occurs when someone drinks too much water, thereby diluting electrolytes (namely sodium). This is also a serious and potentially deadly condition that needs immediate medical attention.
Considering all this, an athlete that is in that sweet spot of happy hydration is an athlete that can perform their best. Enter pre-hydration, AKA hyperhydration.
This is different than euhydration, over-hydration, or hyponatremia. Pre-hydration is a strategic way to start well-hydrated before your event. You are adequately hydrated, but with an edge.
Glycerol is a substance that allows the body to hold on to a bit more fluid than you normally would. It is called a plasma expander. For this reason, until recently it was a banned substance for competitive athletes by the World Anti-Doping Agency (WADA). In January of 2018, it was officially removed from the banned list.
Here’s the backstory: if an athlete is taking a substance that will expand their plasma volume, it would potentially dilute everything else (including illegal substances) in their blood. This might mask drugs or other substances during a doping test. More recent scientific studies have concluded that the dilution and masking effect is minimal and this isn’t a concern. In addition, the average person can consume glycerol on a daily basis unintentionally, which makes it hard to avoid. Also, glycerol is a natural substance in the body. It is the backbone of triglycerides. Endurance athletes that exercise for long periods of time, like an ultra-marathoner, use fat as part of their fuel source. This fat burning, called lipolysis, can result in elevated plasma glycerol levels. So an athlete in this situation, if tested for glycerol, may test positive. For these reasons, WADA has removed glycerol from the banned substance list.
So what does this mean for you? Well, you don’t have to over-think hydration. Most people in normal circumstances would not need to use glycerol to pre-hydrate. However, an athlete that is about to embark on a really long endurance event, especially one that takes place in heat or humidity, might want to try this. As with all new things, test it first in training! Nothing new on race day EVER!
Also, if you are at the level where you would be considering pre-hydration strategies with glycerol, seek out the help of a sports dietitian. Your hydration and nutrition shouldn’t be a DIY affair. It’s worth the time to get professional guidance.
The dosed in most studies is 1.2-1.5 g/kg body weight. It is often taken with 500 mL of aspartame-sweetened water (like Crystal Light) to make it taste better. Spreading out the dose over two hours also enhances fluid retention. Athletes stayed well-hydrated after four hours or longer of exercise. Taking 7.5g of salt with the glycerol seemed to decrease urine production (thereby retaining water longer) which was a good thing. In this study, the dose was 7.5g of salt with 30mL/kg body weight (fat free mass) and 1.4g/kg fat free mass glycerol.
Athletes found improved thermoregulatory responses (meaning body temperature regulation) when exercising in the heat, decreased thirst, improved endurance capacity, decreased heart rate, and improved peak power capacity, and improved time to exhaustion compared with a group that pre-hydrated with just water.
One drawback is that when you have more fluid in your body, you are a bit heavier. Runners in particular like to feel light, and there is a perception that if you are heavier you will go slower. One study looked into this, and the running economy with the runners using glycerol for pre-hydration was the same as the other runners. Another concern is that side-effects can include headaches, dizziness, and nausea.
The take home message is that hyperhydration can be a useful strategy for endurance athletes during a long event, over four or more hours, especially in the heat or humidity. For normal everyday training it’s not needed. Be sure to seek advice from a qualified medical professional to guide you in your hydration strategies. Don’t try to pre-hydrate with glycerol on your own.
Some selected articles on pre-hydration:
Goulet EDB, Rousseau SF, Lamboley CRH, Plante GE, Dionne IJ. (2008). Pre-exercise hyper hydration delays dehydration and improves endurance capacity during 2h of cycling in a temperate climate. Journal of Physiological Anthropology, 27(5): 263-271.
van Rosendal SP, Coombes JS. (2013). Glycerol use in hyper hydration and rehydration: scientific update. Medicine Sport Sci, 59, p. 104-112.
Goulet EDB, De La Flore A, Savoie FA, Gosselin J. (2017). Salt + glycerol-induced hyperhydration enhances fluid retention more than salt- or glycerol-induced hyperhydration. International Journal of Sports Nutrition and Exercise Metabolism, DOI:10.1123/ijsnem.2017-0310
~Disclaimer: This is general nutrition information. Consult your doctor before changing any nutrition or fluid intake.