Can Glycerol Improve Performance?

We have a previous article about fluid intake and performance if you want to delve deeper:

"For every liter of water we lose, our body temperature increases by approximately 0.3°C. The blood volume that our heart pumps out each minute decreases, leading to an average increase in heart rate by 8 beats per minute at a given exertion level (Ref: Gisolfi, C.V, Fluid Balance for Optimal Performance, 1996, not in digital format). An increase of just a few heartbeats over time can cause the sensation of fatigue to arrive much sooner than planned. Therefore, adequate fluid intake is very important for performance, especially if you're active for more than 2 hours and/or on days when the temperature has exceeded 25°C."

A supplement that's sometimes discussed in this context is glycerol—a substance that can increase the body's fluid retention and thus potentially improve your performance in heat.

But does it really work in practice? And if so, for whom and in which situations?

In this article, we explore what glycerol is, how it affects the body, what the research says about performance effects, as well as the practical advantages and disadvantages for endurance athletes.

Glycerol (also known as glycerin) is a simple molecule (C3H8O2) that is chemically classified as an alcohol and naturally occurs in the body. It serves as a primary building block in triglycerides, which are fats stored in adipose tissue and used as an energy reserve.

As fat breaks down in the body, glycerol is produced, which can be utilized in various metabolic processes, such as the formation of new glucose in the liver.

In terms of sports nutrition, glycerol is notable because it has a significant osmotic effect — it binds water. If you're into gardening and decorate with plants at home, you might have used glycerol to keep the leaves preserved a bit longer by retaining more moisture in the plant.

When glycerol is consumed along with a substantial amount of fluid, it can:

• Increase fluid retention in the body
• Reduce urine production
• Increase plasma volume (blood volume)

This phenomenon is often called hyperhydration.

Normally, the body maintains fluid balance with precision. If you consume large amounts of water without electrolytes or other osmotic substances, a significant portion of it will be quickly expelled through urine. That's why it's advisable to drink a carbohydrate/salt solution (e.g., a sports drink) instead of plain water, especially in hot conditions when you're sweating a lot.

However, since glycerol is osmotically active, it draws in water and helps retain fluid within the body's compartments, particularly in the blood and between cells. As a result, you may have more fluid in the body/blood, fewer bathroom breaks, and potentially better temperature regulation.

Several studies have shown that glycerol can increase the body's fluid volume by about 300–700 ml compared to just drinking water.

During intense physical activity in the heat, multiple things occur simultaneously:

• Sweating results in fluid loss
• Blood volume decreases
• Body temperature rises more quickly
• The heart has to work harder to deliver oxygen

When blood volume decreases, cardiovascular stress increases. Heart rate and perceived exertion rise despite the same workout intensity. By boosting the body's fluid reserves before starting, there's a theoretical opportunity to delay dehydration, stabilize blood volume, improve temperature regulation, and just have a body that's slightly less cardiovascularly stressed. Or, to be honest, we all know it won't be less stressed; we'll just push harder and perform better #pushfurther

This is the mechanism behind the interest in glycerol.

Glycerol was synthesized as early as the late 1700s. However, research on glycerol and performance is considerably younger (without being new), having begun in the 1990s and taking off in the early 2000s.

A consistent finding is that glycerol combined with fluid increases the body's fluid retention and reduces urine production compared to water intake.

In a study by Montner et al. (2001), hyperhydration with glycerol significantly increased total body fluid and improved time to exhaustion during exercise in heat.

Earlier studies, including those by Lyons et al. (1990), demonstrated that glycerol can:

• Boost plasma volume
• Enhance temperature regulation

However, the results are not entirely consistent. The study by Lyons et al showed no clear performance effects, only effects on fluid retention itself. It should be noted, though, that the majority of research examining performance markers also observes performance improvements compared to a control group, primarily in hot environments and/or during longer endurance activities (+60 min).

The classic protocol in research is:

1–1.2 grams of glycerol per kg of body weight combined with 20–25 ml of fluid per kg of body weight, taken approximately 60–90 minutes before starting.

For an athlete weighing 70 kg, this means approximately:

• 70–80 g glycerol
• 1.4–1.8 liters fluid

This is quite a large volume of liquid, which can be logistically challenging in practice. You'll retain the fluid in your body for about 2-4 hours from this point.

How does Glycerol-induced hyperhydration compare to Sodium-induced hyperhydration, such as a classic salt load where the kidneys retain more water?

Often, a salt load can be more advantageous than a glycerol load. It typically involves consuming 7-10g of sodium chloride (table salt) alongside 1-1.5L of fluid 60-120 minutes before starting. A common strategy is to also consume carbohydrates, combining fluid retention with the replenishment of glycogen stores.

A salt load has been shown to bind 300-600ml of fluid in the body, which is similar to Glycerol loading but with a lower risk of stomach issues.

Based on the research, the effects seem to be most relevant when:

1. Competing in heat (+25 degrees)

2. Long-duration endurance efforts (over 60-90 minutes)

3. Limited fluid availability

The third point might not be very relevant for the majority of races. But if you're gearing up for Cykelvasan or Vätternrundan this summer where an extra 300-700ml weight in the form of bound fluid doesn't significantly impact you, it could be a good idea if you know you often get dry/thirsty and can't bring more water on the frame/in your pockets, or if you're the person who needs to pee three times but the group you're with doesn't quite plan to stop 😀

A little side note (if you were thinking about it) is that glycerol does not work as a carbohydrate loading. It's too inefficient, as it's actually not a carbohydrate but an alcohol. A small portion of glycerol can, via gluconeogenesis in the liver, help maintain blood sugar. But if you're consuming carbohydrates during activity and have plenty of liver glycogen, the glycerol will barely be noticeable. So there are no such advantages with glycerol, unfortunately.

For many athletes in Sweden, heat stress isn't always the biggest limitation. The effect of glycerol is probably minimal or nonexistent when competing in cool climates, if the event duration is under 60 minutes, or during competitions where there are hydration stations. In those cases, it's likely unnecessary for you to carry 500ml of fluid yourself when water or sports drinks are served continuously.

In these situations, a good hydration plan during the activity is usually more than sufficient. For example, drinking a cup of fluid at each station, sipping from the bottle every 20 minutes, or something similar.

Most studies report relatively mild side effects, but some do happen.

• Gut issues
• Bloating
• Nausea
• Stomach discomfort
• Headache (likely related to changes in fluid distribution.)
• Temporary weight gain

As fluid is retained in the body, body weight can rise by 0.5–1.0 kg. For certain athletes, like runners, this additional weight can somewhat offset the performance benefit.

Glycerol has had a rather unique history in anti-doping.

From 2010 to 2018, glycerol was banned by the World Anti-Doping Agency because it could alter urine concentration and thereby mask doping substances.

Since 2018, glycerol is permitted again, as it was found that this effect was limited in practice.

The collective research offers a nuanced view. Glycerol is effective, but only under the right circumstances—typically when it's hot and fluids are not readily accessible during the race or training. In these cases, the benefits outweigh the drawbacks, such as an increase in body weight of 0.5-1kg.

In summary, glycerol is a fascinating tool for hyperhydration and can boost performance in hot conditions, but its effectiveness is situation-specific and not universal.