Also known as: Glycerine, Glycerin, HydroMax, Glycerol monostearate, GMS
EnduranceGood evidence
A sugar alcohol used to hyperhydrate before exercise in heat, pulling extra water into the body and holding it there. Genuinely effective for a narrow purpose, and with an anti-doping history worth knowing.
Typical dose
1.0–1.5 g per day
60-90 minutes before the session, with 25-30 ml of fluid per kg
What it is
Glycerol is a sugar alcohol forming the backbone of every triglyceride. It is used widely as a food humectant and in pharmaceuticals, and is sweet-tasting and syrupy.
In sports use it is taken before prolonged exercise in heat, to increase total body water before the session begins. HydroMax is the common branded powder, typically around 65 per cent glycerol on a carrier, which handles better than pure liquid glycerol.
Glycerol monostearate, sold in some pump products, is a different compound used as an emulsifier and does not do the same job. The naming causes genuine confusion.
How it works
Glycerol is absorbed and distributes through body water, raising the osmotic pressure of body fluids. That increased osmolality reduces urine output and encourages the body to retain water it would otherwise excrete.
Taken with a substantial volume of fluid, the result is hyperhydration: total body water above normal before the session starts, which provides a buffer against the losses that follow.
The benefit is specific to situations where dehydration is the limiting factor. Starting a hot marathon with extra body water is useful; starting a gym session with it is not, and this is why glycerol is a niche tool rather than a general one.
What the evidence shows
Hyperhydration itself is well demonstrated. Glycerol taken with fluid reliably increases fluid retention and total body water compared with fluid alone, and this finding is consistent.
Performance evidence is more mixed but leans positive in the specific context it is designed for: prolonged endurance exercise in heat, where dehydration would otherwise limit performance. Meta-analyses report modest benefits under those conditions.
In temperate conditions or shorter sessions, benefits are absent, which is what you would expect. The pump and muscle fullness claims made in bodybuilding contexts have very little supporting evidence.
Dosing
Dose is per kilogram of bodyweight: 1 to 1.5 g per kg, taken with 25 to 30 ml of fluid per kilogram, over 60 to 90 minutes before the session. For an 80 kg athlete that is 80 to 120 g of glycerol and around two litres of fluid.
That fluid volume is not optional. Glycerol without adequate fluid does nothing useful, since there is no water to retain.
Expect to gain one to two kilograms of body water, which is the point but is worth knowing if you are weight-sensitive. Practise it in training before using it in competition, since the volume of fluid alone takes getting used to.
Hyperhydration persists for several hours after loading
Timing
60-90 minutes before the session, with 25-30 ml of fluid per kg
Competition status
Previously prohibited in sport as a plasma expander; removed from the prohibited list some years ago. Tested athletes should verify the current position with their governing body before use.
Category
Endurance
Safety and side effects
The most common effects are digestive: nausea, bloating and a feeling of fullness, largely from the sheer volume of fluid involved. Headache and dizziness are also reported.
The weight gain from retained water is expected rather than adverse, but it does mean glycerol is unsuitable before any weigh-in.
Competing athletes should be aware that glycerol was previously prohibited in sport as a plasma expander and was removed from the prohibited list some years ago. Status can change, so anyone subject to testing should verify the current position with their governing body rather than relying on this page.
Who should avoid it
Speak to your doctor before use if you have been told your kidneys are not working normally, or if your blood pressure is being managed. Unsuitable before a weigh-in due to water retention. Practise in training rather than trialling on competition day. Competing athletes should verify current anti-doping status with their governing body.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
