When fine-tuning your nutrition and hydration strategy for the UTMB®, choosing your energy drink is a key milestone well before race day. Here are a few guidelines to understand the differences and choose based on the situation.
What is an isotonic drink?
The term isotonic drink refers to osmolarity, or tonicity. Osmolarity is measured in milliosmoles per litre of solution and quantifies the concentration of dissolved particles, mainly sodium and carbohydrates. The standard benchmark is blood plasma osmolarity, between 275 and 300 mOsm/L.
There are three main categories, depending on how fluid moves between the gut and the bloodstream:
- A hypotonic drink has a lower concentration than blood plasma, facilitating rapid water absorption from the gut into the bloodstream. It hydrates fast, but provides fewer calories per sip.
- An isotonic drink provides a neutral balance: neither drawing nor retaining fluid significantly, while delivering a balanced supply of both water and energy.
- A hypertonic drink has a higher concentration than blood plasma, temporarily drawing water from the bloodstream into the gut before absorption. It hydrates more slowly, but delivers far more energy for the same fluid volume consumed.
Which should you choose? It depends on your limiting factor
This is the point most articles miss: none of the three is inherently better than the others. The right choice depends on your limiting factor at any given moment: hydration or energy.
| Context | Limiting factor | Suitable drink |
|---|---|---|
| Heat, heavy sweating | Water | Hypotonic or isotonic |
| Moderate conditions, long effort | Both | Isotonic |
| Cold, night, low sweat rate | Energy | Hypertonic , with plain water on the side |
| Frequent water points | Carried weight | Hypertonic concentrated, diluted by drinking plain water |
| Post-effort recovery | Glycogen replenishment | Hypertonic |
Two specific scenarios deserve focus, as they directly impact your UTMB® race strategy.
Night sections at high altitude. Temperatures drop and you sweat less, but energy demands stay high. A more concentrated drink delivers the carbohydrates you need without forcing down litres of fluid your stomach does not want. Conversely, in warm valley sections, hydration becomes the priority.
The concentrate-plus-water strategy. Since aid stations offer plenty of water, many experienced runners carry a highly concentrated flask and drink plain water alongside it. Mixing happens naturally in the stomach, substantially reducing carried weight. This is also the exact principle behind energy gels: a gel is hypertonic by nature, which is why taking it with water is strongly recommended.
Choosing your energy drink
An energy drink is formulated with mineral salts and carbohydrates. It comes pre-mixed in bottles, or as tablets or powder to dissolve in water. There is no single best option: the main decision factors remain taste and your digestive tolerance.
Powder or tablet options offer a decisive edge in ultra running: they are lightweight, let you refill your flask at every aid station with the water provided by the race, and crucially let you adjust concentration depending on race conditions.
When it comes to dosing, the baseline is to follow manufacturer guidelines, as the formula is balanced for that specific concentration. But adjusting it deliberately is not a mistake: it is a calculated choice, provided you know what you are doing and drink plain water alongside a more concentrated mix. The real issue is inaccurate mixing done by mistake in the dark with a poorly rinsed flask.
Test your drink during training over several weeks, including in concentrated form if you plan to use it that way. Always carry plain water alongside it to alternate and prevent taste fatigue.
Frequently asked questions
What is the difference between isotonic, hypotonic, and hypertonic drinks?
An isotonic drink has an osmolarity close to blood plasma, between 275 and 300 mOsm/L. A hypotonic drink is less concentrated, hydrating faster but supplying less energy. A hypertonic drink is more concentrated, absorbing more slowly but delivering significantly more carbohydrates per sip.
Should you avoid hypertonic drinks?
No. It is particularly effective in cold weather, at night, or when water is readily available at aid stations: it supplies plenty of energy without adding extra fluid volume. It is less suitable in intense heat, when hydration becomes the primary limiting factor. Energy gels, in fact, are naturally hypertonic.
Can you intentionally mix your powder more concentrated?
Yes, this is a standard ultra strategy to cut pack weight, provided you drink plain water alongside it and have tested it in training. The concentration itself is not the problem—running into digestive distress because you didn't test it is.
Should you choose powder or ready-to-drink options?
Powder is lighter, lets you refill flasks at aid stations, and allows you to adjust concentration as the race unfolds. Ready-to-drink options remove the need for mixing. The main criteria remain your digestive tolerance and taste.
Find out more
- Nutrition and hydration strategy for the UTMB®
- Energy gels and bars for the UTMB®
- Gut training for ultra-trail
- Aid stations and nutrition during the UTMB®
Photo credits: Clément Dumas, Delphine Daniélou.
