When deciding on the nutrition and hydration strategy you will adopt for the UTMB®, choosing an isotonic drink is an important decision to make well in advance of the race date. Here is some information to help guide you in selecting your isotonic drink and getting used to it easily.
What is an isotonic drink?
The term isotonic drink refers to the osmolarity, or tonicity, of a drink. Osmolarity is measured in mOsm per litre of solute, or mOsm/L, and quantifies the number of osmotically active particles in a solution — in particular sodium and glucose. The reference baseline for comparing the osmolarity of a solution is that of blood plasma, which sits between 275 and 300 mOsm/L: a drink is described as isotonic when its osmolarity is close to that of plasma, hypotonic when it is lower, and hypertonic when it is higher.
This concept is essential in the context of hydration: when two solutions separated by a permeable membrane have identical concentrations of osmotically active particles, water movement and particle transfer are optimal. Conversely, a hypotonic solution will absorb particles from the blood plasma, while a hypertonic solution will enrich the plasma too abruptly.
Choosing your isotonic drink
Simply put, an isotonic drink is a drink enriched and precisely dosed with mineral salts and carbohydrates — two essential nutrients in running and sport in general. These drinks come in bottles with the mixture already prepared, or as tablets or powder to mix with water. There is no universally better option; the main deciding factor is taste and your ability to tolerate the drink without affecting your digestion. In short, choose the one you prefer. Some are also available in savoury flavours, for those who quickly tire of sweetness. It is of course important to test your isotonic drink during your preparation. As your training progresses, make sure you tolerate it well and that the taste doesn't become unpleasant over time. To help with this, some brands develop different flavours — it can be a smart approach to have drinks of varying tastes that follow the same formula and come from the same brand.
Also make sure you always carry water with you and alternate from time to time. After a while, unpleasant sensations such as a sticky feeling in your mouth or stomach cramps can occur. After dozens of kilometres deep in the mountains, that's the last thing you need — so run plenty of tests and switch as soon as you feel a drink doesn't suit you. For drinks made from powder, be careful to follow the recommended dosage precisely, otherwise your drink will never be truly isotonic and will offer little benefit during effort. To find out more:
