When you decide what nutrition and hydration strategy to 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 and steer you in selecting your isotonic drink and getting used to it easily.
What is an isotonic drink?
The term isotonic drink refers to osmolarity, or the tonicity of a drink. Osmolarity is measured in mOsm per litre of solute, or mOsm/L, and counts the number of osmotically active particles in a solution — in particular quantifying sodium and glucose. The reference baseline for comparing the osmolarity of a solution is that of plasma, which sits between 275 and 300 mOsm/L: a drink is considered 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 because when two solutions separated by a permeable membrane have identical concentrations of osmotically active particles on each side, 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
An isotonic drink, simply put, is a drink enriched and perfectly balanced with mineral salts and carbohydrates: two nutrients that are essential in running and sport in general. These drinks are available in bottles with the mixture already prepared, or in tablet or powder form to mix with water. There is no single best option — the main criterion comes down to taste and your ability to tolerate the drink without affecting your digestion. In short, you will choose whichever one you prefer. Some also come in savoury flavours, for those who quickly tire of a sweet taste. It is of course important to test your isotonic drink during your preparation. Check through your training sessions that your body absorbs it well and that the taste does not become too unpleasant over time. To that end, some brands develop different flavours: it can be a smart move to have drinks in varied flavours that follow the same recipe and come from the same brand.
Also make sure you always have water with you and alternate from time to time. Unpleasant sensations after a while include a sticky mouth or stomach aches. After dozens of kilometres deep in the mountains, that is far from ideal — so carry out plenty of tests and switch as soon as you feel a drink is not working for you. For drinks made with powder, make sure you follow the recommended dosage carefully, otherwise your drink will never be isotonic and will offer little benefit during exercise. To find out more:
