At a purely scientific level the electrolyte minerals, sodium and potassium, are involved in conducting electrical signals to/from muscles; calcium and magnesium are essential for the contraction and relaxation of the muscle fibres. These minerals work together to maintain normal electrical potentials and to coordinate muscle contraction/relaxation. Dietary basics are essential, but depending on your sporting activity and environment, maintaining optimum hydration, electrolyte balance and muscle glycogen levels may require assistance in the form of purpose designed sports drinks containing electrolytes.
In hot and humid conditions, sweat losses can be considerable – even when the duration and intensity of exercise are fairly modest. In such conditions, the main priority is fluid and electrolyte mineral replacement. Some carbohydrate replacement is also advantageous.
In cooler, less humid conditions and where the exercise duration is longer leading to significant reductions in muscle glycogen (for example over 1-1.5 hours), carbohydrate replacement becomes more of an issue, although fluid and electrolyte replacement is still vital.
The question of when you take on fluid and electrolytes can differ greatly from person to person. It’s useful to know how much you sweat but generally thirst is a good indicator that you need to start drinking more.
It is possible to work out your sweat rate and use this as a rough guide on how much to drink during exercise. Note your weight (naked) before and immediately after a long training session. By subtracting your weight post-exercise from your pre-exercise weight, you will be able to see how much fluid you have lost. You can then use this as a guide as to how much fluid you lose per hour; dependent on weather conditions, fitness level, exercise intensity and duration etc. Then, you can plan your nutrition strategy accordingly.
You must begin your endurance event fully fuelled and ready. Thereafter, you know that events over 90 minutes will require further fuel; ensure you take on this fuel, allowing for sufficient absorption time.
Correct hydration and fuelling strategies can be the key to a successful performance and avoiding feelings of lethargy and fatigue post event. By researching all of the advice from a range of companies you can develop your own strategy and find a consensus of opinion on how and when to take on nutrition.
When running, the ability of the body to ingest as much fluid as you sweat is almost impossible. Most rates of ingestion have not been seen above 1.3L/hr without leading to bloating and discomfort. When cycling however this can be increased because of the lesser abdominal pressures (1). From a triathletes perspective, it is worth knowing your sweat rates for both cycling and running as that can be different in sport and intensity. If you can start the run hydrated, then the issues surrounding discomfort with large volumes of fluid becomes less.
Without replacing electrolyte content (in particular sodium chloride), replacing fluid levels is near enough pointless as it is these salt levels that control fluid retention.
It has also been shown that repeated ingestion of fluids (including some carbohydrate) increases the rate of gastric emptying (2) due to the fact the emptying rate is higher with a fuller stomach. In other words taking on board repeated quantities of carbohydrate, salts and fluid leads to a higher overall quantity absorbed; hence the ability for your body to develop hyponatraemia, where too much water is absorbed without substantial electrolyte replacement. In hotter conditions, you can use this information to your advantage!
References
(1) Noakes, 2003;
Noakes T, (1985,2003) Lore of Running, USA, Oxford University Press
(2) Ryan et al 1989
Ryan, A.J., Bleiler, T.L., Carter, J.E., Gisolfi, C.V. (1989). Gastric emptying during prolonged cycling exercise in the heat. Medicine and Science in Sports and Exercise 21, 51–58.