Q. Is an isotonic sports drink the best way to fuel a day’s walking?
A. It’s a bit of a buzzword, isotonic drink. What you are looking to do is, replace minerals lost through excessive perspiration like sodium, potassium, magnesium and, chloride. Also, muscles use up calcium and magnesium during prolonged exertion. And your blood sugar dips, so you need a relatively fast releasing sugar, at least when you are climbing a mountain.
When compared against hypotonic and hypertonic drinks it was found that: Hypertonic were rich in minerals, while hypotonic were more focused on replenishing the lost fluids. Quite obviously, isotonic drinks were found to hold a firm mid ground between these two.
You can make your own mineral rich energy cocktail to carry with you: coconut water is particularly good for the mineral, grape juice for fast-releasing sugars and apple juice for more sustaining energy.
I recommend eating apples for walking and bananas for climbing. Most so-called isotonic drinks are just glucose and water, with a few added minerals. Quite acceptable, and welcome, if you’ve just climbed a mountain, but otherwise (or anyway) I’d stick to these other natural ‘isotonic’ foods and drinks.
References  Bonetti, Darrell L.; Hopkins, Will G.; Jeukendrup, Asker. Effects of Hypotonic and Isotonic Sports Drinks on Endurance Performance and Physiology (pdf). Sportscience . 2010, Vol. 14, p63-70. 8p. 3 Charts, 1 Graph. ^Back to Top^