You can make your own mineral-rich energy cocktail to carry with you: coconut water is particu-larly good for the minerals, 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.