Depends, how long are you training? I'm not familiar with the effort required for mountain climbing, but eating a varied meal beforehand would help. Personally

Depends, how long are you training? I'm not familiar with the effort required for mountain climbing, but eating a varied meal beforehand would help. Personally, a peanut butter sandwich, banana, half an avocado (depending on the size of the avocado), tall glass of milk, and celery with hummus will prepare me well for the following few hours. Adjust this for yourself as needed. A healthy balance between fat, fiber, carbs, and protein is what you're looking for. I can't give any any hard numbers but a good macro-nutrient balance is the first step to a healthy diet, with the second step being fresh food.