Whittaker Mountaineering Guide Team member, Caroline George on 'what to eat' in the mountains:
There is no one way to eat in the mountains. What might work for one person may not work for another, so it's important to figure out how your body reacts to different food and drinks before you actually start on the climb. As a general guideline, it's good to eat a few healthy and slightly more caloric meals during the days preceding a climb to stock up on energy. During your approach to camp, stop every hour to drink and nibble on a light snack. Once at camp, rehydrate thoroughly. I personally try to drink a warm beverage because it performs the double task of hydrating me and that of keeping me warm. Supper should consist of an easily digestible meal so that it doesn't prevent you from sleeping. Also, the stress of the climb might tie your stomach in knots, which could in turn make the digestion process harder. Be mindful of how much you eat the night before the climb.
On D-Day, a small cup of oatmeal and a warm drink is enough to get me going in the morning. While climbing, take advantage of every break to drink and snack. As it's not always possible to stop every hour, I keep something to eat in my pants or jacket pocket in case of a sudden craving. We all react differently to temperatures, the stress of the ascent, the weather, the altitude, etc: some people need to eat a lot, others have no appetite. What matters however is to eat a little bit every hour: don't eat your whole sandwich in one go because it will literally take your breath away when you start ascending again. A little bite at a time will do the trick.
This is what I would typically take on a climb with me:
Approach Snack: a big cookie, an orange, nuts (I like Maple Pecans because they are sugar coated so it gives me an instant kick), bars, a sandwich.
To rehydrate at camp: an electrolyte powder (Nunn, Gatorade, etc.), a soup or some broth.
In the evening: mashed potatoes (super light to carry) and a package of tuna fish. Some chocolate and an herbal tea for desert.
The morning of the climb: 1.5 small package of Quick Oatmeal and some tea/coffee.
On the climb: a few bars, nuts, chocolate, gels or cliff blocks, a thermos full of a warm drink or a platypus depending on the temperatures. I bring food that I am really excited to eat so that I can motivate to eat them on the mountains.
After the climb: a nice meal, lots and lots of liquid and if you're afraid you might be sore, take an Ibuprofen.