Nutrition: What Do I Need?
I make no claims to be a nutrition expert: these notes are based on my experience, the experience of others and my reading on the subject. I've tried to collate information largely specific to walkers, hikers, backpackers and mountaineers. Information on general nutrition may not cover the specific needs of the outdoors person and the whole area of sports nutrition can be oriented towards those involved in very intensive activity (eg running).
The food we eat contains various essential components, such as carbohydrates, proteins, fats, vitamins, minerals and fibre. It's important to get a balanced diet and to ensure you eat enough of the right food to keep you going. The longer the period you are planning for, the more important the overall balance is: if you miss some vitamins and minerals on a day hike you can catch up easily, on a 3-week backpack you need to be a lot more careful. However, eating a good range of foods will usually ensure a sensible balance.
Carbohydrate is particularly important for the outdoors person: it provides the energy used by our muscles. Complex carbohydrates (from cereals, pasta, rice, etc) release energy slowly, which is exactly what's needed. Sugar, by comparison, gives a rush, but disappears quickly, sometimes resulting in a crash as your blood sugar level plummets.
Fats get a bad press, but in fact they are a useful food source for the outdoors. Unsaturated fats from vegetable sources are often recommended, especially nuts, which apparently have the fats packaged up very well for our bodies to use (I knew those squirrels were onto someting good!). I've also seen reports that having a reasonable proportion of fat in your diet actually helps the body consume its own fat resources!
Proteins are used to repair and grow muscles: you spend all day punishing your muscles so be kind and give them a chance to recover!
From the information I've seen, a reasonable diet for a 2-week backpacking trip, for example, might comprise 50% to 60% carbohydrates, 20% to 30% fats and 15% to 20% proteins. Using a good variety of foods to provide this should ensure you get enough vitamins, minerals and fibre too.
Food planning for long journeys without frequent resupply has its own challenges. Working out the calories likely to be used per day on a hillwalking trip could give totals ranging from 2,500 to 8,000 for a male adult, depending on how strenuous the route is. If you then translate those numbers into actual food the amounts can get alarming! In practice, many folk have found that there is a limit to how much food they can realistically carry and realistically eat; these limits can often result in fewer calories eaten than burned. Probably the best example of this is polar exploration, where travellers can lose very significant amounts of body weight despite consuming a lot of calories.
It's not uncommon for walkers doing long trails, such as the Pacific Crest Trail in the USA, to plan on carrying less food than actually needed, and then binge eat when they get to a town: some binges are legendary! Whether this is good for you is debatable, but it's one solution to a practical problem.
My own experience is that on a typical 10-day backpacking trip I will carry enough food so that I am never hungry and feel pleasantly satisfied after a meal, but I will have lost a few pounds in weight by the end (which in my case is no bad thing at all). I'm probably not eating more than 2,500 calories per day at the most, yet I could well be using twice that.
So I guess what I'm trying to say is don't get too hung up on working out calorie use. Rather, do some experimenting and see how much food you will realistically consume: you will fairly soon find a balance that works well for you. One interesting fact that comes from looking at standard equations to work out calorie requirement is that they are based partly on your body weight; so if you want to reduce the calories of food you need to carry, losing some weight will help!
Some types of trip are special cases. At higher altitudes, appetite drops and it can be a real struggle to get enough food down. Mountaineers typically find that this is one of the limiting factors on how long they can stay high on a peak. Polar travellers can use very high fat diets and consume outrageous numbers of calories: read some of Ranulph Fiennes' books for an insight into this.