Food is essential to the human body because it contains the things the body needs to stay alive. Nutrients are important, more specifically calories. Calories are the fuel that keeps the body moving. If you burn more calories than you take in, you will lose weight. If you take in more than you burn, you gain weight. There is a bit more to it than that, but when it comes right down to it, simplicity will work.
Many people have a hard time finding the critical balance between their level of activity and their calorie intake. In my experience it is because if I'm not doing anything, my mouth can be working. However, I find that if I'm doing a lot of activities, I tend to forget to eat. This is an issue when it comes to survival. You need to eat.
It is true that the human body can live on for weeks without food. Would you want to try? No. You would be so weak that simply eating food could kill you.
Should you find yourself a good three day's walk away from your house, you will need to have enough food on hand to last those three days. How much food? Well, that depends greatly on your metabolism, dietary habits, what type of food, and whether or not you are going to walk over easy terrain or up and down some steep hills.
A simple rule of thumb is that for every mile walked, you burn X amount of calories. Weight x Distance = Calories burned. It should be noted that if you walk slow, you will burn more calories than if you walked a normal pace because each time you step, you have essentially no momentum to carry you forward. However, at faster speeds, you also burn more calories because you utilize more muscle groups to keep up the pace. Also note that the weight of your pack will greatly impact your calories burned per mile.
Why is all this important? Because in order to find out how much food you need, you first need to know how many miles you might have to walk. Let's use my minimum 25 miles. I weigh around 300lbs. So, if I walked at a steady rate of 3 miles per hour (which is actually kind of slow at only 1 mile every 20 minutes), I would burn +-159 calories. Take that 159 calories and multiply it by 25 miles and we have 3975 calories burned. That's enough to drop one lbs of weight, plus some. However, you also burn calories when you are at rest, when you sleep, and every time you inhale and exhale. Heck, your heart is just a muscle that burns calories just to pump blood.
Where did I get that 159 calorie number? I got if from a chart.
Yeah, so what if I found it on the Internet? These are all just estimations anyway, and your results will vary.
What is important to know is that your body uses your energy reserves in the form of fats first. But once you burn off fat, you start to burn off muscle. Doesn't sound so bad at first, but once your heart rate reaches a certain point, you burn both fat AND muscle at the same time. So be cautious.
So, what foods then? Well, my favorite trail meals are from Mountain House Foods. They come in freeze-dried pouches that you simply put boiling hot water into and presto! Instant meal! My absolute favorite, the Chicken and Mashed Potatoes has 250 calories and 32 grams of protein. Protein is ready to use fuel as well as the calories. There is also a lot of sodium in this meal, which may or may not help to replenish electrolytes lost due to dehydration.
Energy bars are another thing to keep in your kit. I always have a Clif Bar or two on my backpacking trips. They have a crap load of everything, and 230 calories to boot. Carbs are also fuel, and they are loaded with them.
Now, if you can't stand carbs, then stop reading. Honestly, in an emergency situation, are you really going to care about your carb intake? Heck, you may not know where or when you will get your next meal, so you'd better pack them in while you can.
Another thing you could carry is a Mayday Food bar. Each bar has 3600 or 2400 calories. Obviously not something you'd eat in one shot, but you could just munch on it as you walk or break off a piece when you rest.
Don't run around and eat three big squares a day. You want to eat as soon as you wake up, and just keep munching every hour or two while you are on the move. This keeps your energy levels elevated and tells your body to metabolize. If you wait too long before eating breakfast, your body goes into starvation mode and will not metabolize as quickly because it doesn't have anything to jump start it. This also means that all those calories you eat get turned into fat instead of burned off as energy.
Remember that energy bars, Mayday bars, Protein bars, or whatever they call them these days require that you be moving. The Mayday bar has 400 calories per 3 ounce serving! That same serving has 17 grams of fat and 55 carbs! Normally, this isn't something you want to eat right before you go to sleep. However, if it is really cold outside, you should eat something right before bed. The energy will help keep your core temperature up while you are sacked out for the night.
Pro tip: If you wake up cold, grab a handful of nuts and eat them. Eating Oreo cookies is okay in small servings too. The sugars, protein and other calories will actually help warm you up. I tested this in a snow cave on Mt Rainier. It was 15 degrees outside, and just barely above freezing inside.
Your food should be highly nutritious and loaded with fuel (calories, protein, carbs, and some sugars). It should be small enough to get into a small pack. Don't store boxed foods because they don't have a long shelf life and are comprised mostly of AIR. If you get something like the Mayday bar (five year shelf life) or the Mountain House meals (7-12 year shelf life), then you keep rotation of food to a minimum. Foods like energy bars, beef jerky, or dried fruits and nuts should be rotated twice yearly. DO NOT store any food that requires refrigeration. Stay away from fresh fruits, veggies, and meats. They will spoil FAST.
Storing freeze-dried and airtight packaged foods may not seem appealing, but neither does an earthquake or some other catastrophe. What I suggest is that you buy different types of rations, freeze-dried meals, and various energy bars and weigh the palatability vs. energy. Don't just buy something because it is loaded with calories. If you don't like it, you might not eat it. You may even put off eating until absolutely necessary because said monster bar tastes like dog doo. If you must make concessions of nutrition and energy for taste, then do so. Just don't store a butt-load of Snickers bars because they taste better than Clif Bars. Be realistic. Storing one or two candy bars isn't a bad idea, but don't load an entire pouch with them. They are basically pure sugar. And while sugar is good for a boost, sugar will also make you crash when taken in large amounts. Don't crash! If you eat a candy bar, save part of it and only eat a nibble. This way, you get the burst of energy without the side-effect of a crash in about, say... 30 minutes!
Any backpacker store, or outfitter will have these freeze-dried meals and other types of food designed specifically for hikers. Just date them with the day you bought them and the date of expiration. When the expiration date draws near, pull them and take them with you on your next hike. Just remember to replace what you take from your 72 hour kit.