Experiencing extreme weather situations can be fun, but you must increase your calorie intake in order to operate at peak performance in such situations.
In my experiences with military training, me and my "battle buddies" were exposed to tough conditions including sleep deprivation, extremes of temperature, physical exertion to exhaustion, and critical thinking exercises in the midst of these circumstances. There is certainly no comparison between the ordinary military training that we experienced and the extraordinary training that elite units such as the Special Forces and Navy Seals have to endure. Nevertheless, we gained valuable experience about surviving in a field environment in various weather conditions. The truth about field survival is that you will not often find yourself grubbing for berries in order to make it out of starvation (this is the popularly romanticized situation). The challenge of field survival is merely to operate normally in a different environment.
Most Army Basic Training centers and bases are located in the Deep South: the two that are highest in latitude are Ft. Leonard Wood, Missouri and Ft. Knox, Kentucky. Ft. Benning, Georgia is the southernmost Basic Training base. Because of this, they tend to have extremely hot summers. Much of Basic Training consisted of patrolling through wooded or sandy terrain under blazing sunshine, shooting weapons on ranges, and spending nights in the woods for several days or a week at a time. In these environments, Basic Training candidates receive high doses of sunshine and heat. When this is combined with "smoking," that is, forced exercise until exhaustion, it is imperative that the body adapt to these environmental changes.
Most likely, most ordinary people will never approach this level of activity even if they do decide to hike the Appalachian Trail or spend a few days camping in the woods. I know this because we witnessed numerous preventable situations of when a soldier lost the ability to train properly because of some common mistakes. Yet nothing about the training was so strenuous that the average person could not withstand it; the problem was that many of us were inexperienced and did not know what habits we should be practicing in a field environment.
Each person has a basal metabolic rate that is determined primarily by weight, age, and gender. There are some variations, however; some individuals naturally have a higher metabolism. This rate is the basic level at which a given person's body burns calories and nutrition. When we are sleeping, this rate is at its lowest (this is why it is not recommended to eat shortly before sleeping). Progressively higher levels of activity yield higher metabolic rates. The rate of caloric burn is linked to heart and respiration rates; doctors recommend exercise as a good method of losing weight because it boosts the metabolic rate over a period of time.
The minute you enter the woods or a field terrain, you will start burning more calories. This isn't a magic idea. The reason that you burn more when you are actively engaged in an outdoor environment for more than a day is that your basic level of activity is higher. Also, you do not have the temperature controls of a garrison (or building) environment, so you do not have the ability to recover from work. The body also operates at a higher tempo in the field, possibly because it reignites ancient survival instincts that bump up your breathing and heart rate and increase adrenal gland activity. All of this burns calories.
The military MREs (Meal, Ready to Eat) are often colloquially referred to by soldiers as Meals Rejected by Ethiopians, or Meals Refusing to Exit. Indeed, the first time I consumed MRE's for several days, it took four days before I was able to pass a bowel movement (I have heard of people supposedly taking as much as six!) The standard recommendation for MRE consumption is to completely consume preferably 3, but at least 2 per day while in the field. Each MRE contains from 2,000 to 2,500 calories, which means that soldiers are recommended to eat between 4,000 to 7,500 calories per day while in the field. This is a huge amount! But soldiers are not any different than regular people. You should consider your calorie intake as well when you are out in the woods for two days or more. Below are the contents of a "Chili and Macaroni" MRE. This is not a favorite among many soldiers, although it cannot compare to the hated "Cheese and Veggie Omelet."
Although it is true that you must make dietary changes in ordinary weather, it is fortunate that hunger usually drives you to eat more without thinking about it. In extreme weather situations, however, your body's instincts do not operate as well. This is a big problem, because it can mean that you unintentionally deplete your energy.
In the summer, you must drink more water! A metabolic increase means that you are getting hotter and sweating more than is ordinary, and your body is going to lose water at a faster rate than at any other time. Drink more than you think you need, but do not forget to couple your water intake with electrolytes such as sodium, potassium, magnesium, and even some sugar. Think about drinking at least one quart of water per hour of heavy activity, but no more than two. In Basic Training, we drank "victory punch" which is essentially Powerade, but mized at several times the normal concentration. Also, don't forget to use sunscreen, because sunburns can suck away even more water from your body.
In the winter, you need to focus on eating more. The body feels very slow in cold weather, and it tends not to want to do anything. Eating can often feel like a chore when it is cold, but it is important to realize that food is harder to digest when it is cold! Therefore, the body must burn even more calories to digest the food properly, and it must burn many of those to keep your body warm. Shivering takes energy. You must also hydrate in the winter, even though cold water is likely to seem unpalatable. The cold deceives you, and you won't realize how dehydrated you are.
In training, we noticed that the people that fainted or suffered from heat sickness usually had some familiar eating patterns: not enough good food, and sometimes not enough of even the bad food. They were also bad about hydration. You should try to get good solid calories from proteins, nuts, peanut butter, bread, carbohydrates, and vegetables, and avoid bad ones from sugars. A sugar rush and crash is the last thing you want in the woods. We had only one go down in the cold in North Carolina (at 14-17 degrees Fahrenheit, not so cold compared to some places). But we did have people faint almost every day in the summer heat of South Carolina, especially on the oven-hot weapons ranges. Taking care of your body with enough food and water is imperative if you want to last in unusual terrain. You will undoubtedly have to pay attention to your body and eat more in order to operate at optimum condition.