This is from an article I found online - “Your metabolism is in effect your body’s furnace. Calories are the fuel that heats this furnace; the hotter your furnace burns, the more calories it uses up just to keep up with basic functions like digestion, breathing and so on. Your activity level and the food you eat affect your metabolism. The more activity and exercise you do, the faster your metabolism runs, even when you aren’t being active. Activity also builds muscle, and muscle burns more calories than fat. In other words, the more muscle you have, the faster your metabolism. If you don’t put enough food in your body, your “furnace” doesn’t have anything to keep it going. It goes into a sort of hibernation mode by slowing down and using fewer calories. So, yes: If you eat too little, you can slow your metabolism”
I felt that this post was a great analogy of how important it is to feed your body the right amount of quality foods. As people focus on losing weight, the first thing they want to do is drastically cut their calories. Although it is true that you need to reduce your caloric intake, most people go to the extreme and try to eat as little as possible. They will skip meals, eat nothing but grapefruit, cut out all carbs but load up on fat, all of which stalls the metabolism.
The body needs a set amount of calories just to maintain biological functions, in women this number is generally 1200 calories. It is important to make sure that you are eating a minimum of 1200 calories, and if you are doing intense exercise, you need to add at least 300 calories to that number. If you are not getting enough calories, you can become lethargic and moody. Once your body has burned its fat stores, your body will begin breaking down muscle tissue and using it for energy.
However, some people have become so accustomed to eating so few calories, that it is difficult for them to eat more food. In order to increase your food intake, eat smaller meals, every two hours. If you eat natural nutrient dense foods, such as whole grains, vegetables, fruit and protein, you need not worry too much about gaining weight. If you are unsure how many calories you are taking in each day, and are underweight, chart what you are eating for 3 consecutive days. If you are falling below the 1200 mark, you need to increase your calories by eating adding more nutrient dense foods to your diet, such as those suggested above. Give your body a couple of weeks to adjust to your new eating pattern.
If your weight is stable, and you are trying to gain muscle, then you also need to increase your calories. To gain muscle, you need to eat 1-1.5 grams of protein per pound of bodyweight. Also, increase you consumption of whole grains and healthy fats, as well as fruits and vegetables. However, if you are eating more calories in order to build muscle, you will need to strength train at least 3 times a week with heavy weight, otherwise, your extra calories will not be burned and will likely be stored as fat.