Metabolism is how your body burns up food and things you eat, turning them into fuel and fat. Does your body have a slow metabolism? Read on to learn more.
There is a scientific ratio known as your Basal Metabolism Ratio, or BMR. This is the number of calories you burn "doing nothing" - just being alive. Let's say you are 5'5" and 150 pounds, for sake of argument. There would be an average number of calories that a person that size would burn every hour just with normal body functions. If you only burned half that amount, you would have a metabolism problem. Your metabolism, at a very basic level, would not be burning calories properly and therefore would gather up fat.
There are medical conditions that can cause this to happen. Some examples are Cushing's syndrome and hypothyroidism. I have a friend with hypothyroidism. She eats like a rabbit and exercises daily - and is heavy anyway. Her body simply doesn't burn calories properly.
Your BMR is affected by how much muscle is in your body. Let's say again that you are 5'5" and 150 pounds - and 40 pounds of that is muscle. Muscle burns a lot of calories just by existing. You would burn a lot of calories every hour, just laying on the couch. Now let's say you are 5'5" and 150 pounds - but only 5 pounds of that is muscle. If you laid on the couch, your body would burn FAR less calories to keep you alive. Your basic calorie burning rate would be much lower.
Metabolism, and therefore your BMR, slows down naturally as you get older. You might be able to eat 2 pizzas a day and not gain weight as a teenager - but when you're 40 years old, doing the exact same thing would cause the pounds to build. You need to be aware of how your body is changing each year, and adjust your eating habits to match.
Note that some studies show that older people who have slower metabolisms can regain their youthful metabolisms by exercising daily and building their muscle mass back up - so it's definately important to stay active!