Most of what controls your metabolism is not under your control, some folks have fast metabolisms and others have slow ones. Also, as we age our metabolism slows down. Scientists have estimated that metabolism slows about 5 percent per decade, beginning at age 40 as we lose muscle mass and increase body fat. Muscle burns more calories [1] than fat, so when you lose muscle mass your caloric needs decrease. Luckily you are not completely at the mercy of your genetics - There are a few things you can do to keep your metabolism up.

Basically, metabolism is the total of all body processes that burn calories [2] - your basal metabolic rate (calories burned by simply being alive) plus your activity factor since we all know that exercise burns calories.

The basal metabolic rate includes the energy we need for body processes, including digestion. About 10 percent of our calories are used to process the food we eat. This phenomenon, known as the thermic effect of food, is influenced by how much, how often and what we eat. In addition, food can directly affect metabolism by altering the way the body functions, which changes the amount of energy it needs.

According to Joy Bauer, nutritionist and author of the book Food Cures [3], there are ways to maximize your metabolism. Curious to know what they are? Then

• Eat at least 1,000 calories per day (I would say 1,200 calories, myself). Although it is generally true that eating a low-calorie diet will help you take off weight, if you eat too few calories, your metabolism will get slower and slower as it tries to conserve energy. As your metabolism crashes, the weight you take off will most likely creep back on over time. Plus, you'll be more likely to binge on junk food if you reduce your calories by too much.
• Eat every four to five hours. A regular meal schedule helps keep your body working to digest and absorb foods. Between breakfast and bed, aim to eat a meal or snack every four to five hours. Try to eat breakfast within 90 minutes of rising - People who regularly eat a healthy breakfast are more likely to control their weight. If you wait to eat until you're ravenous, you're more likely to overeat later in the day. Also, breakfast helps fire up your metabolism after a full night on a slow simmer.
• Eat protein with every meal. All foods contribute to the thermic effect, which means that all foods – carbohydrates, fats and proteins – help to give metabolism a gentle nudge higher when we eat them, but protein has the greatest thermic effect of all. In addition, protein can increase metabolism by helping to maintain and build muscle mass.

Source [4]