Metabolism is the sum of all chemical processes that involve converting the food you eat into energy for physical activity and bodily functions. Energy is used to maintain bone strength, make new skin cells, grow your hair, regulate body temperature, keep your heart beating and a myriad of other functions. Your metabolism is genetically determined, but the choices you make in your lifestyle can impact how well your metabolism functions.
Basal Metabolic Rate
Your basal metabolic rate, or BMR, is the minimum number of calories your body needs for breathing without any other muscle movement. When you refer to a slow metabolism or your metabolism slowing down, you are discussing your body's ability to burn calories from the food you have eaten. Your gender, size, age and amount of lean body tissue -- such as your bones, organs and muscles -- contribute to your basal metabolic rate. Your energy needs to carry out these basic processes stay consistent throughout your life. Your BMR accounts for approximately the largest portion of the calories your body burns in a day, up to 75 percent per day, according to MayoClinic.com.
Factors that Influence Metabolism
Factors that influence your metabolic rate include the amount of physical activity you engage in and the amount of calories burned to digest the food you eat, called thermogenesis. The energy spent digesting your food accounts for up to 10 percent of daily calories burned, which is part of your metabolic rate. Being physically active accounts for the rest of the calories your body burns in a day. The less food you eat and the less physically active you are, the slower your metabolism becomes. When you take in less food and become less active, your body balances out less activity and food to maintain your BMR.
Aging
According to Johns Hopkins, as you age, your metabolism slows down 2 percent each decade after the age of 30. This reduction in your metabolism is due to muscle-mass loss. Between the ages of 50 to 70, you have the greatest loss of muscle mass. This accounts for body composition change and greater accumulation of body fat as you age. Less muscle means fewer calories are burned as part of your metabolic rate; however, this explains only a small part of weight gain experienced as you age.
Starvation Diets
In the effort to lose weight rapidly, people may restrict their calorie intake to levels that slow their metabolism. Your metabolism compensates by slowing to adjust for the small amount of food you are consuming. The calories provided aren't sufficient enough to maintain your BMR, so your body compensates to save your vital organs. Once you resume eating your normal calorie intake, your metabolism returns to your regular BMR.
Slow Metabolism to Blame?
People often want to blame weight gain to a slowing metabolism. Hypothyroidism is a medical condition in which your thyroid organ doesn't produce enough hormones to regulate your metabolism properly. Hypothyroidism isn't the cause for most people who experience weight gain as they age. Most adults experience weight gain as they age because of increased caloric intake and decreased physical activity, says MayoClinic.com. Your metabolism doesn't change easily.
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