What is metabolism ?

Every cell in your body requires energy to function―whether it’s delivering nutrients to your brain, pumping oxygen from your lungs to your muscles during a long power walk, or producing infection-fighting white blood cells deep in your bone marrow. Metabolism is the name for the system by which the body converts the calories in food to energy (blood sugar) to perform these and many other functions.
Many factors contribute to your metabolism, including heredity. You’re born with an internal speedometer that regulates your base metabolic rate (BMR), the pace at which your body uses energy when you’re at rest. BMR accounts for approximately 60 percent of the total energy an average person expends in a day. (The rest is used in digestion, exercise, and non-exercise activities―showering, chopping vegetables, or fidgeting.) “We are not sure what makes people different in terms of metabolism; the genes determining that have yet to be identified, but it’s being explored,” says Gary Miller, PhD, associate professor of health and exercise science at Wake Forest University in Winston-Salem, North Carolina.
However, even if your metabolic rate is governed by genetics, it’s not immutable. “Metabolism can be changed,” Miller says. “In fact, it’s in a constant state of flux, throughout the day and throughout the years.”

How your metabolism changes with age

Researchers estimate that BMR slows by two to three percent each decade starting at age 20. Changes in body composition are a key factor to this slowing effect. As we age, muscle mass tends to decrease. Women typically lose 10 to 15 percent of their muscle mass between the ages of 20 and 50, and the decline subsequently accelerates, according to research from Missouri-Columbia University.
Less frequent exercise. Inactivity is the main culprit in muscle loss. Left unchecked, muscle is replaced by fat. Because muscle is far more active metabolically than fat, the rate at which you burn energy slows. “Muscle burns calories while fat stores them,” says Peter D. Vash, MD, director of medical and scientific affairs at Lindora Medical Clinic in Costa Mesa, California.
General slow-down. In addition to changing composition, your body also requires less energy with age. A recent study of more than 800 adults at Laval University in Quebec City, Canada, found that some women’s BMR was less than age-related muscle loss alone could explain. Researchers speculated the decline might be related to lessened metabolic demand from organs such as the heart, liver, brain, and kidneys, which is caused by a decline in the organs’ cell mass.
Hormone changes. Metabolism can also be slowed by natural changes in levels of hormones, such as estrogen and testosterone, says Nick Flynn, PhD, associate professor of biochemistry at Angelo State University in San Angelo, Texas. He notes changes associated with menopause may cause an increase in body fat and slowed metabolism.
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