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Is My Metabolism Slowing Down?

Posting Date: 10/18/2000

Q: Why am I getting fatter on the same amount of activity and food? Is my metabolism slowing down?

A: People talk about metabolism as if it were a simple thing. They use the word so readily and so frequently, you would think that its meaning was quite clear. But if you look up "metabolism" in a textbook of medicine, you'll find hundreds of pages on the subject.

Why is it that a word that seems perfectly understandable to most of us requires years of study by the medical profession? It must have a deeper, more complex meaning beyond the layman's definition. Basically, metabolism is ALL the chemical reactions that occur in your body -- brain, liver, digestive tract, muscles, heart, lungs, and every other tissue or organ. You can see that a thorough understanding of metabolism would require years of study.

But, the public equates metabolism with calorie burning. When your mother says, "You know, dear, your metabolism slows down as you get older," she's not thinking about complicated physiology. She just knows that you don't burn as many calories when you're older.

So, in order to understand how your body?s ability to burn calories changes over time we need to first understand what burns calories. Let?s break down an average day for a woman of average size to find out what makes up her calorie burning ability.

Basal Metabolic rate (BMR): 800 calories
The number of calories the body uses just to exist, to lie in bed and breathe.

Resting Metabolic Rate (RMR): 100 calories
The impracticality of BMR has generated a more useful measurement called resting metabolic rate (RMR). The person is asked to rest in a comfortable chair, relax as much as possible, and simulate the energy level of reading a book.

Fidgeting: 100 calories
Some people fidget when you ask them to relax. They never stop moving. They scratch; they cross and re-cross their legs. For such people we add a new metabolism category, called fidget.

Digestion: 100 calories
The act of eating and digesting food burns up a few calories.

Daily Activities: 100 calories
Because we don?t sit still all day, but move around doing daily chores, driving the car, getting dressed, we add the category of daily activities.

Exercise: 250 calories
Let?s say our sample woman goes to aerobics class for exercise.

Post-exercise: 100 calories
Don?t forget that she will burn calories after the class is over as her muscles replace glycogen and she recovers from the exercise.

Our sample woman burns 1,550 calories in a day.

All of these calorie-burning processes make up metabolism. When we say metabolism is slowing down as if it were a single process or a single switch that controls one process, we?ve missed the mark. Thousands of metabolic processes are happening every minute of every day. Certainly they don't ALL slow down at once.

BMR is most likely not the culprit, but what does slow down with age is all of the other things that go into making up metabolism. If you are not exercising, your need for glycogen goes down. You may also be unconsciously slowing down in other ways. Perhaps you are eating less and therefore you digest less. Are you able to sit and have a conversation without fidgeting? This further decreases your metabolism.

Can you pinpoint which metabolic process has changed to make you fat? No! You can't, and your physician can't, and your mother can't. But it doesn't matter, because almost all of them can be increased if you add exercise to your life. Don't focus on the number of calories burned during exercise -- think about the metabolic consequences of exercise.

Adapted from Smart Exercise: Burning Fat, Getting Fit by Covert Bailey. Copyright 1994 by the Covert Bailey Revocable Trust, published by Houghton Mifflin Company. All rights reserved.