Your Body's Metabolism

Your body acts like an engine that is constantly in operation. Food is its fuel. It burns the fuel to keep going. But your body's engine actually consists of millions of living cells, which are themselves tiny engines. Each cell in your body requires energy to stay alive. Food provides the energy.

Your body derives the energy and nutrients it needs from foods. Your body is a marvelous engine that keeps you alive by converting food into energy through a vast number of chemical processes.

Lean Lingo

A calorie of food refers to the amount of energy available to the body from the oxidation or digestion of food. When used as a calorie of food, it means that when a food is oxidized in the tissues of the body, it releases 1,000 times that amount of energy to be used by the body.

Your metabolic rate measures the rate at which your individual body is using up energy (as measured in calories) to stay alive. It's not a rate that remains constantly the same. Your metabolic rate varies throughout the day and throughout your life, depending on your level of activity and other factors.

Your basal metabolic rate (BMR) describes the rate at which your body uses energy (also measured in calories) in a totally relaxed mode. It is usually measured in the morning after a comfortable night's rest, when you are relaxed in bed, before breakfast. (Sounds nice, doesn't it?!)

A Closer Look at Metabolic Rate

John, a 28-year-old mechanic whose hobby is mountain biking, claims to have a “high metabolism.” He probably believes that his basal metabolic rate is higher than the norm. He may be right. We don't all have the same BMR. The following are known to influence basal metabolic rate:

  • Age. The BMR gradually decreases with age due to inactivity and lower muscle mass.

  • Sex. The BMR is generally a little lower in women than in men.

  • Sleep. Inadequate sleep over time will decrease your BMR.

  • Exercise. Systematic exercise will increase your BMR.

  • Nourishment. Prolonged undernourishment will decrease your BMR.

  • Thyroid hormone. Poor thyroid functioning decreases your BMR.

  • Stress Levels. High stress levels reduce your BMR.

  • Dehydration. Inadequate water intake decreases your BMR.

  • Body fat percentage. A high body fat percentage decreases your BMR; a low body fat percentage increases it.

John, because he's young, muscular, and athletic, probably does have a higher basal metabolic rate than Jim, a middle-aged accountant who struggles to push the lawn mower on Saturdays. John's higher BMR will help him burn calories faster than Jim does … even when he's asleep. His engine is idling more quickly; it uses up fuel more quickly.

But John may have been referring to the fact that he burns through a lot more calories every week because his overall metabolism operates at a higher level. The physical activities he engages in, both at work and in leisure, burn calories more quickly than sedentary activities. His engine is working harder; it uses up fuel more quickly.

The rate at which you burn calories is lowest while you sleep and greatest when you physically work hard. Here's a guide that describes the approximate calories burned during various types of work. Keep in mind that these “burn rates” will vary from person to person, so use this as a guide to the relative energy used up for each level of activity.

ActivityCalories per Hour
Very light work or sitting at rest100
Light work120
Moderate work175
Heavy-duty work375

As you can see, energy stored in the body is metabolized much more quickly when physical exertion increases. So let's compare the calories burned in a typical day by John and Jim.

8 hrs of sleep at 65 cal/hr520
8 hrs of work as a mechanic at 175 cal/hr1,400
4 hrs of everyday light activity at 120 cal/hr480
2 hrs of mountain biking at 375 cal/hr750
2 hrs of eating, TV, and so on at 100 cal/hr200
Calories burned during digestion 335
Total requirement for 24 hours3,685
8 hrs of sleep at 65 cal/hr520
8 hrs of work at a desk job at 120 cal/hr960
4 hrs of everyday light activities at 120 cal/hr480
4 hrs of eating, TV, and so on at 100 cal/hr400
Calories burned during digestion 189
Total requirement for 24 hours2,549

Without even considering that John probably has a higher basal metabolic rate because of his regular physical activity, you can see the dramatic difference in caloric requirements because of the lifestyle differences between John and Jim. John can eat 1,100 more calories in a day than Jim without gaining weight.

More on: Exercise Tips


Excerpted from The Complete Idiot's Guide to Healthy Weight Loss © 2005 by Lucy Beale and Sandy G. Couvillon. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

To order this book visit the Idiot's Guide web site or call 1-800-253-6476.


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