The human body is a living machine that needs fuel -- energy provided by food we eat -- to stay alive. We use up calories all the time, even when resting or asleep. The amount of energy the body uses to stay alive is called the
The human body is a living machine that needs fuel -- energy provided by food we eat -- to stay alive. We use up calories all the time, even when resting or asleep. The amount of energy the body uses to stay alive is called the basic metabolic rate. Any activity -- even sitting at a desk, typing, or driving -- requires extra energy which adds to the basic requirement for energy. Vigorous exercise or heavy physical work burns five to seven times as many calories per hour as resting.
Calorie Balance and Weight Loss
Fat is your body's energy store. In simple thermodynamic terms, if you take in more calories than your body needs, the excess will be stored as fat. If your body needs more calories than you take in, it will use up the stored reserves and you will loose fat. The only way to loose fat is thus to create a calorie deficit, either by taking in fewer calories or by using up more, and ideally both. The calories you burn to simply stay alive, those burnt in regular activities and those you use up through exercise all combine to make up the energy needs. The standard tables and calculators of energy expenditure list the total of calories burned during an activity: the basic amount to keep the body going and the extra needed to fuel the movement.
Basic and Resting Metabolic Rate
Your weight is the single most important factor that affects your energy needs: the heavier you are, the more calories you need to stay alive and the more calories you will burn during any activity. The basic energy needs of a body are usually referred to as Basic Metabolic Rate or BMR, and sometimes as Resting Metabolic Rate or RMR. These can be measured precisely in a laboratory setting or estimated using standard equations which take into account your sex, age and height as well as your weight. The difference between the two is relatively small and either will provide a rough estimate of your body's energy needs when resting. An average woman of 30 who weighs 150 pounds burns approximately 1,500 calories per day, or 63 calories per hour, simply by staying alive. A man of 40 who weighs 200 pounds will need 1,900 calories, or approximately 80 calories per hour, to just be.
Activities and Excercise
Energy expenditure goes up with activity levels. Even just sitting at a desk or driving adds significantly to calories that we burn resting: the same 150-pound person who expends 63 calories per hour at rest will use up double that amount when working at a desk. Get up and move and the calorie consumption jumps up farther: doing light cleaning uses up 170 calories per hour, while even moderate walking at 2.5 miles per hour burns 200 calories. Exercise can multiply the basic calorie needs by more than a factor of 10. Even leisurely swimming, our 150-pound woman will burn 400 calories per hour, and if she's running at 6 miles per hour, she will burn close to 700 calories per hour.
Daily Energy Needs
You can obtain your approximate BMR by using an online calculator and then very roughly estimate your daily calorie needs by multiplying your BMR by a factor of 1.2 if you lead a sedentary life with little activity, or by 1.55 if you get moderate amount of exercise three to five times a week. If you want a more accurate estimate, instead of using a simple multiple, list all the activities over a period of 24 hours and add up the total energy expended. For example, a person who weighs 150 pounds will burn approximately 490 calories sleeping for 8 hours and another 980 calories in a day's worth of desk work. A half an hour's jog at 5 miles per hour will add 300 calories, an hour of grocery shopping, 155 calories. Record your activity over a week and then average to calculate your typical daily calorie needs.
Factors Affecting Metabolic Rate
In addition to weight, age and height, the metabolic rate -- both resting and during exercise -- is affected by other factors. Body composition, meaning the proportion of fat and muscle, is a significant one. Muscle needs more energy to function than fat, which is why if you compare an average man and an average woman of the same weight and height, the man's calorie consumption will usually be higher than the woman's because men tend to have a higher proportion of muscle. Age also affects metabolic rate: the older we get the less calories we burn per pound of body weight. There are also individual differences due to genetic factors and possibly even personality. For example, people who are more extroverted and less neurotic tend to naturally walk faster and expend more energy when exercising at top capacity. Those who fidget also burn more energy.
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