Ways to Measure Energy Use

Oxygen is used to produce the energy needed to contract a muscle, initiate a nerve impulse, repair a bone, or carry out any number of cellular functions. Oxygen goes from the lungs to the blood and finally to the tissues where it is used. For each liter of oxygen used, the body produces about five calories of energy. Knowing this, it is possible to measure the amount of energy produced during the course of a day simply by measuring oxygen consumption. On the basis of these measurements we are able to estimate the amount of energy a person uses for different activities and provide an overall estimate of what a person might expend in one day.

When we are sitting at rest we expend about one calorie per hour for every kilogram (2.2 pounds) of body weight. The value of one calorie per kilogram per hour is one resting metabolic unit or 1 MET. METs have already been used to quantify your CRF. METs are also used in expressing the energy costs of activities; for example, running at six miles per hour requires 10 METs, or 10 times the rate of energy expended at rest. Before discussing the energy costs of physical activity, however, we need to focus on the energy associated with sitting at rest, the largest energy-producing task in the lives of most people! Given that we produce about one calorie of energy per kilogram of body weight per hour at rest, you can estimate your resting energy expenditure (also called basal metabolic rate, or BMR) by doing the following:

1. Multiply 24 calories by your body weight in kilograms (pounds divided by 2.2) or

2. Multiply 11 calories by your body weight in pounds

For example, for a 150-pound person, 150 pounds · 11 calories per pound = 1,650 calories. This, of course, represents only resting energy expenditure. To obtain an estimate of overall energy expenditure, you must add the energy cost of your other activities. This is done by adding from 400 to 800 calories, depending on whether you are sedentary or very active. So, if our 150-pound person is sedentary, the estimated total energy expenditure is 2,050 calories per day (1,650 calories + 400 calories).

Remember that this is a rough estimate and like any estimate, it may be too high or too low for you. Use it as a guide, but if you find yourself gaining weight even though you are eating just enough food to meet your estimated energy expenditure, decrease your estimate by about 10 percent and see if you can maintain weight on that new estimate. Keep in mind that if you reduce your caloric intake to a very low level, your body responds by decreasing its resting metabolic rate to protect its limited energy stores. In such a circumstance the above formula will result in an overestimation of energy expenditure.