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If a person needs more energy than they consume, such as when doing vigorous work, the body must draw upon the chemical energy stored in fat. So exercise can be helpful in losing fat. However, the amount of exercise needed to produce a loss in fat, or to burn off extra calories consumed that day, can be large, as [link] illustrates.
If a person who normally requires an average of 12,000 kJ (3000 kcal) of food energy per day consumes 13,000 kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra 1000 kJ?
Solution
[link] states that 400 W are used when cycling at a moderate speed. The time required to work off 1000 kJ at this rate is then
Discussion
If this person uses more energy than he or she consumes, the person’s body will obtain the needed energy by metabolizing body fat. If the person uses 13,000 kJ but consumes only 12,000 kJ, then the amount of fat loss will be
assuming the energy content of fat to be 39 kJ/g.
| Activity | Energy consumption in watts | Oxygen consumption in liters O 2 /min |
|---|---|---|
| Sleeping | 83 | 0.24 |
| Sitting at rest | 120 | 0.34 |
| Standing relaxed | 125 | 0.36 |
| Sitting in class | 210 | 0.60 |
| Walking (5 km/h) | 280 | 0.80 |
| Cycling (13–18 km/h) | 400 | 1.14 |
| Shivering | 425 | 1.21 |
| Playing tennis | 440 | 1.26 |
| Swimming breaststroke | 475 | 1.36 |
| Ice skating (14.5 km/h) | 545 | 1.56 |
| Climbing stairs (116/min) | 685 | 1.96 |
| Cycling (21 km/h) | 700 | 2.00 |
| Running cross-country | 740 | 2.12 |
| Playing basketball | 800 | 2.28 |
| Cycling, professional racer | 1855 | 5.30 |
| Sprinting | 2415 | 6.90 |
All bodily functions, from thinking to lifting weights, require energy. (See [link] .) The many small muscle actions accompanying all quiet activity, from sleeping to head scratching, ultimately become thermal energy, as do less visible muscle actions by the heart, lungs, and digestive tract. Shivering, in fact, is an involuntary response to low body temperature that pits muscles against one another to produce thermal energy in the body (and do no work). The kidneys and liver consume a surprising amount of energy, but the biggest surprise of all it that a full 25% of all energy consumed by the body is used to maintain electrical potentials in all living cells. (Nerve cells use this electrical potential in nerve impulses.) This bioelectrical energy ultimately becomes mostly thermal energy, but some is utilized to power chemical processes such as in the kidneys and liver, and in fat production.
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