Just as it takes more fuel to run a big machine than a little one, so it takes more energy for a large person than a small one; therefore we must know the weight of the one whose food requirements we wish to calculate, as well as the amount of energy required to do different kinds and amounts of work. The following table will help in calculating the approximate fuel requirements of any grown person. The food needs of children and young people under twenty-five will be discussed later.
Approximate Energy Requirements of Average-sized Man
| Occupation | Calories per pound per hour |
| Sleeping | 0.4 |
| Sitting quietly | 0.6 |
| At light muscular exercise | 1.0 |
| At active muscular exercise | 2.0 |
| At severe muscular exercise | 3.0 |
Light exercise may be understood to include work equivalent to standing and working with the hands, as at a desk in chemistry or cookery; or work involving the feet like walking or running a sewing machine. Many persons, as students, stenographers, seamstresses, bookkeepers, teachers, and tailors do little or no work heavier than this.
Active exercise involves more muscles, as in bicycling compared with walking, or exercise with dumb-bells as compared with typewriting. Carpenters, general houseworkers, and mail carriers do about this grade of work while on duty.
Severe exercise not only involves a good many muscles, but causes enough strain to harden and enlarge them. Bicycling up grade, swimming, and other active sports would be included in this kind of exercise. Lumbermen, excavators, and a few others do even heavier work than this.
Knowing the weight of a grown man or woman, and something of the daily occupation, as in the case of a professional man, we can estimate the probable energy requirement somewhat as follows:
Sleeping, 8 hours; 8 × 0.4 Calories = 3.2 Calories per pound.
Sitting quietly (at meals, reading, etc.), 8 hours; 8 × 0.6 Calories = 4.8 per pound.