All activity of the human body, whether in the maintenance of its functions or in the performance of labor, is work. These two forms of work may be classified as physiologic and mechanical. Nothing in nutrition is more important than the relation of food to work. _Children_ are practically in constant motion during their waking hours, and their demand for food energy is from two to three times as much per unit of weight as that for adults. _Sex._—Men and women of the same age and weight, doing the same kind and amount of work, require the same amount of food. The fact that men usually require more food than women is because, as a rule, they weigh more, are more active physically, and perform more external work. _Temperament._—Persons of a nervous type, being more active, use more energy than the phlegmatic, and, therefore, require more food. _Brain Workers._—A man whose work is sedentary and chiefly mental does not need so much food as a man doing muscular work. The amount of carbohydrates required is less, and the amount of fat rather more than for the man doing light muscular work. Ranke’s diet for the brain worker is: Protein, 100 grams; fats, 100 grams; carbohydrates, 240 grams; giving an energy value of 2310 calories. The diet should not be bulky, but light and easily digestible. Excess of food and heavy foods are especially bad for brain workers because they produce heaviness, dulness, and drowsiness. Spiced and rich foods upset the alimentary functions, whereby the circulation is flooded with the products of imperfect metabolism to the detriment of the brain. Trained workmen will do a given amount of labor on less food than untrained, because when persons take up mechanical operations with which they are unfamiliar, or undertake work which exercises a new set of muscles, a unit of work accomplished costs more in food energy than when the muscles have been trained to do a particular thing. Very strenuous exercise, like athletic contests and unnaturally slow movements, are both wasteful of food energy. Again, a continuance of the same labor for hours, and a state of fatigue, whether it comes after a longer or shorter time, causes an increase in the energy expenditure per unit of work performed. Economy in the use of energy that food supplies to the body is most fully secured when the movements of the body are at the natural rate; when periods of intense effort do not occur, and when the labor is not too long continued and is not carried to the point of fatigue. _Standard for Daily Dietaries (American)—Atwater._ Fuel Protein. value. Nutritive Grams. Calories. ratio.