NUTRITION 



4316 



NUTRITION 



of older children and grown people there is no 

 such accurate rule. Instinct, to be sure, leads 

 people to take food and thus to maintain their 

 lives. For the savage, who leads a care-free 

 existence, this is sufficient guide, for he is not 

 inconvenienced by being obliged to carry 

 around with him more food than he needs. 

 With civilized man, however, who plans his 

 life carefully, the case is different. He wants 

 the largest possible amount of energy for his 

 work and his pleasurable pursuits and is un- 

 willing to expend any of it carrying about or 

 digesting substances which are of no use to 

 him. For this reason, if for no other, educated 

 people are making careful studies of bodily 

 needs and of the nature of various kinds of 

 foods, and are bringing together the facts thus 

 discovered to form a science of human nutri- 

 tion. 



This science is comparatively new, and is 

 even less developed than most sciences, but it 

 includes a few well-proved facts which have 

 been established by careful, painstaking studies 

 in public and private laboratories. It includes 

 also reliable statistics about the rate at which 

 healthy children grow, the relation between 

 the height and the weight of healthy grown 

 people, and the general signs of bodily well- 

 being. This growing science, or system, of clas- 

 sified knowledge and of proved facts is, among 

 intelligent people, rapidly taking the place of 

 those statements about food and diet which are 

 based only on guesswork. 



One fact which has been definitely estab- 

 lished concerns the amount of energy needed by 

 people of different ages and occupations. This 

 was determined by measuring the heat produced 

 by these people in a given length of time, for, 

 of course, the greater the energy expended, the 

 more the heat produced. The studies were 

 made with a piece of apparatus called a calo- 

 rimeter, or heat measurer, first used in America 

 in the United States Department of Agriculture, 

 and later in the Carnegie Institution, the Rus- 

 >(!! Sage Research Laboratory, and elsewhere. 

 Though difficult to make and to operate, a 

 calorimeter is simple in theory. It consists of 

 a room or enclosed space so built and insulated 

 that no heat can escape through the walls or in 

 any other way except where it can be meas- 

 ured. It is supplied with an inlet and an out- 

 let for air, and also with a coil like an ordinary 

 steam radiator, through which water continu- 

 ally circulates while the experiment is going on. 

 All the heat given off in this room passes either 

 to the air or to the water in the coil and can 



be accurately measured. By this means the 

 heat output, and therefore the energy require- 

 ment of different people, has been determined 

 when they are lying down, sitting, standing, 

 running, and doing many different kinds of 

 work. 



Heat Given off in Physical and Mental Work. 

 The amount of heat given off daily by a man 

 of average weight doing moderate muscular 

 work is about enough to raise seven and one- 

 half gallons of water from freezing to boiling 

 temperature. Investigators need more accurate 

 forms of statement than this, and have agreed 

 to give the name calorie to the amount of heat 

 necessary to raise one kilogram (a little more 

 than a quart) of water from to 1 Centi- 

 grade, or from 32 to 33.8 Fahrenheit (see 

 CALORIE). A man at moderate muscular work 

 gives off about 3,000 calories; a woman weighs 

 about eight-tenths as much as a man, and if 

 she is equally active needs about eight-tenths 

 as much fuel. The energy required by children 

 depends partly on their weight and partly on 

 the degree of muscular activity. Little chil- 

 dren need far less than grown people, but an 

 active boy over fourteen years of age may 

 need as much as a man who leads a sedentary 

 life. By experiments with the heat measurer, 

 it has been shown that mental work does, not 

 increase the amount of fuel consumed. A per- 

 son gives off no more heat when trying to solve 

 a difficult problem than when his mind is quite 

 unoccupied, provided, of course, that muscular 

 activity is the same in both cases. A person 

 who does only mental work most of the time 

 uses up, therefore, less energy than a manual 

 laborer, and for this reason he needs less food, 

 even though he may spend an hour or two 

 every day at tennis, golf or any other form 

 of active exercise. 



How much food does it take to provide the 

 amount of energy needed by the laboring man, 

 the student, the clerk, or the child? That de- 

 pends upon the kind of foods taken. The 

 amount of heat produced by the different food- 

 stuffs when they burn by eggs, for example, 

 or by bread or by butter and consequently 

 the amount of energy stored in them has 

 also been determined by experiments with the 

 calorimeter, or heat measurer. By this means 

 it has been found that all foods have different 

 fuel values. A pound of butter yields about 

 three times as much heat as a pound of bread, 

 about eleven times as much as a pound (or a 

 pint) of milk, twelve times as much as a pound 

 of apples and forty times as much as a pound 



