Studies of American diets indicate that, with the exception of iodine (see 

 pages 100-101), the foods usually eaten contain adequate supplies of the re- 

 maining salts essential to our protoplasm. 



Chemical Regulators We have seen that various inorganic substances 

 play an important role in the building and in the activities of protoplasm. 

 In addition, how^ever, mineral salts appear to be important because their 

 relative cojjcentration in the cells and body fluids affects osmosis and the 

 distribution of material (see page 87). The rhythmic contraction and re- 

 laxation of heart muscle depends upon certain proportions of calcium, 

 sodium and potassium in the body fluids (see page 99). When the supply 

 of calcium is too low, body muscles become tense and rigid; some convul- 

 sions are caused in this way. Other salts affect the oxidation of food in the 

 cells. When the concentrations or proportions of these salts fluctuate too 

 much, the metabolism is disturbed. 



How Can We Plan a Diet to Suit Our Body Needs? 



More than Day by Day We usually know immediately whether our 

 food pleases us or when our hunger has stopped. If something goes wrong 

 with the digestion, we soon discover it. But we may continue a very long 

 time on a diet that is seriously lacking in essentials, without realizing it. For 

 this reason it is important that everybody acquire food preferences and food 

 practices guided by reliable knowledge of daily needs. Such knowledge 

 rests upon studies of what people do actually eat and upon experiments 

 with the diet and its effects on college students, soldiers and other people, 

 and on various animals. 



In discussing metabolism and life needs so far, we have said very little 

 about food: we have considered only such abstractions as Calories, proteins 

 or vitamins. When we sit down to a meal we see none of these things; we 

 are confronted instead with various breadstuffs, fruits, vegetables, meats, 

 and the like. We know that some of the foods which we use contain more 

 of the essentials than others (see illustrations, pp. 126 and 127). How can 

 we translate the products of the food factories and the kitchen into proteins 

 and Calories and vitamins ? The information which we need for such trans- 

 lating has been furnished by research workers in government laboratories, 

 in hospitals, in universities, and in other institutions. It is available to us in 

 convenient tables that have been prepared by various experts. 



Food Groups^ We group or classify the common foods according to 

 what they furnish in our diet. 



^See Nos. 4 and 5, p. 136. 

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