THE RELATION OF DIGESTIBILITY TO DIET 335 



vitamin D) when treated with ultra-violet rays also have the power 

 to cure rickets. This is thought to be due to the presence in the 

 foods of a substance known as ergosterol, which becomes a powerful 

 antirachitic agent upon oxidation. Ergosterol is also present in 

 our skin, so when we are exposed to the ultra-violet rays of the 

 sun, it is probable that vitamin D is actually produced in the skin 

 and thus helps in the cure of rickets. Some investigators in the 

 University of California have found that when rats are fed a diet 

 lacking in certain foods, such^as wheat germ, which contain vitamin 

 E, they will become sterile and will not produce young. And 

 latest of all is the belief that pellagra, a dread disease in many 

 parts of the South, is a deficiency disease due to lack of a vitamin 

 called G. 



Self-Testing Exercise 



Among the best sources of vitamin A are (1), (2), 



(3), and (4) (5) . Among the best sources of 



vitamin B are (6), (7), and (8). Among the 



best sources of vitamin C are (9) (10), and 



(11) (12). The best source of vitamin D is (13) 



(14). Rickets may be cured by including (15) 



(16) in the diet. Scurvy is caused by lack of (17) 



(18). Beriberi is caused by lack of (19) (20). 



Lack of vitamin A causes an (21) (22) . 



PROBLEM m. WHAT IS THE RELATION OF WORK, ENVIRON- 

 MENT, AGE, SEX, AND DIGESTIBILITY OF FOODS TO DIET? 



The relation of work to diet. It has been shown experimentally 

 that a man doing hard, muscular work needs more food than a 

 person doing light work. The exercise gives the individual a 

 hearty appetite; he eats more and needs more of all kinds of 

 food than a man or boy doing light work, for he needs more food 

 for the extra energy release. 



The relation of environment to diet. The temperature of the 

 body is maintained at 98.6° in winter as in summer, but much 

 more heat is lost from the body in cold weather. Hence we need 

 more heat-producing food in winter than in summer. We may 



