348 ANIMAL BIOCHEMISTRY 



metabolism of tyrosine, in providing a suitable siq^ply of Fe++ from 

 the iron storage system, in the general maintenance of the necessary 

 cellular oxidation-reduction potential, in certain terminal oxidations 

 in plants, or in the synthesis of collagen or hyaluronic acid. In no 

 case is the evidence clear cut. 



Requirements 



Apparently only the primates and guinea pigs need a dietary supply 

 of ascorbic acid. Other species, including domestic animals and fowl, 

 synthesize their own. Human daily requirements have been fixed 

 at about 75 mg. for adults and from 30 mg. upward for children. 

 Pregnant and lactating women should receive 100 and 150 mg., respec- 

 tively. English workers, however, find that 10 mg. daily prevents all 

 detectable symptoms of deficiency in adults and believe that 30 mg. 

 provides an adequate margin of safety. There is no reason to challenge 

 this lower value, nor is there any danger from a large quantity since 

 massive doses are not toxic. 



Ascorbic acid is widely distributed in fresh foods of all kinds, espe- 

 cially fruits and vegetables. The best of the common sources include 

 strawberries, oranges, lemons, grapefruit, Brussel sprouts, cabbage, 

 and spinach. Unpasteurized milk and fresh meat are relatively good 

 sources. 



Since this vitamin is a good reducing agent, it is lost under oxidizing 

 conditions like aeration and heating in the presence of oxygen in 

 neutral or alkaline solution. Thus many cooked or canned foods con- 

 tain little ascorbic acid. Others, like canned tomatoes, are quite 

 adequate when properly processed to limit the presence of air and 

 metallic catalysts from the processing machinery. Certain enzymes 

 also catalyze the oxidation of ascorbic acid and should be destroyed 

 by blanching. 



Pasteurized milk has lost much of its vitamin C, and the remainder 

 is destroyed on standing. Hence the usual milk supplies are poor 

 sources. Human milk contains three to four times as much ascorbic 

 acid as cow's milk even when fresh from the cow and is obviously a 

 much better source for infants. Synthetic vitamin C is now widely 

 used in enriching canned foods, particularly fruits and juices. This 

 procedure is especially important in baby foods since bottle-fed chil- 

 dren receive so little ascorbic acid from milk. 



Deficiency 



Lack of sufficient vitamin C causes scurvy, which is characterized 

 by anemia, sore gums, loosening or loss of teeth, damage to capillaries 



