BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



197 



Excretion 



Sodium, potassium, calcium, magnesium, phosphorus, and chlorine are 

 excreted mainly as inorganic salts in both urine and feces. Those salts 

 that are readily soluble are excreted chiefly in the urine ; those salts that 

 are less readily dissolved are excreted largely in the feces. Iron, copper, 

 zinc, manganese, and cobalt are excreted mainly in the feces. 



The excretion of sulfur is almost entirely by way of the kidneys. In 

 this respect it resembles the excretion of nitrogen. Several forms of 

 sulfur are contained in the urine. From 60 to 90 per cent of the total 

 sulfur is excreted in the form of sulfates; from 5 to 15 per cent as ethereal 

 sulfates; and 5 to 20 per cent as neutral or unoxidized sulfur. Taurine 

 and cystine are forms of unoxidized sulfur contained in the urine. 



Variations and losses of mineral elements in foods 



Unless the food that is eaten is analyzed, it is practically impossible to 

 say whether or not the requirements listed above have been satisfied. 

 Different samples of food vary so greatly in their mineral content that 

 any calculation based on existing data is extremely uncertain. For 

 example, Peterson, Elvehjem, and Jamison found that 18 samples of 

 cabbage from various parts of the United States showed variations 

 (from the lowest to the highest) of 96 per cent for calcium, 118 per cent 

 for phosphorus, and 246 per cent for iron. Such extreme variations 

 would probably not appear in a mixed diet over a long period of time. 



A second difficulty in the calculation of the mineral content of the diet 

 results from losses incurred in the cooking of the food. Such losses vary 

 with the kind of food, the method of cooking, and the length of time of 

 cooking. If cabbage is boiled and the water is discarded, 72 per cent of 

 the calcium, 60 per cent of the phosphorus, and 66 per cent of the iron 

 may be lost. The average losses for 16 vegetables reported by Peterson 

 and Hoppert were: 



Calcium 

 Method of cooking percent 



Steaming 10.7 



Pressure-cooking 12.0 



Boiling 20-32 



Differences in the requirements of individuals, variations in the vitamin 

 D content of the diet, variations in the composition of the food, and 

 losses in the preparation of food make it imperative that a liberal allow- 

 ance be made in excess of any calculated figures. 



