glycogen. In short, the organic acids liber- 

 ated by deamination are utilized by the body 

 in one manner or another. 



Action of the Liver on Fats. True fats are 

 absorbed mainly in an emulsified form and 

 cannot pass directly into the cells of most of 

 the tissues. But certain cells, especially in the 

 liver, are able to engulf fat globules — by a 

 process that resembles phagocytosis. In the 

 liver these special cells are called Kupffer 

 cells. The Kupffer cells project partially into 

 the capillaries, and in this strategic position 

 they take in fat globules and other granular 

 material from the passing blood stream. 



The fats are partially catabolized in the 

 liver before passing to the other body tissues. 

 The liver chemically alters the fatty ma- 

 terials, increasing their capacity to be ab- 

 sorbed and oxidized by other tissues. A 

 partial desaturation of the fat molecules re- 

 sults from the action of liver dehydrogenases; 

 and the double bonds thus introduced into 

 the carbon chains make the fats more sus- 

 ceptible to further oxidation. Moreover, the 

 partially oxidized intermediary products of 

 fat catabolism are more soluble in the blood 

 and lymph and more easily distributed 

 throughout the body. 



Other Functions of the Liver. The liver 

 has a number of other special functions. Or- 

 ganic toxins (p. 315), produced by bacteria 

 in the large intestine, are absorbed into the 

 portal blood. But the liver prevents these 

 compounds from reaching toxic levels in the 

 general circulation. A variety of special en- 

 zymes are present in the liver cells, and these 

 convert the toxic products into nontoxic 

 compounds. In some cases the toxin is merely 

 combined with some other substance (for 

 example, benzoic acid is combined with an 

 amino acid, glycine) to form a nontoxic com- 

 pound, which later is eliminated by the kid- 

 neys. However, the detoxifying reactions in 

 the liver are very complex and varied. 



Other hepatic functions, such as the syn- 

 thesis of prothrombase and the secretion of 

 bile (p. 323), have been mentioned previ- 

 ously; still other functions, such as the stor- 



Absorption, Metabolism, and the Diet - 343 



age of vitamins A, D, and B 12 (APA), will 

 be mentioned later. 



GENERAL METABOLISM: A 

 BALANCED DIET 



On any long-term basis, the daily intake of 

 food must be sufficient (1) to provide the 

 energy expended by the body; (2) to provide 

 the proper kinds of matter from which the 

 body assembles and synthesizes its essential 

 components. The first of these objectives is 

 fulfilled by catabolism, and the second, by 

 anabolism — each taken as a whole. 



The Energy Requirement: Destructive Me- 

 tabolism. The only source of energy avail- 

 able to the human body is the decomposi- 

 tion of organic compounds, such as proteins, 

 carbohydrates, and fats, and the derivatives 

 of these substances. Carbohydrates and pro- 

 teins each yield about 4 Calories of energy, 

 and fats 9 Calories, per gram utilized in 

 metabolism. Provided the final products of 

 decomposition are the same, these energy 

 quantities are constant, regardless of the in- 

 termediary course of metabolism, and regard- 

 less of whether the reactions occur inside or 

 outside the living body. 



The most direct method of determining 

 the energy expenditure of an individual is 

 to measure the total heat output of the body. 

 All other forms of energy (movement, elec- 

 tricity, etc.) that are generated during me- 

 tabolism are finally and totally converted to 

 heat, which is given off to the environment. 



Measuring the heat output of the human 

 body — a procedure called direct calorimetry 

 — is very difficult. The subject must be con- 

 fined for a long period in a heat-insulated 

 chamber, which is equipped with compli- 

 cated devices for collecting and measuring 

 the total heat given off by the body. Such ex- 

 periments have been done, and they prove 

 conclusively that (1) if the caloric content of 

 the utilized foods is just equal to the energy 

 expended, the body weight remains un- 

 changed; (2) if the calories of the food are 

 less than the expended energy, the body 



