800 XIV. NUTRITIONAL VALUE OF FATS 



caloric density of the high-fat diets, which enable the animal Avith the high 

 metabolic level to obtain sufficient caloric supply to meet the energy needs, 

 the beneficial effect of fat in this abnormal condition cannot be questioned, 

 and is, in fact, one of the most striking examples of the value of dietary 

 fat. On the other hand, fat may prove a detriment in hypothyroidism. 

 It was shown by Kennelly and Maynard'^^ that, when 3^oung rats are given 

 sufficient thiouracil to retard growth, to decrease body temperature and 

 oxj^gen consumption, the effects are more pronounced when the substance 

 is given to animals on a high-fat diet than when it is fed to animals re- 

 ceiving a low -fat regimen. Thus, a high -fat diet tends to alleviate the 

 effects of increased thyroid activity, but it augments the disturbances 

 due to depressed thyroid action. Templeton and Ershoff^^ reported that 

 both carbohydrate and fat are superior to protein (casein) in permitting 

 the survival of hyperthyroid rats (see page 853) . 



b. X-Irradiation. Fats have a protective effect against x-irradiation 

 injury in rats which seems to be primarily a function of the essential fatty 

 acid content. For a discussion of this relationship the reader is referred to 

 Chapter XIII. 



c. Cold Stress. Cold offers an excellent example of stress for studies of 

 nutrition, since it produces a marked increase in the level of metabolism. 

 More calories are required under such conditions. Mitchell and co- 

 workers^* reported that, when human subjects were exposed to severe cold, 

 a high-fat diet was superior to a high carbohydrate regimen in maintaining 

 general psychomotor performance and visual efficiency, as well as in 

 permitting a maximum speed in tapping tests. It was likewise observed 

 that high-fat rations were superior to high-carbohydrate regimens in 

 maintaining normal internal tissue temperatures in the cold environment, 

 provided the internal between meals did not exceed two hours. It is 

 believed that this is related to a decrease in heat emission, rather than to 

 an increased heat production in the subjects ingesting the high -fat regi- 

 men. The results of the effect of high-fat and low-fat diets upon the re- 

 sistance and acclimatization of animals to cold are somewhat equivocal, 

 and are therefore not discussed here.'^*^ 



d. Hepatectomy. Since the liver is responsible for performing a wide 

 variety of fimctions, any interference with its activity will result in a 

 critical form of stress. Although Mann^^ was able to keep dogs alive for 



'8 H. H. Mitchell, N. Glickman, E. H. Lambert, R. W. Keeton, and M. K. Fahne- 

 stock, Am. J. Physiol., 146, 84-96 (1946). 



'** A. C. Burton and O. G. Edholm, Man in a Cold Environment, Monogr. Physiol. 

 Soc. (London), Arnold, London, 1955, p. 148. 



" F. C. Mann, .4m. J. Physiol., So, 285 (1921 ). 



