McCay, Maynard, Sperling, and Osgood {121) 

 found that the maximum lifespan of their animals 

 occurred when rats weighed between 350 and 450 

 grams. No animal with a body weight exceeding 

 450 grams at any time in life lived more than 810 

 days. In considering nutrition during the latter 

 half of life, these authors reported that the degree 

 of fatness of the body is the most important 

 factor as far as lifespan is concerned. 



Silberberg and Silberberg (171) reported the 

 results of feeding male mice a stock diet containing 

 5 percent fat with or without an additional 25 

 percent lard. The mean lifespan of the mice 

 consuming the fat-enriched diet throughout their 

 life was 107 days shorter than that of the stock-fed 

 animals. When the high-fat regimen was initiated 

 at the age of 6 or 12 months, the lifespan was also 

 shortened but less so than when this regimen was 

 started at weaning. When fed for 5-month 

 intervals, the results varied, depending on the 

 period of life during which the fat-enriched diet 

 was consumed. The differences observed were due 

 to high fat and not to the caloric intake. 



French, Ingram, Uram, and others (69) com- 

 pared the results of ad libitum feeding of a diet 

 containing 22.7 percent fat, chiefly corn oil (20 

 percent), with one containing 3.4 percent fat and 

 20 percent sucrose in place of the corn oil. The 

 lifespan of male rats fed the high-fat diet was 

 markedly decreased. Decreased lifespan was cor- 

 related with increased efficiency of utilization but 

 not with caloric intake. These authors indicate 

 that this decrease in longevity may have been due 

 to the fat as such, or may have been the result of 

 the improved growth rate conferred by the high- 

 fat diet. 



According to Comfort (4-2), heredity may be as 

 important as dietary reduction in determining 

 lifespan. Although the lifespan of laboratory 

 animals can be increased by eliminating specific 

 heritable diseases, inbred stock rats tend to reach a 

 shorter lifespan than do random-bred animals, 

 and the author suggests that information on the 

 mechanism for vigor in hybrids may prove of 

 significance in studies of aging. Sperling, Loosli, 

 Barnes, and McCay (176) also reported data im- 

 plicating inheritance as a factor in survival. Six 

 and one-half percent of the litters of rats investi- 

 gated by these authors accounted for 27 percent of 

 the short-lived rats that died within 500 days; 18 

 percent of the litters accounted for 47 percent of 

 the animals dying after 700 days. Lane and 

 Dickie (111) presented data showing the shortened 

 lifespan that results from excessive food consump- 

 tion by genetically obese mice fed ad libitum 

 and, as the result of long-term restriction of 

 food intake of these mice, increased their lifespan 

 by more than 300 days. 



Summary 



In general, growth was good on all of the experi- 

 mental diets under investigation, and no evidence 

 of any dietary deficiency was apparent. Young 



rats tended to grow more rapidly when the level 

 of fat was 17 to 19 percent than when the diet 

 contained 9 percent fat or less. With some diets, 

 growth rate was associated with caloric intake; 

 with others, such as those containing high levels 

 of egg, milk, beef, or peanut butter, it appeared to 

 be associated with efficiency of utilization. 



Rats fed stock rations generally attained their 

 maximum weight at a relatively early age and 

 maintained a constant weight thereafter. On the 

 semipurified diets and the various modifications of 

 it, rats tended to continue to gain throughout their 

 healthy lifespan. Rats exceeding 800 grams in 

 weight were frequently obtained, especially on the 

 diets containing milk or peanut butter. 



The results reported provide further evidence of 

 the many factors that need to be considered in 

 evaluating the effect of diet on the lifespan of the 

 rat. Survival of rats varied even on diets of simi- 

 lar fat and protein content. The tendency to 

 excessive consumption of certain diets may explain 

 some of the differences in survival observed. The 

 extent of weight gain and the amount of a diet 

 that can be tolerated by the adult rat without ad- 

 verse effects appear to vary with diet. Diets con- 

 taining high levels of egg or egg yolk resulted in a 

 shortened lifespan, but the longer life of rats fed 

 100 percent egg indicates that other dietary in- 

 gredients were also contributing to the response to 

 the SPE diet. The results obtained when stock 

 and SPE diets were reversed at 250 days contribute 

 further evidence that the age period under study 

 may be a contributing factor in the response to 

 diet. The results of feeding the same diets to 

 BHE and Wistar rats emphasize the importance 

 of recognizing inherited characteristics in evaluat- 

 ing dietary response. 



Histology and Size of Selected Organs 



Histology of kidney 



When subjected to gross and to microscopic 

 examination, the kidneys from 113 rats fed the 

 stock diet, 99 fed SP 8 HVO diet, and 201 fed 

 SPE diet, provided information on the influence 

 of diet, age, fasting, and weight loss on this organ. 



Rats maintaining weight on stock, SP 8 

 HVO, and SPE diets. — In table 14 are sum- 

 marized for different age groups the results of 

 gross and microscopic examination of the kidneys 

 from rats that were maintaining weight on these 

 three diets at the time of sacrifice. The results 

 for fasted and nonfasted animals have been com- 

 bined. There appeared to be a tendency toward 

 somewhat higher ratings for the presence of hyalin 

 casts when rats were sacrificed without fasting, 

 but the differences were too small to warrant a 

 separation of the results without more data to 

 establish the significance of this trend. 



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