butter grew more rapidly and used their diet more 

 efficiently for growth than did those fed the 

 semipurified diet. Many of the rats continued to 

 gain in weight as long as they remained healthy 

 and some became extremely large, particularly 

 when the diets contained milk or peanut butter. 

 The differences in body weight observed were not 

 consistently related to food intake or to level of 

 dietary fat. 



The lifespan of BHE rats fed the various experi- 

 mental diets differed widely. At death, kidney 

 damage was a frequent finding regardless of diet. 

 The extent of the damage varied among diets and 

 generally paralleled the length of survival. The 

 shortest average lifespan and the most extensive 

 kidney damage were observed with diets contain- 

 ing 25 percent egg. When rats fed the egg- 

 containing diet, SPE, during the first 250 days of 

 life were then placed on stock ration, their lifespan 

 was significantly longer than when the SPE diet 

 was fed throughout life. When the feeding of this 

 diet was delayed until the rats reached 250 days 

 of age, length of life was also prolonged. Survival 

 was longer with a diet consisting of 100 percent 

 whole egg than with one containing 25 percent 

 egg. Some extremely long-lived rats were ob- 

 tained when the milk-containing diet was fed. 

 When the dietary fat was HVO, animals tended 

 to live longer than when the fat was lard or butter. 



No evidence was obtained of any nutritional 

 deficiency in the diets under investigation. 

 Neither level of fat nor level of protein explained 

 the differences observed. The results, particu- 

 larly those with 100 percent whole egg or egg 

 yolk in contrast to those with 25 percent egg, 

 suggest that the rate of the development of 

 untoward changes in the tissues was related to 

 the particular combination of nutrients under in- 

 vestigation rather than to any one dietary in- 

 gredient. 



Research is needed to determine the specific 

 combination of nutrients responsible for acceler- 

 ating changes in the tissues which result in early 

 death, and to establish whether or not the age 

 span during which the diets are fed is a critical 

 factor in determining the response to such diets. 

 The possibility also should be explored of estab- 

 lishing criteria that would detect at an early age 

 possible adverse effects of specific nutrient 

 combinations. 



A factor that appears to complicate interpre- 

 tation of the longevity data is the tendency for 

 some rats to eat excessively and to gain at a very 

 rapid rate. Animals weighing 600 grams or more 

 by the time they reached 200 days of age died at 

 an early age regardless of diet. 



Comparative studies of rats fed controlled as 

 well as ad libitum amounts of food are needed to 

 permit a more accurate assessment of the data 

 on longevity in relation to diet, as well as to deter- 

 mine the possible adverse effect of excessive 

 food consumption at various stages in the life 

 cycle. 



Microscopic examination of the tissues indicated 

 that the kidney was the organ most frequently 

 found to appear abnormal, and that kidney damage 

 was observed in rats showing no obvious signs of 

 ill health, as well as in moribund rats. Some diets 

 obviously hastened the onset of lesions and also 

 appeared to influence the type of degenerative 

 changes observed. A kidney weighing more than 

 1.8 grams usually showed evidence of degenerative 

 change, and extensive damage was apparent in 

 kidneys weighing more than 3 grams. The in- 

 fluence of diet on the composition of the kidney 

 depended chiefly on its influence on the size of this 

 organ. Enlarged kidneys generally contained a 

 relatively high percentage of protein and a low 

 percentage of fat. Although high ash values were 

 also found frequently, calcium deposition as deter- 

 mined microscopically did not necessarily parallel 

 the percentage of ash in the kidney. 



Microscopic examination of the livers revealed 

 little evidence of degenerative changes in this 

 organ, although both histological and chemical 

 measurements showed a high fat content in the 

 livers of rats on diets containing high levels of egg. 



The kidneys as well as the livers from rats 

 subjected to a 17-hour fast before sacrifice were 

 generally smaller than those from nonfasted rats. 

 The increasing difference with age between the 

 weight of these organs from fasted and nonfasted 

 rats fed stock diet suggests that the physiological 

 activity of these organs is decreasing with age. 

 Calcium deposits in the kidney also appeared to 

 depend upon the fasting state of the rat at the 

 time of sacrifice. 



Comparative data on the tissues of fasted and 

 nonfasted rats at different intervals throughout life 

 might well contribute information on the aging 

 processes. 



The size of the adrenal and thyroid glands 

 seemed to be influenced by diet when comparisons 

 were made for animals that were maintaining weight 

 when sacrificed. The influence of diet on the 

 thyroid weight was apparent even in relatively 

 young rats, whereas the influence of diet on the 

 adrenal was seen chiefly in older rats. In mori- 

 bund rats, large adrenals and thyroids were a 

 frequent finding regardless of diet. 



In rats that were maintaining weight at the 

 time of sacrifice, serum cholesterol levels varied 

 with age and with diet and appeared to bear no 

 consistent relation to level of dietary cholesterol 

 or fat. With the stock diet, serum levels were 

 generally low at all ages. With the various 

 modifications of the semipurified diet, serum 

 cholesterol levels tended to be high in older rats. 

 Some extremely high levels were observed, even 

 at a relatively early age, in rats fed the cholesterol- 

 containing egg diets. Cholesterol levels also 

 tended to be high in rats fed the diet containing 

 20 percent peanut butter, although the cholesterol 

 content of this diet was low. In moribund rats, 

 serum cholesterol levels exceeding 200 mg./lOO 

 ml. were observed frequently regardless of diet, 



92 



