bulletin, but animals that were obviously obese 

 were obtained with several of the experimental 

 diets. Marshall, Hildebrand, Dupont, and Wo- 

 mack {126) observed for adult rats an apparent 

 difference in the conversion to body fat of calories 

 from different diets, even on comparable calorie 

 intake. Apparent differences in the utilization of 

 calories by the adult rat fed some of the diets 

 such as SPM or SPPB may be related to the 

 extent of the conversion of calories from these 

 diets to body fat. 



Discussion. — Mickelsen, Takahashi, and Craig 

 {132) observed exceedingly obese rats when the 

 Osborne and Mendel strain of animals were fed ad 

 libitum a diet containing 60 percent Crisco. The 

 largest animal weighed 1,655 grams. Sprague 

 Dawley rats or NIH black rats fed these high-fat 

 diets became heavier than stock animals but not 

 so heavy as the Osborne and Mendel strain. 

 Obese rats approaching 1,000 grams in weight were 

 also observed when the Osborne and Mendel strain 

 were fed the authors' "best" low-fat regimen for a 

 period of 60 weeks. The weight curves of these 

 rats fed the low-fat diet (3 percent) were similar 

 to those already discussed for BHE rats fed many 

 of the experimental diets. A reduction in growth 

 rate occurred at about 15 weeks, with many of the 

 rats continuing to gain slowly throughout the 60- 

 week experimental period. Some rats showed a 

 spurt in body weight gain at the 20th or 30th 

 week. This "best" low-fat diet of Mickelsen, 

 Takahashi, and Craig {132) was a relatively simple 

 diet with casein as the chief source of protein and 

 with a protein level of 25 percent. Sucrose was 

 the carbohydrate in this diet, amounting to 66 

 percent. 



The sucrose content of the semipurified diet and 

 its various modifications discussed in this publica- 

 tion was high (39 to 52 percent) and may be re- 

 sponsible for the ready acceptance of these diets 

 by BHE rats. Food intakes frequently were as 

 much as 19 or 20 grams daily even at a relatively 

 early age. The high digestibility and correspond- 

 ingly low fecal bulk observed by Marshall, Hilde- 

 brand, Dupont, and Womack {126) with similar 

 diets make it possible for rats to consume excessive 

 amounts of these diets without apparent digestive 

 disturbances. In general, the results obtained 

 with BHE rats provide further evidence that 

 obesity in normal male rats may be produced by 

 diet and that rats may overeat voluntarily if sup- 

 plied with diets that are sufficiently acceptable and 

 that can be consumed in relatively large amounts. 

 The failure to obtain equally large rats when the 

 stock diet was fed was probably due to a limitation 

 in the amount of this diet that could be consumed 

 because of the large fecal bulk and the poor digesti- 

 bility observed with this diet {126) . 



Longevity 



In table 12 are summarized data dealing with the 

 survival of rats on all of the experimental regimens 

 under investigation. In addition to the average 



results for the more extensive investigations with 

 SP 8 HVO and SPE diets, data are also presented 

 for individual series to permit a direct comparison 

 of the response of littermates to those diets for 

 which only limited information is available. 



SP 8 HVO diet. — A group of 53 rats representing 

 five experimental series on the semipurified diet 

 provides data on the influence of this diet on 

 longevity. The average age of the animals at 

 death was 629 days, with 50 percent dead by the 

 end of 616 days. Fourteen rats survived more 

 than 700 days. Figure 4 shows the percentage of 

 rats that died -within different age intervals on this 

 diet. The highest death rate occurred between 

 600 and 700 days. 



SPE diet. — Similar data were obtained for 85 

 rats representing eight experimental series on 

 SPE diet (table 12). The lifespan of these rats 

 was considerably shorter than that observed with 

 the semipurified diet; average age at death was 

 464 days, with 50 percent dead by 449 days. 

 Only one of the 85 rats survived more than 700 

 days. As shown in figure 4, the maximum death 

 rate for these rats occurred between 400 and 500 

 days of age. 



Protein-fat-containing foods. — Two series 

 of rats were fed the diets containing milk, beef, 

 or peanut butter. In addition, comparative data 

 were obtained on the response of littermates to 

 SP 8 HVO or SPE diets. The results for the 21 

 littermates fed SP 8 HVO or SPE diets were, in 

 general, similar to those already discussed for the 

 larger number of animals fed these diets. The 

 shortest lifespan was observed for animals fed 

 SPE diet, Rats fed the SPPB diet also tended 

 to die at an early age. Although the average 

 age at death was approximated the same — about 

 580 days— for rats fed SP S HVO, SPM, or SPB 

 diets, the number of rats dying within comparable 

 age intervals was not the same. 



Figure 5 presents data for SPM, SPB, and 

 SPPB diets similar to that seen in figure 4 for 

 SP 8 HVO and SPE diets. On SPM diet there 

 were several early deaths but 8 of the 21 rats 

 survived more than 700 days, in contrast to only 

 3 of the littermates fed SP 8 HVO diet. On SPB 

 diet only 1 rat died before reaching 400 days of 

 age; the maximum death rate occurred between 

 600 and 700 days; 4 rats survived beyond 700 

 days. On SPPB diet the maximum death rate 

 occurred at a somewhat earlier age, between 500 

 and 600 days, with 5 of the 21 rats dead before 

 400 days of age. 



SPE diet with selected supplements. — Rats 

 fed SPE diet supplemented with choline, vitamin 

 B 6 , or vitamin Bi 2 , alone or in combination, were 

 generally as short lived as on SPE diet alone, and 

 there was no evidence that any of the supplements 

 investigated had a measurable effect on the 

 longevity of these animals. The addition of 

 cholesterol or ascorbic acid also seemed to exert 

 little influence on the length of life of rats fed SPE 

 diet. 



22 



