WEIGHT GAIN 

 (Grams per week) 



FOOD INTAKE 

 (Grams per week) 



120 



60 











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f / 









40 



Jl 



\ 

 \ 

 \ 



\ * 



Vv 



DIET FOOD 

 INTAKE 



SP 8 HVO A-a 



WEIGHT 

 GAIN 





— 



20 



I 



I . 1 I 1 



SPE o--o 



*• i ^~- 



, I . I 



• — • 













, *" l 



1 t 



- 80 



40 



40 



80 



200 



240 



120 160 



DAYS ON DIET 



Figure 1. Average weight gain and food intake of rats fed SP 8 HVO and SPE diets. 



280 



Body weight and age — individual rats fed 

 SP 8 HVO and SPE diets.— Many of the rats 

 continued to gain as long as they appeared to 

 remain healthy, increasing their food intake when- 

 ever there was a tendency for their body weight 

 to remain constant. This response is best illus- 

 trated in figure 2, which shows the change of 

 weight with age of three individual animals. 

 Curve 1 represents data for a rat that was fed SPE 

 diet and died before he reached 400 days of age. 

 This animal gained rapidly for 350 days, at which 

 time a precipitous weight loss occurred in spite of 

 a food intake averaging 17 grams per day. Curve 2 

 shows the continued gain for 550 days of a rat 

 fed SPE diet and the rapid loss in weight that 

 occurred during the 25 days before death. This 

 animal had decreased his food intake slightly from 

 14 to 12 grams daily. Curve 3 represents the 

 body weight of a rat that was fed SP S HVO diet 

 and was still gaining when S00 days old. A 

 marked decrease in food intake from 19 to 9 grams 

 daily paralleled the decrease in body weight ob- 

 served between 800 and 900 days of age, at which 

 time the animal was obviously moribund. 



Average body weight and age (stock, SP 8 

 HVO, and SPE diets). — Data for weight changes 

 throughout life were obtained for 53 rats fed SP 8 

 HVO diet and for 74 fed SPE diet. The tendency 

 for continuing increase in weight with age noted 

 for individual rats (fig. 2) was not apparent in the 

 average weight curves of rats fed these two diets 

 (fig. 3). The third curve in figure 3 represents 

 cross-sectional data available for a large group of 

 rats from the stock colony that were sacrificed at 

 different age intervals. Animals fed the stock 



12 



diet tended to be smaller than those fed SP 8 HVO 

 or SPE diets and showed little change in average 

 weight after 250 days. Of these animals 52 per- 

 cent weighed between 450 and 500 grams; 2 per- 

 cent exceeded 600 grams. In contrast, 59 percent 

 of the rats fed SP 8 HVO diet and 61 percent of the 

 SPE-fed rats weighed 600 grams or more, and 21 

 and IS percent, respectively, exceeded 700 grams in 

 weight. Rats fed SPE diet appeared to have 

 reached their maximum weight by 350 days, 

 whereas those fed SP 8 HVO diet attained a 

 comparable average weight between 500 and 600 

 days of age. The lower average weights observed 

 for the older surviving rats fed SP 8 HVO and 

 SPE diets seem to be due to the early death of 

 many of the heavy rats, and will be discussed 

 further in relation to the longevity data. 



Discussion. — Reports in the literature on 

 weight changes in the rat throughout life have been 

 chiefly for diets of natural foods suitable for raising 

 stock animals. Changes made in the diets of 

 stock animals based on increasing knowledge of 

 their nutritional requirements have resulted in an 

 appreciable increase in their size. Mendel and 

 Hubbell {128) have reported a gradual increase in 

 rate of growth of their stock ("Yale" strain) rats 

 over a period of 25 years, with the most marked 

 change occurring when the diet of Anderson and 

 Smith (9) was introduced. This diet produced 

 adult animals weighing about twice as much as 

 rats on the earlier stock diets. This change was 

 attributed to diet rather than to selective breeding, 

 and the improved growth rate was accompanied 

 by superior reproductive performance. Mayer 

 {127), also using the "Yale" strain of rats, reported 



