84 Discussion 



with 87 per cent for restricted rats. Incomplete data indicate that 

 the onset of lesions was delayed nine to twelve months. 



Rockstein: What was the maximum weight attained ? 



Berg: The body weight of restricted rats was 25 per cent lower 

 than maximum weight of ad libitum-fed animals. 



Rockstein: Is the protein content restricted to the same extent as 

 the caloric value of this diet ? 



Berg: The protein content of the diet was 20 per cent and was the 

 same for the restricted diet. 



BourUere: Have you measured the basal metabolic rate in both 

 restricted and unrestricted animals of the same age ? 



Berg: No, I have not. 



Comfort: In view of the interesting similarities between the life- 

 span curves in man and in rats, it pays to remember, when consider- 

 ing dietary restrictions, that there are differences in growth patterns 

 between them. Your rats show virtually determinate growth, but 

 under some conditions the rat grows in weight and in bone length for 

 most of its life. This is a very different situation from that in man. 



Berg: After 170 days, skeletal growth practically ceases in the rat 

 though some of the epiphyses remain open for the entire lifespan. 

 There is no evidence of osteogenesis in the cartilage plate of the 

 tibial epiphysis of old rats. Increments in body weight of ageing rats 

 are due largely to fat accumulation associated with prolonged 

 inactivity. 



Comfort : But my point was that in man you could not, I imagine, 

 restrict growth. The effect of underfeeding on the growth pattern 

 and on the appearance of sexual maturity in man may be different 

 from the effects you can produce in rats. McCay kept his rats 

 infantile for over 1,000 days; I doubt whether a comparable effect 

 could be produced in man. 



Berg: I think that if we had a comparable inbred strain of glutton- 

 ous men, and could perform a similar experiment, we might obtain 

 results corresponding to those in the rat. 



Tanner: I am accustomed to dealing with growth data rather than 

 with data dealing with the other end of the lifespan. But methodo- 

 logically there are very great similarities. We must consider, for 

 example, the implications of the use of chronological age in all these 

 data. One possible interpretation of your data could be as follows. 

 We think of children or animals growing in the same way as we 

 think of trains moving along a railway line. You can think of the 

 various diseases as trapdoors on the railway line. You can either 

 consider that those trapdoors have been moved nearer the start so 

 that the train gets to them earlier, or you can consider that the 



