The Biology of Senescence 



In McCay's experiments the dietary restriction was confined 

 to reduction of calories. The undernourished majority in the 

 world at the present time derive no benefit in longevity from 

 their circumstances. But it is not impossible, as Edmonds sug- 

 gested in 1832, and as Sinclair (1955) and McCance and 

 Widdowson (1955) have repeated, that adult life may be 

 shortened by the pursuit of excessively rapid growth during 

 childhood. Human puberty can be accelerated by overfeeding 

 (Bruch 1941), and there is already evidence that while the 

 maximum mean height of Englishmen has not increased during 

 the last century, it is now reached no less than five years earlier 

 (Morant 1950), and the loss of height with increasing age shows 

 a parallel advance. 



In 1948, Evans, Simpson and Li confirmed with pure growth 

 hormone Wiesner's (1932) original finding that rats could be 

 kept in continuous growth throughout life by injections of 

 pituitary growth hormone. Wiesner had reported some im- 

 provement in the condition of old male rats under the influence 

 of growth hormone. The experiment of Evans, Simpson and Li 

 was not designed to study the effect of growth on longevity, and 

 they found that continued growth from hormone administra- 

 tion in rats itself leads to death from an increased incidence of 

 tumours. The 12 animals in the original experiment of Evans, 

 Simpson and Li were killed at 647 days for histological pur- 

 poses. With small doses of the purified hormone, 'drug-resist- 

 ance 5 to the growth-promoting and nitrogen-retaining effects 

 develops (Whitney, Bennett, Li and Evans, 1948). In dogs, and 

 cats, continued administration of growth hormone after growth 

 cessation produces not growth but diabetes, while in others 

 (man) epiphyseal fusion prevents continued body growth after 

 sexual maturity. Although the response of rats both to retarda- 

 tion and to growth hormone is apparently atypical, and cer- 

 tainly differs from that which might be expected in man, the 

 possibility exists of comparing in a mammal the effects on rate 

 of senescence of ( 1 ) retarded growth to the full specific size, 

 (2) of accelerated growth up to, and beyond, the specific size, 

 and (3) of growth beyond the specific size, but beginning in old 

 age. Apart from the complication of tumour production, the 

 general endocrine effects of growth hormone are likely to be 



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