High fat, high protein and high carbohydrate diets have been compared in ex- 

 periments with subjects who lived for long periods in cold chambers. Cold tolerance 

 was highest on the high fat diet, although the high carbohydrate diet was almost as 

 good. High protein diet was markedly inferior. Mitchell, Gluckman and their col- 

 leagues, who were responsible for these experiments, suggest that the high fat diet 

 may owe its value to the laying down of fat in the subcutaneous tissue. 



The thickness of the subcutaneous layer of fat can be of considerable impor- 

 tance as regards insulation. The thermal conductivity of huma'n fat is from V2 - % 

 that of muscle. The difference in conductivity within the body may be even greater 

 as muscle is a much more vuscular tissue than fat. Recently my colleagues Dr Pugh 

 and Dr Hatfield have investigated the effects of immersion in cold water, and were 

 particularly interested in the performance of long distance swimmers. During the 

 last war considerable information was obtained of the survival at sea of shipwrecked 

 sailors. Molnar collected this information and his figures showed the time during 

 which survival was likely at various sea temperatures. His figures suggest that at a 

 water temperature of 15°C, there would be few survivors after five hours' immersion. 

 On the other hand, Channel swimmers may spend from 10- 20 hours in the water, and 

 measurements made last year during the race across the Channel showed that the 

 water temperature, except along the coasts, was approximately 15°C. Observations 

 were made on a number of the competitors in this race, and one volunteered for fur- 

 ther experiments. All the competitors examined were extremely fat with a subcu- 

 taneous layer up to three times that normally expected. Comparisons were made of 

 the rate of cooling of the Channel swimmer and control subjects. In well stirred 

 water kept at 15"^ normal subjects shivered violently and the oxygen consumption 

 rose up to seven times the resting rate. In spite of the violent shivering, rectal tem- 

 perature fell and one subject had to be removed after 40 minutes. The swimmer, on 

 the other hand, remained lying in the water reading a paper, with only very mild 

 shivering and quite comfortable. His metabolic rate was only doubled and there was 

 no fall in rectal temperature. From studies made of the tissue gradients it was clear 

 that the great difference was largely explained by the insulation of the subcutaneous 

 fat. 



It is possible, therefore, that the high calorie, high fat diet, which is preferred 

 in cold regions, may owe some of its value to the increased development of subcu- 

 taneous fat. 



The mechanisms which may be responsible for increased tolerance to cold, de- 

 veloped during long exposure to cold,can include a small rise in basal metabolic 

 rate, a changed distribution of blood permitting a greater constriction of peripheral 

 vessels, an increased insulation provided by fat, and a diminished local effect of 

 cold, the mechanism of which is as yet unknown. 



The greatest and certainly the most important adaptation to cold environments 

 is not these relatively small physiological adjustments but learning how to live in 

 arctic conditions. The insulation required is provided by clothing, the absence of 

 cold injury is due to the avoidance of risks, and a knowledge of the conditions 



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