Effect of high-fat diet on thermal acclimation with special reference to thyroid activity.

In an attempt to clarify why the seasonal variation of basal metabolic rate (BMR) of Japanese is different from that of Caucasians (white American and Canadian), animal experiments were performed to clarify the effect of dietary composition on thyroid activity, metabolic adaptation to heat, and tolerance to cold, using groups of rats fed on different kinds of diets. Results obtained are as follows:1) By radioisotope techniques with 13II, it was demonstrated that the 131I uptake of thyroid gland is higher and PB131I is higher in the high-fat diet groups than in the control diet rats, while the conversion ratio and the fractional turnover rate of thyroxine are not significantly different between the two groups. Thus it is inferred that the thyroid activity is promoted by feeding rats a high-fat diet.2) The resting oxygen consumption of rats which were reared in the control temperature.(20°C) as well as kept at 30°C for 5 weeks was higher in the high-fat diet group than in the high-carbohydrate group. The resting metabolism was reduced by heat acclimation. The reduction rate of 02 consumption in heat acclimation was less in the high-fat diet group than in the high-carbohydrate group.3) Assuming that an adaptative process of the thyroid gland may be involved in the mechanism of metabolic adaptation to heat and cold and that the experimental results of metabolism in rat may be applicable to the mechanism of metabolism in man, the following explanation was presented for the difference of seasonal variations of BMR between Caucasian (white American and Canadian) and Japanese. Japanese present a seasonal variation of BMR, increasing in winter and decreasing in summer, as an outcome of metabolic adaptation to heat and cold in various seasons. In Caucasians, however, their thyroid activity is promoted by the intake of a high-fat diet. This promotion of thyroid activity may inhibit the reduction of metabolism in heat acclimatization, thus maintaining the BMR at a constant level throughout the year. Thus the appearance of climatic effect in the BMR of Japanese may be due to their habitual high-carbohydrate diet.4) In acute exposure to air of 10°C, the high-fat diet group showed superiority in maintenance of body temperature in the cold, while the high-carbohydrate group showed a most remarkable drop of rectal temperature.