Sex difference in the influence of obesity on the retention of a tracer of ³H-estradiol

The influence of obesity on the retention of a tracer of ³H-estradiol was studied in 15 nonobese premenopausal women, 15 obese premenopausal women (49%-274% above desirable weight), and 27 young men ranging in weight from 5% below to 330% above desirable weight. The women showed a clear-cut inverse linear correlation between the 72 hr excretion of radioactivity and the percent deviation from desirable weight over the entire weight range examined (y = 66 - 0.10x, r = -0.59, P < 0.005); the average excretion in the 6 most obese women (145%-274% above desirable weight) was 45 ± 11 (SD)%, significantly lower than the value of 65 ± 12% in 15 nonobese women (P < 0.025). The obese men showed no correlation whatever between excretion of radioactivity and relative body weight; the average excretion of the 6 most obese men was 55 ± 7, not significantly different from the value of 56 ± 12 in nonobese men. This sex difference makes untenable the hypothesis previously proposed by others that retention of estradiol tracers in obese women (men were not studied) is due to simple solubility of estrogens in fat. Various alternative possibilities to explain the present data are discussed and it is concluded that a possibility worth examining is that the adipose tissue of women contains specific estrogen binding protein (? receptor) while the adipose tissue of men does not.