The Effects of Fasting on Serum and Urinary Gonadotropins in Obese Postmenopausal Women*

Abstract

Ten postmenopausal women, 12–27% over their ideal weight, fasted for 10 days for weight reduction. All lost at least 4.4% of their weight and demonstrated ketonemia, hyperuricemia (if not treated with Allopurinol), ketonuria, and natriuresis. Serum gonadotropin concentrations measured by RIA for 2 control, 10 fasting, and 2 refeeding days did not differ significantly [(LH, 12.1 ± 2.0 (mean ± SE) mlU/ml (LER 907); FSH, 47.6 ± 5.2 mlU/ml (LER 907)]. The mean urinary excretion of LH (measured by acetone precipitation and RIA) on the control days was 109.3 ± 14.5 (±SE) IU/24 h [Second International Reference Preparation of human menopausal gonadotropin (2nd IRP-hMG)], on fasting day 1 it was significantly higher (139.0 ± 30.3), and by day 8 it had reached 427.7 ± 88.5 (P < 0.0025). On refeeding day 2, a significant decrease in LH excretion occurred (LH, 188.1 ± 23.6 IU/24 h). Similar elevations in LH excretion during fasting were demonstrable when the urine was extracted with kaolin and the LH concentration was determined by RIA or bioassay. Urinary excretion of FSH was likewise determined by RIA (acetone precipitation). The mean amount excreted on control days was 47.0 ± 7.3, on fasting day 1 was 50.4 ± 8.3, on fasting day 8 was 196.1 ± 30.0, and on refeeding day 2 was 86.9 ± 21.2 IU/24 h, 2nd IRP-hMG. After extraction of the urine with kaolin, proportional increases in FSH excretion were confirmed by RIA and bioassay. Chromatography of urinary precipitates on Concanavalin A columns likewise suggested that increased amounts of both LH and FSH were excreted. A portion of the excreted hormone was shown to be desialylated. It is concluded that during relatively brief fasts, postmenopausal women excrete large quantities of gonadotropins. Because the serum concentrations of the gonadotropins did not change, it is suggested that the hypergonadotropinuria reflects an inability of the renal proximal tubular cells to reabsorb and catabolize these small proteins. Under such circumstances, the quantities of gonadotropins excreted in the urine reflect neither pituitary secretion nor blood concentration.