SHORT-TERM FASTING AFFECTS LUTEINIZING-HORMONE SECRETORY DYNAMICS BUT NOT REPRODUCTIVE FUNCTION IN NORMAL-WEIGHT SEDENTARY WOMEN

Acute food withdrawal reversibly inhibits the hypothalamic-pituitary-gonadal axis in men and in rhesus monkeys, and it produces defects in LH pulsatility in normal-weight women. However, the clinical effect of short-term nutritional deprivation on the reproductive axis of normally cycling women has not been evaluated. Thus we studied the effect of a 3-day fast during the midfollicular phase on menstrual cycle length, gonadotropin secretory patterns, follicular development, and ovulation. After a baseline ovulatory cycle, 12 women within 15% of ideal body weight were randomized to be fed (n = 7) or fasted (n = 10) on cycle days 7 to 9. Five of the women repeated the study and received the alternate diet. Endocrine and metabolic parameters of fasting and reproductive physiology were measured on cycle days 6 to 12. Fasted physiology was demonstrated by characteristic alterations in growth hormone, insulin-like growth factor I, TSH, and T-3 levels. During fed cycles, the number of LH pulses remained constant on cycle days 6, 9 and 11, whereas mean LH levels, LH area under the curve, and LH pulse amplitude increased significantly over this time (all P < 0.05). In contrast, fasted cycles were marked by a significant decrease in the number of LH pulses on the last day of the fast (cycle day 9, P < 0.05) and by a lack of increase over time of mean LH values, LH area under the curve, and LH pulse amplitude. Follicle development, as assessed by daily ultrasound examination and estradiol measurements, was similar in all cycles and was followed by ovulation in all women; follicular and luteal phase lengths of fasted and fed cycles were similar. We conclude that the alterations in LH secretory dynamics that occur during a S-day fast are not sufficient to perturb follicle development and cycle lengths in normal-weight sedentary women. The resilience of the reproductive axis in these healthy women contrasts with the sensitivity of the hypothalamic-pituitary-gonadal axis to acute nutritional withdrawal in men and in monkeys. We speculate that differences in the set point for sensing fuel availability or in the status of energy balance before the initiation of food deprivation may account for these species-specific and sex-specific variabilities.