EFFECT OF MENSTRUAL CYCLE PHASE ON THE GLUCOSE-INSULIN RELATIONSHIP AT REST AND DURING EXERCISE

The purpose of this study was to investigate the effect of menstrual cycle phase on the insulin/glucose relationship at rest and response to sub-maximal exercise. Eight eumenorrheic women completed exercise sessions (60 min, 70% V̇O₂max) in the follicular (day 7±2) and luteal (day 20±2) phases of their cycles. At 45 min before each exercise session the subject consumend an oral glucose load (OGL) of 1 gkg^-1 body weight in a 400 ml solution. Blood samples were obtained before the OGL (-45 min) immediately before exercise, and at 15 min intervals until the end of exercise (60 min). Results indicated that serum glucose and insulin increased (P<0.01) due to the OGL and decreased (P<0.01) due to exercise. No significant phase differences were observed for the glucose responses: however, insulin levels were greater immediately before exercise in the luteal than follicular phase (47.2±5.8 vs. 39.3±5.1 U·ml^-1, respectively). The insulin-to-glucose ratio (I/G) was calculated and also showed significant changes. The I/G ratio was greater in the luteal than in the follicular phase at 0 min (6.5±0.4 vs. 5.3±0.6 mU·mmol^-1, respectively). These data would suggest that in eumenorrheic women the menstrual cycle impacts the insulin/glucose relationship at rest, but has no effect during exercise. Whether the mechanism of the resting effect is due to changes in pancreatic and/or target tissue functions in unclear.