Menstrual phase and exercise: Intermediary effects on glucose metabolism

There are several reports examining the effects of menstrual phase on glucose homeostasis. The results of these investigations have been unequivocal. The results of the euglycemic hyperinsulinemic clamp studies are consistent with the notion that glucose metabolism is not altered during the menstrual cycle, whereas the results of the hyperglycemic hyperinsulinemic clamp studies are consistent with the notion that glucose uptake deteriorates during the progesterone-dominated luteal phase. The primary role of carbohydrates, both at rest and during exercise, is to serve as an anergy reservoir for the body. It is well established that both the intensity and duration of exercise per se determine the rate of glucose uptake by working muscles. Uptake of glucose increases with exercise of increasing intensity and duration. Exercise training results in an 'overall carbohydrate sparing effect.' Carbohydrate refeedings during exercise increase the level of blood glucose and insulin concentrations and therefore increase (double, in some cases) the rate of glucose uptake in exercising skeletal muscle. The carbohydrate refeedings do not spare muscle glycogen utilization; the effect of the carbohydrate refeedings is to maintain blood glucose. It has been estimated that carbohydrate refeedings can delay fatigue by as much as 30 to 60 minutes but fatigue does ultimately occur suggesting that factors other than carbohydrate availability cause fatigue. Most researchers contend that physical performance is not dramatically altered by the menstrual cycle. Maximal and submaximal V̇O₂, V̇CO₂, heart rate, and blood pressure responses generally remained unchanged during the different phases of the menstrual cycle. V̇E, HLa, T(max), and RPE responses to exercise in highly trained eumenorrheic and eumenorrheic athletes require more detailed study. Furthermore, it may be more appropriate to examine these responses with respect to endurance performance, rather than to a given exercise intensity, to delineate differences in these variables during the menstrual cycle. Endurance tests may be more sensitive in detecting differences in exercise performance related to menstrual cycle phase and thus warrant further study. Whether the differences in the ovarian hormonal milieu between the menstrual phases result in altered glucose metabolism during exercise remains to be undisputedly determined. The more recent data of Nicklas and colleagues and Lavoie and colleagues however, guide us toward the concept of menstrual phase indeed having an effect on exhaustive exercise performance and metabolism.