GENDER DIFFERENCES IN SUBSTRATE UTILIZATION DURING EXERCISE

The selection and utilisation of metabolic substrates during endurance exercise are regulated by a complex array of effectors. These factors include, but are not limited to, endurance training and cardiorespira tory fitness, exercise intensity and duration, muscle morphology and h istology, hormonal factors and diet. Although the effects of these fac tors on substrate utilisation patterns are well understood, the variat ion in substrate utilisation during endurance exercise between males a nd females is not. Because of the extreme heterogeneity in exercise pr otocols and individuals studied, the differences in substrate utilisat ion between males and females remain somewhat inconclusive. Regardless of heterogeneity, if the results from studies are interpreted collect ively, an apparent gender difference in the selection and metabolism o f substrates can be seen in sedentary individuals. However, this diffe rence between genders diminishes as the level of cardiorespiratory fit ness is increased to that of highly trained individuals. During rest a nd lower intensity exercise, the preferential metabolism of lipid occu rs with a concomitant sparing of muscle glycogen. However, as the inte nsity of exercise is increased, the relative contribution of carbohydr ate also increases. The exercise intensity at which the shift from lip id to carbohydrate is determined and regulated by the previously menti oned factors. Because the intensity and duration of exercise play a pr edominant role, the variation in exercise protocols poses a methodolog ical concern when interpreting previous research. When attempting to c ompare the metabolism of substrates during endurance exercise, appropr iate selection and interpretation of measurement techniques are necess ary. Measurement techniques include the nonprotein respiratory exchang e ratio, muscle and fat biopsies and the measurement of various blood metabolites, such as free fatty acids and glycerol. Similarly, in vitr o analysis of lipolytic activity has also been demonstrated in males a nd females in response to varying levels of female gonadotrophic hormo nes. When comparing the substrate utilisation patterns between males a nd females, the area of hormonal regulation has received less attentio n. Often the catecholamine response to endurance exercise is measured; however, the gonadotrophic hormones, particularly those of the female . have received less attention when comparing genders. Indeed, the reg ulatory nature of the female gonadotrophic hormones has been demonstra ted. Collectively, the effects of elevated estrogen, as in the luteal phase of menstruation, appear to promote lipolytic activity. Estrogen- mediated lipolytic activation occurs by apparently altering the sensit ivity to lipoprotein lipase and by increasing the levels of human grow th hormone (somatropin), an activator of lipolysis. Similarly, lipolyt ic activity appears to decrease under situations of lower estrogen lev els (i.e. during the follicular phase of menstruation and in males). I n addition, other cellular mechanisms which may influence substrate ut ilisation include the response of the insulin receptor to varying leve ls of female gonadotrophic hormones. Insulin binding capacity is decre ased in response to elevated levels of estrogen. Parallel to the regul atory effects of the female gonadotrophic levels is the menstrual stat us of endurance-trained females. When females progressively increase t raining volume, menstrual dysfunction becomes increasingly apparent. T hus, those females classified as endurance trained may vary in menstru al function from eumenorrhoeic to oligomenorrhoeic to ultimately ameno rrhoeic. Since menstrual dysfunction may accompany endurance training, the circulating levels of female gonadotrophic hormones diminish to a bnormally low levels. In response to subnormal levels of gonadotrophic hormones, the metabolism of energy substrates will probably change. A dditional areas that may influence substrate utilisation include muscl e morphology and histology. However, differences in the metabolic sele ction of substrates between genders do not appear to be appreciably af fected by these variables. In terms of fibre type distribution, muscle morphology is similar between males and females. Although males have a tendency to retain greater muscle fibre diameter, the overall fibre type distribution is comparable between similarly trained males and fe males. Males reportedly have slightly higher succinate dehydrogenase e nzyme activity regardless of training status. However, malate dehydrog enase activity is similar between genders. Therefore, it appears that muscle enzyme activity is more affected by training status than by gen der. Furthermore, when matched for cardiorespiratory fitness level, ma les and females show few appreciable differences. When considering whe ther there are differences between males and females in terms of the s election and utilisation of metabolic substrates during exercise, nume rous factors are involved: research methodology, training status, musc le morphology and histology, particularly endocrine function.