Gender differences in carbohydrate loading are related to energy intake MARK A. TARNOPOLSKY,1,2 CAROL ZAWADA,2 LINDSAY B. RICHMOND,2 SHERRY CARTER,1 JANE SHEARER,3 TERRY GRAHAM,3 AND STUART M. PHILLIPS2 1 Departments of Medicine (Neurology and Rehabilitation) and 2Kinesiology, McMaster University, Hamilton, Ontario L8N 3Z5; and 3Human Biology and Nutrition, University of Guelph, Guelph, Ontario, Canada N1G ZW1 Received 5 July 2000; accepted in ﬁnal form 6 April 2001
Tarnopolsky, Mark A., Carol Zawada, Lindsay B. Richmond, Sherry Carter, Jane Shearer, Terry Graham, and Stuart M. Phillips. Gender differences in carbohydrate loading are related to energy intake. J Appl Physiol 91: 225–230, 2001.—We demonstrated that female endurance athletes did not increase their muscle glycogen concentration after an increase in the dietary carbohydrate intake (58 3 74%), whereas men did (Tarnopolsky MA, SA Atkinson, SM Phillips, and JD McDougall, J Appl Physiol 78: 1360–1368, 1995). This may have been related to a lower energy or carbohydrate intake by the women or due to an inherent gender difference in glycogen storage capacity. We examined whether well-trained men (n 6) and women (n 6) increased muscle glycogen concentration after an increase in both the relative (58 3 75%) and absolute energy and carbohydrate intake and whether potential gender differences were related to muscle hexokinase enzyme activity. Subjects were randomly allocated to three diets [Hab, habitual; CHO, high carbohydrate (75%); and CHO E, extra energy CHO (1 34%)] for a 4-day period before a muscle biopsy for analysis of total and pro- and macroglycogen and hexokinase activity. Total glycogen concentration was higher for the men on the CHO and CHO E trials compared with Hab (P 0.05), whereas women increased only on the CHO E trial compared with Hab (P 0.05). There were no gender differences in the proportion of pro- and macroglycogen or hexokinase activity. A low energy intake may explain the previously reported lower capacity for women to glycogen load compared with men. sex difference; hexokinase; glycogen; ergogenic aids
AN INCREASING NUMBER OF STUDIES are ﬁnding gender differences in the metabolic response to endurance exercise (6, 10, 15, 17, 22–24). Consistently, studies have shown that women have a lower respiratory exchange ratio (RER) compared with men during submaximal endurance exercise (6, 10, 15, 17, 22–24). Animal data suggest that the female sex hormone 17- -estradiol may mediate these metabolic differences (5, 12, 13, 18). The aforementioned gender differences could have implications for nutritional recommendations for female athletes. For example, our laboratory previously
reported that female endurance athletes did not increase muscle glycogen concentration in response to an increase in dietary carbohydrate intake from 58 to 74% of energy intake, yet men showed an increase with the same dietary modiﬁcation (23). Furthermore, a recent study has also found that women increase muscle glycogen in response to a high dietary carbohydrate intake “but the magnitude was smaller than that previously observed in men” (25). Both groups proposed several mechanisms to explain the apparent gender difference in carbohydrate loading. First, dietary energy intake is lower in women compared with men, even when expressed relative to lean mass (17, 23). This translates into a lower carbohydrate intake for women compared with men when expressed relative to fat-free mass (FFM; the largest pool of glucose disposal). For example, we found that the carbohydrate intake for men was 7.7–9.6 g kg FFM 1 day 1 and for women was 5.9–7.9 g kg FFM 1 day 1, when the subjects increased their carbohydrate intake from 58 to 74% of energy intake (23). The majority of studies examining carbohydrate loading have been conducted using predominantly or exclusively male subjects, and the carbohydrate intake on the high carbohydrate diet was usually...