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Secretory IgA (S-IgA) levels in stimulated salvia were measured before and after a maximal treadmill test in a population of 175 young adults (84 men, 91 women). Habitual physical activity levels of this population were measured by a standardized interview, and scores were obtained for the amount of time and intensity of sports activities and total activity. Before the maximal treadmill test, men and women showed no statistically significant differences in the salivary concentrations of S-IgA, but the concentration of S-IgA in women decreased and in men increased statistically significantly (P less than 0.05) after the treadmill test. Only women showed a significant correlation between absolute values of S-IgA and the weekly amount of time spent on sports activity measured by a standardized interview expressed in minutes per week.

A population of young adults, 92 men and 107 women, with a mean age of 21 (+/- 0.7) years, were retrospectively questioned about their habitual physical activity levels (period of 3 months) and the incidence and duration of upper respiratory tract infection (URI) symptoms (period of 6 months). We hypothesized that the incidence and duration of URI symptoms were inversely related with the level of sports activity and total physical activity. Only in women was a statistically significant (P less than 0.05) negative relation found between the incidence of URI symptoms and the level of sports activity. However, the low Spearman's rho coefficient (-0.18) indicates a very weak relation between both parameters.

In the Amsterdam Growth and Health Study, 103 girls and 97 boys were studied five times on a longitudinal basis over a period of 8 years, covering the teenage years from 12 to 17 until young adulthood at 22/23 years. Measured were anthropometric variables such as height, weight (BW), and body fat, and physiological variables such as maximal aerobic power (V̇O2max) and endurance performance (max slope). During the teenage period, V̇O2max/BW remains constant in boys and decreases in girls whereas endurance performance increases in boys and remains constant in girls. By young adulthood V̇O2max/BW and maximal slope have declined in both sexes, and in the case of females are even lower than at the beginning of their teens. Boys superiority in aerobic fitness and the decline in aerobic fitness in both sexes is mainly caused by the differences in the intensity of daily physical activity level.

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