Evidence for an acute rise of intestinal calcium absorption in response to aerobic exercise.

BACKGROUND: The acute effects of physical activity on intestinal calcium (Ca) uptake and on bone metabolism are not known. AIM OF THE STUDY: To investigate the consequences of an acute aerobic exercise bout on fractional Ca absorption and on biomarkers of bone turnover. METHODS: With the use of a cross over design, eighteen male athletes, aged 25.2 (SE 0.6) years, either had to perform a 60 min run (70 % of maximal speed) or had to rest for 60 min. Intestinal Ca absorption (Fc(240)) was assessed by the use of a stable strontium test. Moreover, calciotropic hormones and serum C-Telopeptide (CTx), a biomarker of bone collagen degradation, and serum C-terminal propeptide of type I collagen (PICP), a marker of bone collagen formation, were measured prior (t(-60)) and 3 hours after (t(240)) exercise or rest. RESULTS: Fc(240) values were significantly enhanced in response to exercise compared to rest (16.2 +/- 0.7 % vs. 14.6 +/- 0.8 %; P < 0.05). PICP values were significantly lower in response to exercise compared to rest: -9.8 % (P < 0.05). Exercise did not influence serum levels of intact parathyroid hormone and calcitriol. Serum CTx levels decreased markedly between t(-60) and t(240) during both intervention periods (both P values < 0.001), the results being in line with the circadian rhythm of serum CTx. CONCLUSIONS: A moderate exercise bout can induce an acute rise in fractional Ca absorption. Moreover, even in endurance-trained young men a moderate exercise bout acutely decreases bone collagen formation, while the physiologic fluctuations of the bone resorption marker CTx remain unaffected.

Exercise-trained young men have higher calcium absorption rates and plasma calcitriol levels compared with age-matched sedentary controls.

The effect of physical activity on human calcium (Ca) metabolism is still not completely understood. Thus, we investigated fractional Ca absorption using a stable strontium test (Fc(240)), calciotropic hormones, and renal Ca excretion in 31 young men with a high activity level (GH) and in 26 age-matched sedentary control subjects (GL). Weekly hours spent on physical activity, obtained with a questionnaire were 15.0 +/- 6.6 (GH) and 1.0 +/- 1.4 (GL), respectively. Serum testosterone levels were significantly lower in GH compared with GL (P < 0.005). Dietary Ca intake (4-day food record) was twice as high in GH compared with GL men (P < 0.001). GH had significantly higher serum calcitriol levels and Fc(240) values than GL (P < 0.001 and P < 0.01, respectively). In a stepwise multiple regression analysis including serum levels of 25-hydroxyvitamin D, calcitriol, testosterone, and dietary Ca intake, only calcitriol was significantly correlated with Fc(240) (P = 0. 017). Twenty-four hour renal Ca excretion was only slightly higher in GH compared with GL (P < 0.05). However, additional Ca losses might have occurred through the extensive sweating of GH, as indicated by a difference of 1.7 liter between fluid intake and renal fluid excretion (P < 0.001). In summary, we observed a higher fractional Ca absorption rate in physically active young men compared with sedentary controls which is probably mediated by calcitriol. The low testosterone serum levels of the athletes were obviously not a limiting factor in Ca absorption efficiency. An additional Ca retention might, however, only be obtained if absorbed Ca exceeded total obligatory Ca losses.