No abstract

Vegetarian diets are associated with factors that may not support bone health, such as low body mass and low intakes of protein; yet, these diets are alkaline, a factor that favors bone mineral density (BMD). This study compared the correlates of BMD in young, non-obese adults consuming meat-based (n = 27), lacto-ovo vegetarian (n = 27), or vegan (n = 28) diets for ≥1 year. A 24 h diet recall, whole body DXA scan, 24 h urine specimen, and fasting blood sample were collected from participants. BMD did not differ significantly between groups. Protein intake was reduced ~30% in individuals consuming lacto-ovo and vegan diets as compared to those consuming meat-based diets (68 ± 24, 69 ± 29, and 97 ± 47 g/day respectively, p = 0.006); yet dietary protein was only associated with BMD for those following vegan diets. Urinary pH was more alkaline in the lacto-ovo and vegan groups versus omnivores (6.5 ± 0.4, 6.7 ± 0.4, and 6.2 ± 0.4 respectively, p = 0.003); yet urinary pH was associated with BMD in omnivores only. These data suggest that plant-based diets are not detrimental to bone in young adults. Moreover, diet prescriptions for bone health may vary among diet groups: increased fruit and vegetable intake for individuals with high meat intakes and increased plant protein intake for individuals who follow a vegetarian diet plan.

Exercise can cause a decrease in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and bone resorption. We used a novel intravenous iCa clamp technique to determine whether preventing a decline in serum iCa during exercise prevents increases in PTH and carboxy-terminal collagen crosslinks (CTX). Eleven cycling-trained men (aged 18 to 45 years) underwent two identical 60-min cycling bouts with infusion of Ca gluconate or saline. Blood sampling for iCa, total calcium (tCa), PTH, CTX, and procollagen type 1 amino-terminal propeptide (P1NP) occurred before, during, and for 4 hours after exercise; results are presented as unadjusted and adjusted for plasma volume shifts (denoted with subscript ADJ). iCa decreased during exercise with saline infusion (p = 0.01 at 60 min) and this was prevented by Ca infusion (interaction, p < 0.007); there were abrupt decreases in Ca content (iCa and tCa ) in the first 15 min of exercise under both conditions. PTH and CTX were increased at the end of exercise (both p < 0.01) on the saline day, and markedly attenuated (-65% and -71%; both p < 0.001) by Ca. CTX remained elevated for 4 hours after exercise on the saline day (p < 0.001), despite the return of PTH to baseline by 1 hour after exercise. P1NP increased in response to exercise (p < 0.001), with no difference between conditions, but the increase in P1NP was not significant. Results for PTH and CTX were similar to unadjusted results. These findings demonstrate that bone resorption is stimulated early in exercise to defend serum iCa. Vascular Ca content decreased early in exercise, but neither the reason why this occurred, nor the fate of Ca, are known. The results suggest that the exercise-induced increase in PTH had an acute catabolic effect on bone. Future research should determine whether the increase in PTH generates an anabolic response that occurs more than 4 hours after exercise. © 2018 American Society for Bone and Mineral Research.

An exercise-induced decrease in serum ionized calcium (iCa) is thought to trigger an increase in parathyroid hormone (PTH), which can stimulate bone resorption. Purpose The purpose of this study was to determine whether taking a chewable calcium (Ca) supplement 30 minutes before exercise mitigates disruptions in Ca homeostasis and bone resorption in competitive male cyclists. Methods Fifty-one men aged 18 to 45 y were randomized to take either 1000 mg Ca (CA) or placebo (PL) 30 minutes before a simulated 35-km cycling time trial. Serum iCa and PTH were measured before and immediately after exercise and a marker of bone resorption (collagen type-1 c-telopeptide; CTX) was measured before and 30 minutes after exercise. Results Serum iCa decreased in both groups from before to after exercise (mean ± SD CA: 4.89±0.16 to 4.76±0.11 mg/dL; PL: 4.92±0.15 to 4.66±0.22 mg/dL; both p≤0.01); the decrease was greater (p=0.03) in the PL group. There was a nonsignificant (p=0.07) attenuation of the increase in PTH by Ca supplementation (CA: 30.9±13.0 to 79.7±42.6 pg/mL; PL: 37.1±14.8 to 111.5±49.4 pg/mL; both p≤0.01), but no effect of Ca on the change in CTX, which increased in both groups (CA: 0.35±0.17 to 0.50±0.21 ng/mL; PL: 0.36±0.13 to 0.54±0.22 ng/mL, both p≤0.01). Conclusion It is possible that ingesting Ca only 30 minutes before exercise was not a sufficient time interval to optimize gut Ca availability during exercise. Further studies will be needed to determine whether adequate Ca supplementation before and/or during exercise can fully mitigate the exercise-induced decrease in serum iCa and increases in PTH and bone resorption.