When extra calcium is absorbed, as much calcium is temporarily stored in the bones, and deported from the bones as soon as there is the opportunity to do so. (1) When little calcium is consumed, dietary calcium absorption rate is greater and there is less calcium to deport. (2)
Due to exercise, bone-cells are destroyed, which stimulates the bones to hold on to more calcium, to be better able to cope with future burdening.
And if there is no loading of the bones, the bones will deport more calcium, since there seems to be no need to hold on to it.
In astronauts bone-mineral density (BMD) averagely decreases 1% to 2% each month. But the lower weight-bearing bones appeared more sensitive than the upper ones. (3) However, BMD of the skull, a non-weight bearing bone, does not decrease at all. (4)
In the MIR 97 mission high calcium intake and vitamin D supplementation led to decreased bone formation and increased bone-calcium deportation. (5) And also calcium absorption is reduced during immobilization. (6) Like bone-calcium absorption is increased when physical active. (7)
Because the bones experienced no burdening in space, there ‘apparently’ was less need for minerals in the bones, and thus BMD decreased.
No matter how much calcium was consumed, the bones did not hold it, because ‘there was no need to do so’. Apparently not just deportation of calcium was increased, but even calcium absorption and bone-formation was decreased, to prevent the need for subsequent calcium deportation.
And the bones appear to be sensitive to this stimulus according to the need for adaptation given earthly circumstances, which is different for the legs, the arms, the head etc.
So the bones “are not stupid”. They hold the calcium they need. And they deport what is not needed.
All the redundant calcium that is consumed, always is to deported too. The more calcium is processed, the sooner the cells who have to do so, will be worn out, eventually causing osteoporosis.
Abstracts of these sources can be found at The National Library of Medicine
(1) Bronner F., et al, Development and regulation of calcium metabolism in healthy girls. J. Nutr. 1998 / 128 (9) / 1474-1480. , O'Brien K.O., et al, Variables related to urinary calcium excretion in young girls. J. Pediatr. Gastroenterol. Nutr. 1996 / 23 (1) / 8-12. , Lee, W.T. et al, A follow-up study on the effects of calcium-supplement withdrawal and puberty on bone acquisition of children. Am. J. Clin. Nutr. 1996 / 64 (1) / 71-77.
(2) O'Brien, K.O. et al, Increased efficiency of calcium absorption from the rectum and distal colon of humans. American Journal of Clinical Nutrition 1996 / 63 (4) / 579-583.
(3) Collet P, et al, Effects of 1- and 6-month spaceflight on bone mass and biochemistry in two humans. Bone 1997 /20 (6) / 547-551.
(4) Miyamoto A, et al, Medical baseline data collection on bone and muscle change with space flight. Bone 1998 / 22 (5 Suppl.) / 79S-82S.
(5) Heer M, et al, Calcium metabolism in microgravity. Eur. J. Med. Res. 1999 / 4 (9) / 357-360.
(6) Branca F., Physical activity, diet and skeletal health. Public Health Nutr. 1999 / 2 (3A) / 391-396.
(7) Zittermann A, et al, Exercise-trained young men have higher calcium absorption rates and plasma calcitriol levels compared with age-matched sedentary controls. Calcif. Tissue Int. 2000 / 67 (3) / 215-219.
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