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Vitamin D and calcium absorption

Vitamin D and calcium absorption is an example of gestalt biology or 1 + 1 = 3.  Both compounds independently appear to enhance bone metabolism.  Vitamin D and calcium absorption together have a Vitamin-D-and-Calcium-Absorption | Review Articlemuch greater benefit.  In a 2007 New England Journal of Medicine article entitled:  Vitamin D Deficiency, Holick quantified the impact of vitamin D and calcium absorption: 

“Without vitamin D, only 10 to 15% of dietary calcium and about 60% of phosphorus is absorbed. The interaction of 1,25-dihydroxyvitamin D with the vitamin D receptor increases the efficiency of intestinal calcium absorption to 30 to 40% and phosphorus absorption to approximately 80%.

In one study, serum levels of 25-hydroxyvitamin D were directly related to bone mineral density in white, black, and Mexican-American men and women, with a maximum density achieved when the 25-hydroxyvitamin D level reached 40 ng per milliliter or more.  When the level was 30 ng per milliliter or less, there was a significant decrease in intestinal calcium absorption that was associated with increased parathyroid hormone.. parathyroid hormone enhances the tubular reabsorption of calcium and stimulates the kidneys to produce 1,25-dihydroxyvitamin D.  Parathyroid hormone also activates osteoblasts, which stimulate the transformation of preosteoclasts into mature osteoclasts. Osteoclasts dissolve the mineralized collagen matrix in bone..."

How do mechanism of action with vitamin D and calcium absorption?   According the Linus Pauling Institute, the two promote calcium balance.

“Maintenance of serum calcium levels within a narrow range is vital for normal functioning of the nervous system, as well as for bone growth and maintenance of bone density. Vitamin D is essential for the efficient utilization of calcium by the body.  The parathyroid glands sense serum calcium levels and secrete parathyroid hormone (PTH) if calcium levels drop too low. Elevations in PTH increase the activity of the 25-hydroxyvitamin D3-1-hydroxylase enzyme in the kidney, resulting in increased production of 1,25-dihydroxyvitamin D. Increasing 1,25-dihydroxyvitamin D production results in changes in gene expression that normalize serum calcium by 1) increasing the intestinal absorption of dietary calcium, 2) increasing the reabsorption of calcium filtered by the kidneys, and 3) mobilizing calcium from bone when there is insufficient dietary calcium to maintain normal serum calcium levels. Parathyroid hormone and 1,25-dihydroxyvitamin D are required for these latter two effects.”

A recent meta-analysis published in the Lancet illustrated the bone density results from calcium alone or vitamin D and calcium absorption.  Tang concluded “The...effect was better with calcium doses of 1200 mg of more than with doses less than 1200 mg (0.80 vs. 0.94; p=0.006) and with vitamin D doses of 800 IU of more than with doses less than 800 IU (0.84 vs. 0.87; p= 0.03).” 

One notable exception in Tang’s meta analysis was a study of elderly women taking AdvaCAL Vitamin-D-and-Calcium-Absorption | Review Articlecalcium.  Daily intake was only 900 mg of AdvaCAL calcium with no vitamin D. The AdvaCAL study participants experienced  fracture risk reduction, albeit among a small study population. Adding vitamin D to AdvaCAL could have further improved those impressive results, based on the known symbiosis of vitamin d and calcium absorption