Measuring Optimal Vitamin D Status
Vitamin D and parathyroid hormone (PTH) interact to regulate calcium metabolism, bone turnover and bone mineral density. PTH stimulates the conversion of 25(OH)D3 by the kidneys to the active form which increases the active absorption of calcium by the intestine. Treatments for osteoporosis in postmenopausal women target metabolic pathways involving parathyroid hormone (PTH) secretion to stimulate bone formation and resorption.
When PTH stimulates the release of calcium from bones, this can maintain calcium homeostasis in the blood. However, when blood 25(OH)D3 concentrations are low, the kidneys do not have a substrate to activate to increase calcium absorption. The net effect is a loss of bone mineral density and low calcium uptake. The question of interest is: what is the optimal blood 25(OH)D3 concentration for bone health?
Zhang and colleagues studied serum PTH and 25(OH)D3 concentrations and bone mineral density in 634 males living in China during the winter months. 59% of these men had insufficient circulating vitamin D concentrations (< 50 nmol/L). Only 12.6% were >75 nmol/L. They report mean PTH concentrations were inversely correlated with vitamin D status when serum 25(OH)D3 concentrations were <25 nmol/L, then gradually increased up to ~50 nmol/L and then gradually decreased again as 25(OH)D3 concentrations increased.
Total hip mineral density peaked at slightly greater than 75 nmol/L and then trended downward at concentrations approached 100 nmo/L. Earlier this year, Bischoff-Ferrari and colleagues reported negative effects on lower extremity function (related to falls) in elderly individuals with 25(OH)D3 concentrations >100 nmol/l .
Zhang and colleagues discuss target 25(OH)D3 concentrations reported in the literature, ranging from 30 to 100 nmol/L, and suggest these could be a reflection of vitamin D polymorphisms in the study populations. They specifically mention geographical and race/ethnicity factors.
Age may also be important. A small study (76 children) finds ~50% of Icelandic children had 25(OH)D3 concentrations > 100 nmol/L at 1y of age (mean = 97.5 nmol/L) whereas the means was 55.8 nmol/L by 6 years of age.
The only way to know your vitamin D status is to have it measured.
Zhang Q, Shi L, Peng N, Xu S, Zhang M, Zhang S, Li H, Zhuang H, Gong M, Wu D, Wang R. Serum concentrations of 25-hydroxyvitamin D and its association with bone mineral density and serum parathyroid hormone levels during winter in urban males from Guiyang, Southwest China. 2016 Br J Nutr doi: 10.1017/S0007114515005383
Thorisdottir B, Gunnarsdottir I, Steingrimsdottir L, Palsson GI, Birgistottir GE, Thorsdottir I. Vitamin D intake and status in 6-year-old Icelandic children followed from infancy. 2016 Nutrients doi: 10.3390/nu8020075
Neer RM, Arnaud CD, Zanchetta JR, Prince R, Gaich GA, Reginster J-Y, Hodsman AB, Eriksen EF, Ish-Shalhom S, Genant HK, Wang O, Mellstrom D, Oefjord ES, Marchinowska-Suchowierska E, Salmi J, Mulder H, Halse J, Sawicki AZ, Mitlak BH. Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis. 2001 NEJM doi: 10.1056/NEJM200104105103441904
Sahota O, Mundey MK, Pan S, Godber IM, Lawson N, Hosking DJ. The relationship between vitamin D and parathyroid hormone: calcium homeostasis, bone turnover, and bone mineral density in postmenopausal women with established osteoporosis. 2004 Bone doi: 10.1016/j.bone.2004.02.003
Bischoff-Ferrari HA, Dawson-Hughes B, Orav J, Staehelin HB, Meyer OW, Theiler R, Dick W, Willett WC, Egli A. Monthly high-dose vitamin D treatment for the prevention of functional decline: A randomized clinical trial. 2016 JAMA Intern Med doi: 10.1001/jamainternmed.2015.7148