Relationship between serum parathyroid hormone levels, vitamin D sufficiency, and calcium intake

Author(s): Steingrimsdottir L, Gunnarsson O, Indridason OS, Franzson L, Sigurdsson G

Abstract

Abstract

Context Adequate vitamin D status for optimum bone health has received increased recognition in recent years; however, the ideal intake is not known. Serum 25-hydroxyvitamin D is the generally accepted indicator of vitamin D status, but no universal reference level has been reached.

Objective To investigate the relative importance of high calcium intake and serum 25-hydroxyvitamin D for calcium homeostasis, as determined by serum intact parathyroid hormone (PTH).

Design, Setting, and Participants Cross-sectional study of 2310 healthy Icelandic adults who were divided equally into 3 age groups (30-45 years, 50-65 years, or 70-85 years) and recruited from February 2001 to January 2003. They were administered a semi-quantitative food frequency questionnaire, which assessed vitamin D and calcium intake. Participants were further divided into groups according to calcium intake (<800 mg/d, 800-1200 mg/d, and >1200 mg/d) and serum 25-hydroxyvitamin D level (<10 ng/mL, 10-18 ng/mL, and >18 ng/mL).

Main Outcome Measure Serum intact PTH as determined by calcium intake and vitamin D.

Results A total of 944 healthy participants completed all parts of the study. After adjusting for relevant factors, serum PTH was lowest in the group with a serum 25-hydroxyvitamin D level of more than 18 ng/mL but highest in the group with a serum 25-hydroxyvitamin D level of less than 10 ng/mL. At the low serum 25-hydroxyvitamin D level (<10 ng/mL), calcium intake of less than 800 mg/d vs more than 1200 mg/d was significantly associated with higher serum PTH (P = .04); and at a calcium intake of more than 1200 mg/d, there was a significant difference between the lowest and highest vitamin D groups (P = .04).

Conclusions As long as vitamin D status is ensured, calcium intake levels of more than 800 mg/d may be unnecessary for maintaining calcium metabolism. Vitamin D supplements are necessary for adequate vitamin D status in northern climates.

The importance of adequate vitamin D status for optimum bone health has received increased recognition in recent years, with higher recommended intake levels being proposed by some investigators.1-3 The ideal intake is not known, and different criteria have been proposed for estimating population requirements. Serum 25-hydroxyvitamin D has been the generally accepted indicator of vitamin D status, but no universal consensus has been reached regarding which serum values constitute sufficiency.4,5 A further difficulty in assigning a universal reference value may lie in the interrelationship between several factors influencing calcium homeostasis, not the least being the 2 nutrients vitamin D and calcium.

An inverse relationship between serum 25-hydroxyvitamin D and serum parathyroid hormone (PTH) is well established, up to a certain level of 25-hydroxyvitamin D, in which little further decrease in serum PTH is observed. The serum level for 25-hydroxyvitamin D corresponding with the PTH inflection point has been interpreted as indicative of optimal calcium homeostasis and proposed as a marker of vitamin D sufficiency.5,6 However, this approach to defining recommended vitamin D intake has been disputed, partly because there is considerable variation in the level of 25-hydroxyvitamin D associated with any given serum PTH concentration, and reported threshold levels have varied greatly from 8 to 44 ng/dL.7,8 This wide range may be in part due to different methods for measuring both serum PTH and 25-hydroxyvitamin D and defining baseline levels,9 and possibly also due to different calcium intakes in study populations since serum calcium regulates PTH release.10 The interrelationship between calcium intake and vitamin D requirements has not been addressed adequately in the past.

The goal of our study was to investigate the relative importance of high calcium intake and serum 25-hydroxyvitamin D for calcium homeostasis in healthy adults, as determined by serum intact PTH.

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