Relationship between serum parathyroid hormone, vitamin D sufficiency, age, and calcium intake
Introduction
Vitamin D deficiency is extremely common among elderly subjects. Together with the accompanying increase in serum parathyroid hormone (PTH) it has been associated with poor bone health, and to a number of other conditions, such as impaired muscle function and some tumours [1]. The correlation between 25-hydroxy-vitamin D [25(OH)D] and PTH is so strong that the point along the serum 25(OH)D continuum at which PTH becomes constant has been used in order to identify the lower desirable concentration for serum 25(OH)D. Estimates of the inflection point varies from > 20 nMol/L [2] to > 50 nMol/L [3] and to 110 nMol/L [4]. A recent consensus of investigators settled on a value of 75 nmol/L [5].
This wide range may be in part due to different calcium intakes in study populations [1]. The interrelationship between calcium intake and vitamin D requirements has been recently documented but it was concluded that calcium intake is not relevant in maintaining normal PTH concentrations in subjects with 25(OH)D higher than 25 nMol/L [6]. The study was performed in relatively young individuals with a mean calcium intake and a mean 25(OH)D levels considerably higher than those found in most South-European countries. In other two studies it was also reported that age may affect the relationship between 25(OH)D and PTH concentrations [7], [8].
To address these issues we analyzed data from a survey on the prevalence of hypovitaminosis D in elderly women in Italy [9] and in osteoporotic women on a low calcium intake but given large supplements of vitamin D. The aims were to determine whether the relationship among 25(OH)D and PTH concentrations differ according to both age and calcium intake. This information may change our definition of ideal vitamin D intake.
Section snippets
Methods
The study population and the methods of the epidemiological investigation have been described in detail elsewhere [9]. Briefly, the study included 850 Caucasian postmenopausal women aged 60–80 years, referred for the first time for osteoporosis risk assessment. Women with malignancies, chronic renal (serum creatinine > 100 μMol/L) or liver disease, or malabsorption syndromes, or who were already on treatment for osteoporosis were excluded. The recruitment period began on 15 February 2000 and
Results
Serum sample reliably stored at − 20 °C was available for 704 subjects of the epidemiological study. In 7 subjects the questionnaire for calcium intake was not obtained, leaving a total of 697 cases for analysis. The more crucial findings of the study population are listed in Table 1 by age group. Both PTH and 25(OH)D were normally distributed only after logarithmic transformation and the logarithmic values for both were used for subsequent analysis. The remaining information are reported
Discussion
In this study we examined the correlation between PTH concentrations and 25(OH)D levels, age and calcium intake. PTH is considered the most reliable marker of inadequate calcium availability and its overproduction can be directly or indirectly linked to bone loss and several other organ disturbances [1]. The strong inverse relationship between serum 25(OH)D and serum PTH has been used for establishing an ideal 25(OH)D lower threshold level. This is generally defined by the 25(OH)D levels at
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