Seasonal changes in calcitropic hormones in Israeli men

Seasonal changes in calcitropic hormones might be expected, being that dietary calcium intake may differ with fluctuations in climate and temperature, and vitamin D is diet- and sunlight-dependent. While there are studies on elderly subjects, prospective data on younger men is limited. The objective of this study was to clarify possible seasonal changes in homeostatic regulators of calcium in Israeli men aged 25-64 years. The study was a prospective follow-up analysis of data collected during June-August 1995 and 1996 (summer) and January March 1995 and 1996 (winter). Subjects were ninety-five industrial male employees with and without occupational lead exposure. The main outcome measures were summer and winter serum concentrations of parathyroid hormone (PTH), 25-hydroxyvitamin D (25-OH-D), and 1,25-dihydroxyvitamin D (calcitriol). Summer and winter values of PTH were similar (38.2 and 39.8 ng/l, respectively). 25-OH-D levels were significantly higher in summer (32.8 ng/ml) than in winter (25.4 ng/ml) after controlling for possible confounders (p < 0.0001). Calcitriol levels were significantly higher in summer (79.1 pmol/l) than in winter (73.5 pmol/l) in univariate analyses, but not after controlling for possible confounders. We conclude that healthy men show considerable seasonal changes in 25-OH-D levels even in Israel, a relatively sunny country all the year round. Summer values of 25-OH-D, were 35% higher than in winter. These fluctuations should be taken into account during evaluation of pathological conditions and in research. Given an adequate diet and vitamin D status there are no seasonal variations in PTH or in calcitriol levels.