OBJECTIVES: To investigate the influence of season and outdoor daylight exposure on serum levels of 25-hydroxyvitamin D (25OHD), intact parathyroid hormone (PTH), ionized calcium, and femoral neck bone mineral density (BMD).
DESIGN: A 12-month, longitudinal, prospective study on a consecutively selected sample of healthy independent older people.
SETTING: Men and women living in their own homes in the inner city of Stockholm (latitude 59.2°N).
PARTICIPANTS: Sixty-four healthy men (n = 13) and women (n = 51), age 79 to 96 (mean age 83.7) were requested to spend at least 3 hours per week outdoors during April through September. Fifteen of these, who failed to comply with the required time outdoors, constituted a separate group. Subjects with serious diseases and subjects with medication that could interfere with vitamin D metabolism or bone mineralization were excluded from the study.
MEASUREMENTS: Daily dietary intakes of calcium and vitamin D were recorded on the basis of the subjects' recall. The participants kept a diary recording the time spent in sunlight and the area of skin exposed. Fasting blood samples were collected and analyzed on four occasions for 25OHD, intact PTH, and ionized calcium. BMD was determined at 6-month intervals in the femoral neck, using dual energy x-ray absorptiometry.
RESULTS: At study start in the spring, 6% of the subjects had 25OHD levels below 10 ng/ml and 78% had levels below 31 ng/ml. Forty-seven percent had intact PTH levels above the upper limit of the reference range (8–51 ng/L). Seasonal variations in the serum levels of 25OHD (P < .001), intact PTH (P < .001) and ionized calcium (P < .001) were observed only in the group with ≥3 hours of weekly outdoor daylight exposure. The 25OHD levels in autumn were positively correlated with the number of hours spent outdoors during the preceding summer (r = 0.63, P < .001). In the whole group and in the group with weekly outdoor exposure of ≥3 hours, there were no significant changes detectable in mean values of femoral neck BMD during the study year. Femoral neck BMD, expressed as Z-score, was significantly and positively correlated with serum 25OHD (r = 0.38, P = .003 after summer; r = 0.37, P = .003 after winter).
CONCLUSION: The seasonal changes in serum levels of 25OHD, PTH, and ionized calcium and the positive correlation between femoral neck BMD and 25OHD indicate that regular outdoor daylight exposure during the summer could enhance calcium homeostasis and possibly bone health, even among very old people living at northerly latitudes.