Vitamin D status of women in the Geelong Osteoporosis Study: association with diet and casual exposure to sunlight.

OBJECTIVE: To assess vitamin D intake and casual exposure to sunshine in relation to serum 25-hydroxyvitamin D (25OHD) levels.

DESIGN: Cross-sectional study of a population-based, random sample of women aged 20-92 years, assessed between 1994 and 1997.

SETTING AND PARTICIPANTS: 861 women from the Barwon Statistical Division (population, 218000), which includes the city of Geelong (latitude 38 degrees south) in Victoria.

MAIN OUTCOME MEASURES: Vitamin D intake; serum 25OHD level; season of assessment; exposure to sunshine.

RESULTS: Median intake of vitamin D was 1.2 microg/day (range, 0.0-11.4 microg/day). Vitamin D supplements, taken by 7.9% of participants, increased intake by 8.1% to 1.3 microg/day (range, 0.0-101.2 microg/day) (P< 0.001). A dose-response relationship in serum 25OHD levels was observed for sunbathing frequency before and after adjusting for age (P< 0.05). During winter (May-October), serum 25OHD levels were dependent on vitamin D intake (partial r2= 0.01; P<0.05) and were lower than during summer (November-April) (age-adjusted mean, 59nmol/L [95% Cl, 57-62] v 81 nmol/L [95% CI, 78-84]; P<0.05). No association was detected between serum 25OHD and vitamin D intake during summer. The prevalences of low concentrations of serum 25OHD were, for <28nmol/L, 7.2% and 11.3% overall and in winter, respectively; and, for <50 nmol/L, 30.0% and 43.2% overall and in winter, respectively.

CONCLUSIONS: At latitude 38 degrees south, the contribution of vitamin D from dietary sources appears to be insignificant during summer. However, during winter vitamin D status is influenced by dietary intake. Australia has no recommended dietary intake (RDI) for vitamin D, in the belief that adequate vitamin D can be obtained from solar irradiation alone. Our results suggest that an RDI may be needed.