The high prevalence of vitamin D insufficiency across Australian populations is only partly explained by season and latitude

Abstract

Background: Inadequate sun exposure and dietary vitamin D intake can result in vitamin D insufficiency. However, limited data are available on actual vitamin D status and predictors in healthy individuals in different regions and by season.

Methods: We compared vitamin D status [25-hydroxyvitamin D; 25(OH)D] in people < 60 years of age using data from cross-sectional studies of three regions across Australia: southeast Queensland (27 degrees S; 167 females and 211 males), Geelong region (38 degrees S; 561 females), and Tasmania (43 degrees S; 432 females and 298 males).

Results: The prevalence of vitamin D insufficiency (<or= 50 nmol/L) in women in winter/spring was 40.5% in southeast Queensland, 37.4% in the Geelong region, and 67.3% in Tasmania. Season, simulated maximum daily duration of vitamin D synthesis, and vitamin D effective daily dose each explained around 14% of the variation in 25(OH)D. Although latitude explained only 3.9% of the variation, a decrease in average 25(OH)D of 1.0 (95% confidence interval, 0.7-1.3) nmol/L for every degree increase in latitude may be clinically relevant. In some months, we found a high insufficiency or even deficiency when sun exposure protection would be recommended on the basis of the simulated ultraviolet index.

Conclusion: Vitamin D insufficiency is common over a wide latitude range in Australia. Season appears to be more important than latitude, but both accounted for less than one-fifth of the variation in serum 25(OH)D levels, highlighting the importance of behavioral factors. Current sun exposure guidelines do not seem to fully prevent vitamin D insufficiency, and consideration should be given to their modification or to pursuing other means to achieve vitamin D adequacy.

Figure 1

Prevalence of vitamin D deficiency (≤ 25 nmol/L), vitamin D insufficiency (26–50 nmol/L), and vitamin D sufficiency (> 50 nmol/L) for women < 60 years years of age in southeast Queensland (latitude 27°S), Geelong (latitude 38°S), and Tasmania (41–43°S) by season.

Figure 2

Prevalence of vitamin D deficiency (≤ 25 nmol/L; solid bars) and insufficiency (≤ 50 nmol/L; stippled bars) and the UV index (•—•) on a cloudless day by month of the year in (A) southeast Queensland, (B) the Geelong region, and (C) Tasmania. Colors of the bars represent the current sun protection recommendations based on the UV index: green (UV index ≤ 2), can safely stay outdoors with minimal protection; yellow (UV index 3–5), wear hat, sunscreen, sunglasses, seek shady areas; orange (UV index 6–7), see yellow and stay indoors between 1000 and 1400 hours (1100–1500 hours daylight saving time); red (UV index 8–10), see orange and stay indoors as much as possible; blue (UV index ≥ 11), see red.

Figure 3

Seasonal variation in actual serum 25(OH)D levels and predicted serum 25(OH)D (solid line) (A, D, G), simulated maximum daily duration of vitamin D synthesis in human skin (B, E, H), and simulated vitamin D effective daily dose (C, F, I) in southeast Queensland (A–C), the Geelong region (D–F), and Tasmania (G–I).