Factors associated with vitamin D status in Australian women

Vitamin D can be sourced from food and produced from skin exposure to sunlight.⁽¹⁾ In Australia and worldwide, vitamin D deficiency is highly prevalent. Data shows that 23% of the Australian population are considered to be vitamin D insufficient (25(OH)D < 50 nmol/L), increasing to 36% during the winter.⁽²⁾ Current estimated average intakes do not meet dietary recommendations.⁽³⁾ A lack of vitamin D, and thus reduced absorption of calcium, results in leaching of calcium stored in the bones leading to poor musculoskeletal health.⁽¹⁾ The aim of this study is to determine the impact that latitude, skin type, diet and sun exposure have on vitamin D status of healthy Australian women and to explore the rates and potential causes of deficiency. This cross-sectional study was conducted in Wollongong, Australia (34.42°S) during winter 2020 and spring 2021 with 100 women (> 18 years, pre- or post-menopausal). Skin types were self-defined. Serum 25(OH)D was measured through liquid chromatography mass spectrometry, dietary intake through a 4-day food record analysed using FoodWorks 10, bone density through dual energy x-ray absorptiometry and sun exposure through a polysulphone film badge worn for 4 days. Participants were aged 41.4 ± 15.5 years and 98 women had valid serum 25(OH)D measurements. Of these, n = 1 (1.0%) was deficient (< 25 nmol/L), n = 14 (14.3%) were insufficient (25–50 nmol/L) and n = 41 (41.8%) were sufficient (> 50 nmol/L), while n = 42 (42.9%) had an optimal status (> 75 nmol/L) of vitamin D. The mean 25(OH)D was significantly different across skin type groups (white compared to moderate brown skin types) (F(4,93) = 2.6, p = 0.04), whilst average sun exposure (SED) of 1.1 ± 1.0 was on the borderline of significantly predicting 25(OH)D levels (F(1, 92) = 3.74, p = 0.06). A significant positive correlation existed between 25(OH)D and total bone mineral density (r(96) = 0.27, p < 0.001). Mean vitamin D intake (n = 94) was 3.1 ± 3.0 μg/day, with the majority of participants (86.2%) having intakes below.⁽³⁾ Logistic regression, controlling for age, showed that an increase in reported calcium intake (OR = 1.0, 95% CI [1.0, 1.0], p = 0.01), average SED (OR = 0.0, 95% CI [0.0, 1.3], p < 0.001) and having a lighter skin type (OR = 7.0, 95% CI [1.1, 43.3], p = 0.04) were significantly associated with reduced odds of deficiency/insufficiency. Serum 25(OH)D was found to be higher than reported in previous Australian data,⁽²⁾ which is favourable given the essential role vitamin D plays in calcium homeostasis and musculoskeletal health. However, reported dietary vitamin D intakes were low. Given the increased risk of melanoma from excessive sun exposure, targeted advice towards vitamin D-rich food sources may be helpful for those individuals with poorer vitamin D status.