An exploratory study on the association between vitamin D intake or synthesis and sleep health in young adult women: an observational study

BACKGROUND: Vitamin D deficiency, which is highly prevalent in modern society, is associated with health issues, including sleep-wake disorders and depression. This pilot study aimed to examine the relationship between dietary vitamin D intake or vitamin D synthesis from ultraviolet exposure and serum 25-hydroxyvitamin D (25[OH]D) levels as an indicator of vitamin D status, as well as their association with sleep health and depressive symptoms. METHODS: Twenty-nine young adult women participated in two surveys conducted in spring and summer. Vitamin D intake was estimated based on responses to a dietary questionnaire. The amount of vitamin D synthesized in the skin through ultraviolet exposure was calculated by estimating ultraviolet exposure over a 2-week period. Serum 25(OH)D concentrations were measured using liquid chromatography-tandem mass spectrometry. Sleep habits were assessed using a 2-week sleep diary, and depression and insomnia symptoms were evaluated using questionnaires. RESULTS: The mean (SD) serum 25(OH)D concentrations in spring and summer were 8.9 (2.9) and 15.5 (4.8) ng/mL, respectively. The median (IQR) dietary vitamin D intake adjusted for energy expenditure based on dietary surveys was 8.5 (6.6, 9.2) μg/day in spring and 6.2 (4.3, 10.2) μg/day in summer (P = 0.40). The median (IQR) amount of vitamin D produced from ultraviolet exposure increased from 10.5 (4.6, 17.4) μg/day in spring to 17.0 (10.1, 34.5) μg/day in summer (P = 0.009). Among those with low serum 25(OH)D concentrations or low vitamin D intake in summer, the delayed midpoint of sleep was observed. No significant correlation was found between changes in serum 25(OH)D concentrations from spring to summer and changes in depression or insomnia symptoms. CONCLUSIONS: All participants exhibited vitamin D deficiency during the spring term. In contrast, during the summer term, serum 25(OH)D levels increased, primarily due to enhanced vitamin D synthesis resulting from ultraviolet exposure of the skin. However, the observed increase in serum 25(OH)D levels from spring to summer did not correlate with any improvement in depression or insomnia symptoms.