Serum 25(OH)D levels varied by week of blood collection, with the highest levels during the summer and autumn months and the lowest levels during winter and spring (). Twenty-seven % of the population in winter and 17% in summer had serum 25(OH)D < 25 nmol/L respectively. These percentages were 75% and 62% respectively, for a cut-point of < 50 nmol/L 25(OH)D
In unadjusted factors significantly (p<0.01) associated with higher 25(OH)D levels were: those whose blood was drawn in the summer months, living in coastal or urban areas, higher vocational achievements, more leisure time physical activity (PA), BMI ≥21, higher intake of fish, poultry, alcoholic spirits, vitamin D (including supplements), protein, polyunsaturated fatty acids, N-3 fats from fish, eicosapentaenoic acids, β-carotene and higher serum levels of retinol, β-carotene and α-tocopherol (cholesterol-adjusted). Missing more than ten teeth and dietary calcium intake were significantly associated with lower 25(OH)D levels.
Distribution of demographic, lifestyle, anthropometric, dietary and serum factors by categories of serum 25(OH)D n=2271 (unadjusted)
The factors which remained significant when entered into the stepwise multiple linear regression (adjusting for confounders) against natural log-transformed 25(OH)D nmol/L were: summer season of blood collection, coastal residence, greater leisure time PA, BMI ≥ 21 kg/m2, greater dietary intake of fish and poultry, higher vitamin D intake, and higher serum α-tocopherol (cholesterol-adjusted), retinol and β-carotene (all positive) and missing teeth (negative). When stratified by season, differences emerged in the relative value of these predictive factors (log-transformed: total r2 = 0.290; summer r2 = 0.345 and winter r2 = 0.243)
In a further investigation of seasonal difference presents results of logistic regression analyses of the factors which are associated with 25(OH)D deficiency (i.e., the odds of being ≥25 nmol/L 25(OH)D). In summer, having no versus leisure time physical activity (OR=2.0; 95% CI = 1.3–3.1), a BMI ≥ 21 kg/m2 versus < 21 kg/m2 (OR= 2.6; 95% CI = 1.3–5.0) and a high versus low vitamin D intake (OR=3.6; 95% CI = 1.5–8.5) were the strongest factors associated with lack of vitamin D deficiency. In winter, the association with dietary vitamin D intake was similar to summer values but the effect of having some leisure time PA was less evident and increased BMI had no effect (OR ≥25 nmol/L 25(OH)D: a high versus low vitamin D intake = 3.6; 95% CI = 1.8–7.3; having some versus no leisure physical activity OR = 1.3; 95% CI = 1.0–1.9; BMI ≥ 21 kg/m2 versus < 21 kg/m2= 1.0; 95% CI = 0.5–1.9). Although not a significant predictor over the whole year (), occupational activity which was of greater intensity versus light activity was associated with ≥25 nmol/L 25OHD, especially in winter (OR = 1.6; 95% CI = 1.1–2.5). Another difference in winter was that higher consumption of fish or poultry (adjusted for vitamin D intake), and having higher serum levels of retinol and α-tocopherol, were more strongly associated with vitamin D deficiency than in the summer months.
Predictors of vitamin D status (25(OH)D ≥ 25 nmol/l) in winter (n=1241) and summer (n=1030) in middle-aged Finnish men
Estimates from linear regression analyses () were similar to all the above for both summer and winter. presents a non-linear association between BMI and 25(OH)D. It appears in these data that this distribution approximates an inverted U shape with either very low or very high BMI measures associated with lower levels of 25(OH)D (< 40 nmol/L).
Predictors of vitamin D status (25(OH) D nmol/L) over the whole year (n=2271): in winter (n=1241) and summer (n=1030)
Association between Body Mass Index (kg/m2) and unadjusted mean serum vitamin D ((25(OH)D nmol/L)