BMC Public Health 2009, 9:22 http://www.biomedcentral.com/1471-2458/9/22 type I statistical error was allowed in all analyses (p < 0.05). The relationships between serum 25(OH)D con- centration and PTH in summer were studied with the nonlinear least-squares regression method for optimal vitamin D cut-off determination. The locally weighted polynomial regression modelling (LOWESS) was used to model the winter PTH and vitamin D data. Determinants of 25(OH)D were studied using the multiple linear regres- sion method with backward selection using 25(OH)D (models for summer and winter separately) as the dependent variable. The initial variables included in the regression analysis were age, gender, BMI, fresh milk con- sumption (as dL/day), other milk product consumption, caffeinated and alcoholic beverage consumption, smok- ing (both as a factor and number of cigarettes/day), vita- min D supplement usage, sun-bathing habits and menopausal status (in women). All variables with p < 0.1 were included in the final models. Statistical analysis was performed using the R software package version 2.7.0 (R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria). Results The mean age of the study group was 48.9 ± 12.2 years (range 25–70 years). All participants were mobile, com- munity dwelling Estonians and Caucasians by race. The study group characteristics are outlined in Table 1. The mean 25(OH)D concentration in winter was 43.7 ± 15.0 nmol/L (17.2 ± 5.9 μg/L). A significant increase was observable in 25(OH)D concentration in summer to 59.3 ± 18.0 nmol/L (23.3 ± 7.1 μg/L) (p < 0.0001). The average increase of 25(OH)D for men was 17.9 nmol/L (relative increase 40%) and for women 13.4 nmol/L (30%), result- ing in a difference in seasonal increase of 4.5 nmol/L (p = 0.01, 95% CI 0.6–8.2). Gender difference in the seasonal parathyroid hormone change was not significant. Eight subjects (2%), all women, were regularly using vitamin D supplements (over the counter vitamin D or cod-liver oil; 200–800 IU of vitamin D/day). Those subjects taking sup- plements had a higher serum concentration of vitamin D in winter (53.9 nmol/L in supplementers vs. 43.5 nmol/L in non-supplementers, p = 0.053). Eight percent (n = 29) of subjects had vitamin D defi- ciency in winter, with equal gender distribution (15 women and 14 men). In summer less than 1% (n = 2; a single woman and a man) were still vitamin D deficient. In winter 73% of subjects (133 men and 135 women) had 25(OH)D level under 50 nmol/L, indicating insufficiency. In summer 29% (n = 93) of subjects were vitamin D insuf- ficient. In winter 97% and in summer 87% of the subjects were below the optimal vitamin D level of 75 nmol/L (Figure 1). The mean 25(OH)D in winter for subjects refusing the follow-up in summer was lower (39.9 nmol/ L) than that for subjects with both samples performed (44.4 nmol/L, p < 0.05). Serum PTH had a strong negative correlation with 25(OH)D concentrations both in winter and in summer (rp = -0.18 and rp = -0.19 respectively, p < 0.001). In sum- mer the nonlinear least-squares regression shows an approximate cut-off value (PTH reaches a plateau) for 25(OH)D around 80 nmol/L in both younger (aged under 50 years) and older individuals (50 years and above). In winter there is no plateau effect in either of the age groups and PTH persists to decline even around the highest vitamin D values measured in the period (Figure 2). Hyperparathyroidism was present in 23 patients in winter (15 women and 8 men, mean 25(OH)D 35.9 nmol/L) and in 15 subjects in summer (8 women and 7 men, mean 25(OH)D 42.8 nmol/L). In winter 74% (17 of 23) and in summer 80% (12 of 15) were aged 50 or over. In winter 78% (18 out of 23) of the subjects with hyper- parathyroidism were vitamin D insufficient and 26% (6/ 23) were vitamin D deficient. In summer 50% of these subjects were vitamin D insufficient, but none showed serum levels of deficiency. Three of the subjects with hyperparathyroidism had the condition persistent throughout the year. There was a difference in the 25(OH)D level in summer between pre- and post-menopausal women (55 nmol/L Table 1: Characteristics and vitamin D status indicators of the study group (mean +/- SD). Females (n = 200) Males (n = 167) Age BMI 25(OH)D winter (nmol/L) PTH winter (pmol/L) 25(OH)D summer (nmol/L)* PTH summer (pmol/L)* * n = 316; 138 males, 178 females Mean 49.2 28.5 44.6 4.3 58.4 3.9 SD 12 6.9 15.8 1.8 17.7 1.7 Mean 48.5 27.9 42.7 3.8 60.5 3.6 SD 11.6 4.6 14.0 1.9 18.5 1.7 p value NS NS NS p < 0.05 NS NS Page 3 of 8 (page number not for citation purposes)