The strengths of thisstudy includeits longitudinal design and the use of serum 25OHD concentration as an indicator of vitamin D status, reflecting vitamin D obtained from diet, supplements, and cutaneous synthesis (8). Also, several limitations are related to the use of serum 25OHD.First,25OHD levels in one individual fluctuate from one season to another. Blood samples were collected only once, and this took place throughout the year except for high summer, which means that the 25OHD levels for individuals might notbefully comparable.However, adjustment for time when blood samples were collected did not notably change the results. Second, the serum samples were stored at 20°C before assessing 25OHD. It cannot be excluded that the 25OHD concentrations might have changed during storage (9). In one study (10), the average plasma 25OHD concentration was significantly lowered during storage at 18°C, whereas another study (11) did not observe any notable changes.Our earlier study indicated good stability for selenium and-tocopherol (12), which gives indirect evidence about the stability of 25OHD in our serum samples. Third, we observed low average serum 25OHD concentrations compared with those of other cohorts