Tracking of serum 25-hydroxyvitamin D during 21 years

BACKGROUND/OBJECTIVES Our objective was to evaluate the degree of tracking for serum levels of 25-hydroxyvitamin D [25(OH)D] over time, by using data from three previously conducted surveys of the Tromso study collected in the years 1994/1995 (Tromso 4), 2007/2008 (Tromso 6), and 2015/2016 (Tromso 7). SUBJECTS/METHODS Subjects with valid 25(OH)D measurements in all three surveys were included. 25(OH)D z-scores were used to adjust for seasonal variation. Z-scores and sextiles were used to illustrate tracking of 25(OH)D. RESULTS 1702 subjects (572 males, 1130 females) fulfilled the inclusion criteria. Median (5th, 95th percentiles) age for these subjects was 55 (33, 65) years in Tromso 4, and mean (SD) 25(OH)D levels were 57 (18) nmol/L, 59 (19) nmol/L, and 72 (21) nmol/L for Tromso 4, Tromso 6, and Tromso 7, respectively. There was significant tracking of serum 25(OH)D over the 21 years period between the surveys of the Tromso study. The correlation coefficient r between 25(OH)D z-scores from Tromso 4 and Tromso 6 was 0.40, and declined to 0.29 for the correlation between Tromso 4 and Tromso 7. Twenty-six percent of the subjects in the lowest 25(OH)D z-score sextile in Tromso 4 were in the three highest sextiles of 25(OH)D in Tromso 7. Similarly, 35% of those in the highest sextile in Tromso 4 were in the lowest three sextiles in Tromso 7. CONCLUSIONS The degree of tracking for serum 25(OH)D declines over time, and the use of a single serum 25(OH)D measurement as an indicator of the vitamin-D status is questionable if used in long-lasting observational studies.