Vitamin D status in a Norwegian population: any link to lung function?

Abstract

Vitamin D is predominantly synthesized from precursors in the skin after direct exposure to ultraviolet B radiation from the sun. Vitamin D can also be obtained by dietary intake of vitamin D rich food, or by supplement intake. Poor vitamin D status has been reported worldwide. Determinants include unexposed skin, high latitude, winter season, and high body mass index (BMI). Serum 25-hydroxyvitamin D (25(OH)D) is the major circulating metabolite of vitamin D and considered the best marker of vitamin D status. The association between vitamin D status and bone health is well studied, and increasing evidence suggests an association between low serum 25(OH)D level and range of non-skeletal health outcomes including respiratory disorders, such as asthma. This dissertation uses data from the Nord-Trøndelag Health Study (HUNT), one of the largest population based health studies in Norway, with clinical, lifestyle and environmental information on more than 130,000 people collected over 25 years. Using data from HUNT2 (1995-1997) and HUNT3 (2006-2008), we investigated: 1) the prevalence of vitamin D deficiency (serum 25(OH)D<50nmol/L) in a general adult population and factors associated with vitamin D deficiency; 2) the association between serum 25(OH)D level and lung function (LF) in adults with asthma; and 3) the interrelationship between serum 25(OH)D level, smoking and LF changes in a general adult population. We found a high prevalence of vitamin D deficiency in this Norwegian population (40% overall), particularly in winter months (64%). Besides season and high BMI, several potentially modifiable lifestyle factors were associated with vitamin D deficiency. We also found that low serum 25(OH)D level was not associated with airway obstruction in most adults with asthma, with the exception of men with asthma but without allergy status. Finally, we found no clear associations between serum 25(OH)D levels and LF changes in never smokers, whereas we did observe significant associations in ever smokers. Replication and confirmation of these findings via well-designed prospective and intervention studies is required to determine if vitamin D supplementation is a potentially cost effective, safe and straightforward means of asthma control or disease mitigation in ever smokers.