Low levels of serum 25-hydroxyvitamin D are associated with decreasing peak expiratory flow rate in older men but not in older women, according to results from a longitudinal Dutch study.
Although the mediating effects of muscle strength and physical performance were examined as part of the analysis, it remains unclear exactly how vitamin D impacts pulmonary function, according to Dr. N.M. van Schoor and her colleagues at VU University Medical Center, Amsterdam, the Netherlands. The report was published online in the Journal of Clinical Endocrinology and Metabolism.
"With the exception of men having serum 25-OHD levels below 10 ng/mL (in cross-sectional analyses), adding physical performance or grip strength to the model did not lead to major changes in the association between serum 25-OHD and PEFR," the researchers wrote. "Several other mechanisms have been proposed. First, in vitro studies revealed that vitamin D modulates the activity of various defense and immune cells. Second, in a mouse model, it was shown that vitamin D deficiency decreases pulmonary function. This was primarily explained by a deficit in lung volume and could not be explained by alterations in somatic growth."
For the current study, believed to be the first of its kind, Dr. van Schoor and colleagues assessed 1,509 men and women aged 65 or older as of Jan. 1, 1996, who participated in a medical interview during 1995/1996 for the Longitudinal Aging Study Amsterdam, which they described as "an ongoing multidisciplinary cohort study on predictors and consequences of changes in physical, cognitive, emotional, and social functioning in older persons" (J. Clin. Endocrinol. Metab. 2012 April 3 [doi:10.1210/jc.2011-3199]). Data on serum 25-hydroxyvitamin D (25-OHD), peak expiratory flow rate (PEFR), and covariables were available for 596 men and 611 women in the cross-sectional analysis (limited to those who underwent medical interviews during 1995/1996) and for 454 men and 496 women in the longitudinal analysis (limited to those who underwent subsequent measurements in 1998/1999).
At baseline, men had a mean serum 25-OHD of 23.5 ng/mL while the mean level for women was 19.7 ng/mL. In men, a strong correlation between vitamin D status and PEFR was seen for serum 25-OHD levels up to 20 ng/mL. "In the cross-sectional analyses, this association was no longer statistically significant for serum 25-OHD levels between 10 and 20 ng/mL in the fully adjusted model but remained statistically significant for serum 25-OHD levels below 10 ng/mL," the researchers wrote. "In the longitudinal analyses, the relationships remained statistically significant up to a serum 25-OHD level of 20 ng/mL."
Adjustment for physical performance or grip strength did not change these results in the longitudinal analyses. However, in the cross-sectional analyses, the relationship between serum 25-OHD and PEFR was no longer statistically significant after researchers added the potential mediator grip strength.
In women, significant associations between 25-OHD levels and PEFR were observed in the cross-sectional analyses, but only after adjustment for age and season of blood collection – not in the fully adjusted models or in the longitudinal analyses. "The reason for this is unclear," the researchers noted. "It may be more difficult to find a statistically significant result in women, because women showed less variation in PEFR than men. In addition, women had lower PEFR values than men, and fewer women reported obstructive airway diseases at baseline. When excluding women having obstructive airway diseases at baseline, a trend toward significance was observed after additional adjustment for physical performance and grip strength, respectively."
The investigators acknowledged certain limitations of the study, including the fact that causality "cannot be established within the setting of a cohort study, although the observed longitudinal relationship in men does support causality. Second, data on FEV1 and FVC were not available. However, we do think that PEFR is a valid outcome, because it is an effort-independent measure of expiratory flow limitation, which might play a role in the observed association between vitamin D status and respiratory tract infections. In addition, PEFR is related to physical functioning, cognitive functioning, and mortality. Third, we did not have data on fat mass, which may be a more important confounder than body weight."
The Longitudinal Aging Study Amsterdam is largely supported by a grant from The Netherlands Ministry of Health Welfare and Sports, Directorate of Long-Term Care. The authors stated that they had no relevant financial conflicts to disclose.