Aging

The Vitamin D and Mobility Connection

Abstract & Commentary

By David Kiefer, MD

Research Fellow, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona

Dr. Kiefer reports no financial relationships relevant to this field of study.

Synopsis: Low serum 25-hydroxyvitamin D correlates with the development of mobility limitations and disability in older people who were otherwise healthy.

Source: Houston DK, et al. Low 25-hydroxyvitamin D predicts the onset of mobility limitation and disability in community-dwelling older adults: The Health ABC Study. J Gerontol A Biol Sci Med Sci 2012; 9:1-7.

Stretching the boundaries of vitamin D thinking and dovetailing with prior work connecting vitamin D intake or supplementation and serum 25-hydroxyvitamin D (25(OH)D) levels with muscular function and many other medical conditions, the researchers behind this study sought to further examine the relationship between vitamin D and mobility in adults.

The study started with 3075 initially well, community-dwelling men and women, black and white, ages 70-79, from Pittsburgh and Memphis. After excluding those with missing serum 25(OH)D levels or who had limited mobility during the second year of the trial, a total of 2099 people remained to be followed for the 6 years of this analysis. Serum 25(OH)D and parathyroid hormone (PTH) were measured at the year 2 visit. The occurrences of mobility disability and limitations were assessed by an annual clinic visit and phone calls every 6 months. Information about a variety of confounding variables also was collected, such as the season during which blood was collected (addressing the expected higher 25(OH)D levels during the summer), demographics, exercise, smoking status, and dietary supplement and alcohol intake. The breakdown for serum 25(OH)D levels for the 2099 people in the study was as follows: 35% ≥ 75 nmol/L, 36.1% 50 to < 75 nmol/L, and 28.9% < 50 nmol/L. With respect to the mobility part of the equation, 36.3% reported a mobility limitation during the 6 years, while 22.0% reported a mobility disability. The researchers used two statistical models depending on which possible confounding variables were included, which then compared the people in the different 25(OH)D groups with their mobility limitations and disabilities. With these models, the researchers found a greater risk of mobility limitation and disability when the groups with a 25(OH)D < 50 nmol/L and 50 to <75 nmol/L were compared to those with levels ≥ 75 nmol/L (P = 0.02 to < 0.001). Even the models including the most confounding variables showed this trend. With respect to PTH, those participants with an elevated PTH (≥ 70 pg/mL) had a higher risk of mobility limitation and disability than those with a low PTH (< 27 pg/mL). The researchers calculated that people with serum 25(OH)D < 50 nmol/L were at twice the risk of mobility problems, whereas those with serum 25(OH)D < 75 nmol/L were at a 30% higher risk.

Commentary

Hints in the medical literature should have made our response to these results “Of course” or “It seems reasonable.” Although some inconsistencies exist, several prior studies, detailed nicely by the authors in their introduction, have found impaired strength and physical performance and self-reported limitations in mobility in older adults. In addition, the mechanism apart from changes associated with normal aging exists: Vitamin D is important not only for bone health (clearly relevant to mobility), but also to overall calcium, phosphorus, and magnesium metabolism, minerals that are involved with muscle function.1 The results seem convincing enough to add mobility to our list of why adequate vitamin D levels are important for maintenance of good health. Ideally, the researchers would have had more than one laboratory value (rather than just at year 2), and not relied so heavily on self-reported mobility limitations and disability. That said, the size of the trial, the inclusion of numerous possible confounding variables, and statistically significant findings were all positive aspects of this study. Presumably, the next step would be a prospective intervention trial comparing vitamin D supplementation (or dietary intake or sunlight exposure), and the resulting serum 25(OH)D levels, with placebo, for people with low serum levels to determine if such remediation would be of benefit in the realm of mobility.

Putting the results into the units of measure for 25(OH)D levels most commonly used, the cutoffs in this study would be < 20 ng/mL, 20 to < 30 ng/mL, and ≥ 30 ng/mL, arguably still within the lower range of ideal vitamin D levels.2 How might this affect our interpretation of these results? If anything, it would strengthen the demonstrated association; imagine comparing the lowest tertile to a group in the range 40-50 ng/mL that some experts recommend and for which some evidence exists for improved mobility. The authors touch on this issue, lightly criticizing the Institute of Medicine recommendation that, for ideal bone health, people have a serum 25(OH)D > 20 ng/mL.

References

1. Casey CF, et al. Vitamin D supplementation in infants, children, and adolescents. Am Fam Physician 2010;81:745-748.

2. Holick MF. The vitamin D epidemic and its health consequences. J Nutr 2005;135:2739S-2748S.