By Adam S. Grimaldi, Beth A. Parker, PhD, and Paul D. Thompson, MD, FACSM
Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies
Adam S. Grimaldi, B.S., is a fourth-year medical student at the University of Connecticut School of Medicine in Farmington. He also is a research assistant to Drs. Parker and Thompson at Hartford Hospital in Connecticut.
Beth A. Parker, Ph.D., is an assistant professor of health sciences at University of Hartford and director of exercise physiology research at Hartford Hospital in Connecticut. She is a member of ACSM and her research focuses on age and sex differences in vascular function and cardiovascular responses to exercise. Dr. Parker is a principal or co-investigator on several projects involving the pleiotropic effects of
statin therapy on skeletal muscle and cognition.
Paul D. Thompson, M.D., FACSM, is director of cardiology at Hartford Hospital and professor of medicine at University of Connecticut. He is a past-president of ACSM and has presented invited keynote lectures at the ACSM annual meetings, such as the D.B. Dill Lecture. He has authored more than 200 scientific articles on such topics as exercise training and heart disease and sudden death during exercise. Dr. Thompson’s current research, funded by the National Institutes of Health, examines the effects of statins on muscle strength and aerobic performance.
This commentary presents perspectives from Mr. Grimaldi, Dr. Parker, and Dr. Thompson relative to issues central to their cross-sectional study that appears in the January 2013 issue of ACSM’s Medicine and Science in Sports and Exercise® (MSSE).
In recent years, Vitamin D has become a topic of great discussion. Evidence shows that its biologic role goes beyond bone metabolism and calcium homeostasis. For example, elevated concentrations of serum 25-hydroxy vitamin D (25(OH)D) have been shown to be associated with improved cardiovascular outcomes, enhanced immune function, and reductions in cancer risk. Additionally, 25(OH)D levels have been linked to skeletal muscle performance through our historical understanding that long-term vitamin D deficiency can produce a myopathy. In recent years, multiple studies have shown mixed results in favor of a direct relationship between 25(OH)D and muscle performance. Even more recently, low vitamin D levels have been associated with increased non-specific skeletal muscle complaints by patients in primary care practices. This becomes clinically important for those patients located in cold-temperate climates during winter months when vitamin D levels are lowest.
The relationship between 25(OH)D and muscle performance has been investigated in the very old or the very young. In the elderly, it has become clinically relevant in fall prevention and maintaining safe mobility. In the young, studies have shown improved jump mechanics being associated with higher 25(OH)D levels. Overall, such evidence points to a generalized trend toward vitamin D levels being directly related to weight-bearing, antigravity muscle groups.
In our recent MSSE study “25(OH) Vitamin D is Associated with Greater Muscle Strength in Healthy Men and Women,” we hypothesized that our data would reflect the current literature on vitamin D and its relationship to weight-bearing muscle groups. Using computerized dynamometry to test arm and leg muscles in both isokinetic and isometric environments, we found a surprisingly robust relationship with 25(OH)D and arm strength. There existed a relationship between 25(OH)D and leg strength, but only in isometric measures. This study helped to support the current vitamin D and muscle strength literature, in addition to expanding these findings over a broader age range of healthy males and females (20-76 yr). Our study could not explain the reason for vitamin D being strongly related to arm strength over leg strength. We hypothesized that since vitamin D seems to have more effect on the type II muscle fibers, perhaps the arm muscle groups contain a higher density of type II fibers. There could also be a differential expression of the vitamin D receptor (VDR) among different muscle groups that would explain this observation. As of now, these hypotheses remain unanswered in human subjects.
At this time, there is no widely accepted consensus for the optimal serum 25(OH)D concentration. The recommended dietary allowance (RDA) for individuals 1-70 years of age is 600 international units per day. For those over the age of 70 years, 800 allowance (RDA) for individuals 1-70 years of age is 600 international units per day. For those over the age of 70 years, 800 international units per day is recommended. These recommendations came from the Institute of Medicine in 2010 and are doses established for skeletal health. No recommendations for extra-skeletal health doses have been established as the evidence is lacking.
We have concluded that there is increasing evidence for a vitamin D and muscle strength relationship. At this point however, we cannot make the assumption that supplementation with vitamin D will improve muscle strength in healthy individuals. This assertion will require more randomized controlled trials and should not be limited to testing only the weight-bearing, antigravity muscle groups. Data from these studies will be critical in defining an RDA that may address benefits beyond skeletal health.