Association between muscular strength and mortality in men: prospective cohort studyBMJ 2008; 337 doi: https://doi.org/10.1136/bmj.a439 (Published 01 July 2008) Cite this as: BMJ 2008;337:a439
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The study by Ruiz et al (“Association between muscular strength and mortality in men: prospective cohort study” BMJ 2008;337a439) finds a strong association between muscular strength and the risk of cancer and all-cause mortality. Although not stated implicitly, the article leaves the reader to conclude that exercise somehow causes this benefit. Significantly, the possibility that vitamin D is responsible for this association was not explored. Physical activity is one of the strongest predictors for high 25-hydroxyvitamin D plasma levels. Since Ruiz’s testing occurred in Dallas, Texas, it is reasonable to assume that many of the physically fit men in his study regularly exercised outdoors in a location where sunlight is abundant, resulting in higher levels of vitamin D. It is also possible that vitamin D promotes better muscle control and physical performance in general, perhaps because vitamin D metabolites bind to receptors in muscle tissue and promote muscle cell growth.[2,3]
Vitamin D is also thought to reduce cancer rates.[1,4] One recent study evaluated men with naturally-occurring variations in 25-hydroxyvitamin D levels and found that an increase in 25-hydroxyvitamin D levels of 25 nmol/L was associated with a 17% decrease in the incidence of all types of cancer, a 29% decrease in deaths due to all cancers, and a 45% decrease in digestive-system cancer mortalities, even after controlling for factors such as physical activity. The mechanism by which vitamin D protects against cancer has not been fully elucidated, but it has been shown that various tissues have vitamin D receptors and that 1,25-dihydroxyvitamin D influences genes that regulate cell proliferation and apoptosis.
Vitamin D also modulates the immune response and may reduce the susceptibility to respiratory infections. Future studies should control for vitamin D as a potential confounding variable when evaluating the association between physical fitness and mortality. It is premature for men who exercise indoors to anticipate the improved health benefits associated with exercise based on Ruiz’s study of men in Texas.
 Giovannucci E, Liu Y, Rimm EB, Hollis BW, Fuchs CS, Stampfer MJ, Willett WC. Prospective study of predictors of vitamin D status and cancer incidence and mortality in men. J Natl Cancer Inst 2006; 98: 451-9.
 Bischoff-Ferrari HA, Dietrich T, Orav EJ, Hu FB, Zhang Y, Karlson EW, Dawson-Hughes B. Higher 25-hydroxyvitamin D concentrations are associated with better lower-extremity function in both active and inactive persons aged > 60 y. Am J Clin Nutr 2004; 80: 752-8.
 Wicherts IS, van Schoor NM, Boeke JP, Visser M, Deeg DJH, Smit J, Knol DL et al. Vitamin D status predicts physical performance and its decline in older persons. J Clin Endocrin 2007; 92 (6): 2058-65.
 Holick, MF. Vitamin D: its role in cancer prevention and treatment. Progress in Biophysics and Mole Bio 2006; 92: 49-59.
 Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF et al. Review article: Epidemic influenze and vitamin D. Epidemiol Infect 2006; 1-12.
Competing interests: No competing interests