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Relationship Between Serum 25-HO D and Pulmonary Function in the NHANES III
Association does not mean causation
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Michael Pazianas, MD, Associate Professor of Medicine Univ of Pennsylvania
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pazianas{at}mail.med.upenn.edu Michael Pazianas
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In your Dec 2005 issue, Black PN et al 1 analyzed data from the NHANES III and reported a relationship between serum 25-hydroxyvitamin D (25-OH D) and pulmonary function. The authors, in the discussion of their findings and Dr. Wright RJ, in the accompanied editorial, presented an extensive list of possible explanations for the above relationship. However, the possibility that improved strength of the muscles facilitating breathing was not discussed. Indeed, it is now well accepted that vitamin D can improve significantly the muscular strength2. Therefore, the higher FEV1 values in individuals with higher 25–OH D levels could well be due, almost exclusively, to improved strength of the respiratory muscles. Michael Pazianas, MD Associate Professor of Medicine University of Pennsylvania 3615 Chestnut Street Philadelphia, PA 19104 Tel: 215 573 5788 Fax: 215 573 8684 Web (Osteoporosis in pictures): http://mail.med.upenn.edu/~pazianas/ References 1. Black PN, Scragg R. Relationship Between Serum 25-Hydroxyvitamin D and Pulmonary Function in the Third National Health and Nutrition Examination Survey. Chest 2005; 128:3792-3798 2. Venning G Recent developments in vitamin D deficiency and muscle weakness among elderly people. BMJ 2005;330:524-526 |
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Jerome C. Arnett, Jr., physician none, none
Send letter to journal:
jerry.arnett{at}gmail.com Jerome C. Arnett, Jr., et al.
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To the Editor, CHEST: I read with interest the article by Black and Scragg (December, 2005)1 on the relationship between serum 25-hydroxyvitamin D and pulmonary function. The data are from a large observational epidemiological study that was able to correct for up to 10 confounders. It was found that a 4% decrease in pulmonary function (a decrease in FEV1 of 126 ml. and a decrease in FVC of 172 ml.) was associated with a decrease in serum 25- hydroxy vitamin D level. Surprisingly, the association was found for whites and blacks but not for Mexican Americans, nor for those with bronchitis or emphysema. The eight references cited as evidence that diet can influence pulmonary function also were observational epidemiological studies performed over a 13-year period, each of which found only an association between the two. As the authors noted, association does not establish causation. Observational epidemiological studies often have inconsistent or conflicting results, and nearly always are inconclusive, because risk factors may have nothing to do with causes.2 Relative risk does not measure harmfulness, but only the strength of an association between the disease and an exposure factor. These studies are plagued by hidden confounding factors, biases, and methodological weaknesses.3 They do not allow experimental control, and cannot test hypotheses nor prove causation, so they are best used to identify problems for future studies.4 A tightly controlled experiment then is needed to prove that the observed association has identified a necessary cause.5 The randomized controlled clinical trial, though much more expensive and time-consuming, is the gold standard for this requirement, and is the best way to answer the questions these observational epidemiological studies have raised.6 Jerome Arnett, Jr., MD, FCCP Helvetia, WV 15 January, 2006 Bibliography 1.Black PN, Scragg R. Relationship Between Serum 25- Hydroxyvitamin D and Pulmonary Function in the Third National Health and Nutrition Examination Survey. Chest 2005;128:3792-98 2. Skrabanek Petr. Risk-Factor Epidemiology: Science or Non-Science? In Health, Lifestyle and Environment: Countering the Panic. Manhattan Institute, New York, 1991, pp. 47-56 3.Taubes G, Mann CC. Epidemiology faces its limits. Science. Washington, Jul 14,1995;269:164-68 4.Charlton BG. The Scope and Nature of Epidemiology. J Clin Epidemiol 1996;49:623-26 5.Charlton BG. Attribution of Causation in Epidemiology: Chain or Mosaic? J Clin Epidemiol 1996;49:105-07 6.Taubes and Mann, p. 165 |
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