American Journal of Epidemiology Advance Access published online on June 25, 2008
American Journal of Epidemiology, doi:10.1093/aje/kwn166
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Scragg and Camargo Respond to "Physical Activity and Vitamin D"
1 School of Population Health, University of Auckland, Auckland, New Zealand
2 Massachusetts General Hospital, Harvard Medical School, Boston, MA
Correspondence to Dr. Robert Scragg, School of Population Health, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand (e-mail: r.scragg{at}auckland.ac.nz).
Received for publication May 6, 2008. Accepted for publication May 8, 2008.
We appreciate the thoughtful comments by Drs. Birrell and Francis (1) about our article (2) based on data from the Third National Health and Nutrition Examination Survey (NHANES III). We agree that it is most likely that we have underestimated the strength of the positive association between outdoor activity and serum 25-hydroxyvitamin D because of error in measuring both variables and that the second explanation for the association (i.e., outdoor exercising raises vitamin D levels) is more plausible than the first and third explanations.
The invited commentary by Birrell and Francis stimulated us to do further analyses demonstrating that confounding by latitude is not a reason for the positive association between the frequency of outdoor activity and serum 25-hydroxyvitamin D. Table 1 shows adjusted mean 25-hydroxyvitamin D levels by frequency of outdoor activity in the past month, stratified by US region. All regions show a significant variation in serum 25-hydroxyvitamin D across the full range of outdoor frequencies, particularly the Northeast and South regions, which are confined to a narrow range of high and low latitudes, respectively.
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The prevalence of vitamin D insufficiency in the Third National Health and Nutrition Examination Survey has been reported previously by using a definition of serum 25-hydroxyvitamin D at less than 50 nmol/liter (3). The authors found that vitamin D insufficiency varied in age-sex subgroups from 13 to 40 percent during winter in the South (median latitude, 32°N) and from 8 to 34 percent during summer in the North (median latitude, 39°N). To assist the clinical and public health interpretation of our results, we have calculated the percentage of people with low vitamin D levels across the five outdoor activity frequency groups, weighted to the US civilian population (table 1). The percentage of people with serum 25-hydroxyvitamin D at less than 50 nmol/liter varied from 35 percent in those who did no outdoor activity in the past month down to 11 percent in those who were outdoors daily, while the percentage with serum 25-hydroxyvitamin D at less than 75 nmol/liter varied from 68 percent down to 41 percent for the same activity groups. Thus, variation in outdoor activity during the past month was associated with about a 25 percent absolute reduction in the prevalence of vitamin D insufficiency, by either definition, between the least and most active groups. We believe this variation is clinically significant.
Finally, we agree with Birrell and Francis that our analyses should be updated by using data from more recent studies. However, although there is need for more research in nursing home residents, they comprise a very small portion of US citizens. Greater gains in public health are possible from strategies that increase vitamin D levels in the general free-living population, should future studies confirm some of the health benefits currently linked to vitamin D. To properly determine whether vitamin D supplementation is beneficial, these studies need to give vitamin D doses much higher than the typical dose of 400–800 IU/day given in previous studies (4), so that serum 25-hydroxyvitamin D levels are increased to more than 75 nmol/liter or even higher (e.g., 90–100 nmol/liter) (5). Only then will we have a clear picture of how much the strategies to increase vitamin D status, including daily outdoor activities, will affect health.
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ACKNOWLEDGMENTS |
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Dr. Scragg was supported by the Health Research Council of New Zealand. Dr. Camargo was supported by grant R01 HL84401 (Bethesda, Maryland) and the Massachusetts General Hospital Center for D-receptor Activation Research (Boston, Massachusetts).
The authors thank the Centers for Disease Control and Prevention (Hyattsville, Maryland) for making these data available for analysis.
Conflict of interest: none declared.
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References |
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 TOP References |
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- Birrell F, Francis R. Invited commentary: physical activity and vitamin D. Am J Epidemiol (2008) 168. 000–00.
- Scragg R, Camargo CA Jr. Frequency of leisure-time physical activity and serum 25-hydroxyvitamin D levels in the US population: results from the Third National Health and Nutrition Examination Survey. Am J Epidemiol (2008) 168. 000–00.
- Looker AC, Dawson-Highes B, Calvo MS, et al. Serum 25-hydroxyvitamin D status in adolescents and adults in two seasonal subpopulations from NHANES III. Bone (2002) 30:771–7.[Medline]
- Boonen S, Lips P, Bouillon R, et al. Need for additional calcium to reduce the risk of hip fracture with vitamin D supplementation: evidence from a comparative metaanalysis of randomized controlled trials. J Clin Endocrinol Metab (2007) 92:1415–23.
[Abstract/Free Full Text] - Bischoff-Ferrari HA, Giovannucci E, Willett WC, et al. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr (2006) 84:18–28.
[Abstract/Free Full Text]
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