C-reactive protein and physical activity physical exercise (17). However, they had a small sample-size to conclude this finding. Why does physical activity reduce the serum CRP levels? Vari- ous theories have been postulated to explain this (25-27). The serum levels of TNF-α and IL-6 were reduced by physical activity, independent of the adipose tissue content (27). TNF-α and IL-6 induce the production and secretion of CRP (28). So, this may reduce the serum CRP levels, irrespective of fat content. On the other hand, physical activity in- duces anti-inflammatory cytokines, including IL-1 receptor antagonist and IL-10, which are known to hamper the production of CRP (26,27). We did not measure the levels of IL-6 and TNF-α in serum in our participants. Nevertheless, we encourage other researchers to evaluate the serum level of different cytokines in parallel with CRP and physical activity in the future epidemiologic studies. Previous studies gave different weights to the health effects of moderate and vigorous physical activity (29). Some authors suggest that vigorous activity is much stronger in improving cardiovascular health but others argue that moderate activity (which can be tolerated longer in an ordinary man) can be as effective as vigorous activity (30). Our results showed that activities of both vigorous and mod- erate intensity were independently associated with the lower CRP level but the degree of association was higher for vigorous vs moderate in both males and females. Therefore, our study provides a sup- port for the beneficial consequences of activities of both vigorous and moderate intensity. Limitations The principal limitation of the present study is its cross-sectional nature, which precludes the deter- mination of the direction of causality. This study was conducted in accordance with the stepwise guidelines of WHO, and our questionnaire did not gather data on history of cardiovascular diseases, malignancy, or other acute/chronic diseases (22). The decision regarding non-inclusion of history of cardiovascular diseases in the SuRFNCD question- naire comes from the fact that there is no readily- available technique to diagnose these conditions in the cases of negative history. For example, doing exercise testing or angiography on a national scale is not feasible. Therefore, the influence of these factors was not assessed in our study; however, we believe that this limitation does not lessen the im- portance of our findings to show a significant crude association between the CRP levels and physical in- activity in a general population. Also, factors, such as diet or alcohol drinking status, may also be in- fluential. Nevertheless, drinking of alcohol is illegal in Iran, and based on our experience, a very low percentage of our population was drinkers. Volume 30 | Number 1 | March 2012 Esteghamati A et al. The principal advantage of the present study is its large sample-size, and the point that it is represent- ative of the Iranian population. Moreover, we used a standardized international questionnaire to study the different domains of physical activity and sed- entary behaviours. We also evaluated the poten- tial confounding effects of several tertiary factors on the association between physical activity and quantitative CRP. Conclusions In this study, we evaluated the association between different intensities of physical activity and the se- rum CRP level. We showed that both vigorous and moderate physical activities were associated with the lower serum CRP levels. This association was independent of diabetes and body adiposity. We also showed that the time spent in sedentary be- haviours positively correlated with the quantitative CRP levels, independent of physical activity and obesity. The practical message of the study is that any type of physical activity (vigorous or moderate) is associated with lower CRP and can, therefore, be beneficial for a healthier lifestyle. Low activity and sedentary behaviours are independently associated with high CRP and can potentially cause negative health consequences. ACKNOWLEDGEMENTS The study was supported by the Ministry of Health and Medical Education, Tehran, Iran. REFERENCES 1. Fernandez-Real JM, Vayreda M, Richart C, Gutierrez C, Broch M, Vendrell J et al. Circulating interleukin 6 levels, blood pressure, and insulin sensitivity in ap- parently healthy men and women. J Clin Endocrinol Metab 2001;86:1154-9. 2. Tracy RP, Lemaitre RN, Psaty BM, Ives DG, Evans RW, Cushman M et al. Relationship of C-reactive protein to risk of cardiovascular disease in the elderly. Results from the Cardiovascular Health Study and the Rural Health Promotion Project. Arterioscler Thromb Vasc Biol 1997;17:1121-7. 3. Ridker PM, Hennekens CH, Buring JE, Rifai N. C- reactive protein and other markers of inflammation in the prediction of cardiovascular disease in women. N Engl J Med 2000;342:836-43. 4. Ridker PM, Rifai N, Stampfer MJ, Hennekens CH. Plasma concentration of interleukin-6 and the risk of future myocardial infarction among apparently healthy men. Circulation 2000;101:1767-72. 5. Fröhlich M, Imhof A, Berg G, Hutchinson WL, Pepys MB, Boeing H et al. Association between C-reactive protein and features of the metabolic syndrome: a population-based study. Diabetes Care 2000;23:1835-9. 71