The finding that some people who exercised had adverse metabolic responses to regular exercise [1] may be related to air pollution during exercise, which was overlooked in this paper. There is abundant evidence that small particulates (PM 2.5) are associated with increased risk of cardiovascular disease in the United States [2]. The components of PM 2.5 most likely to be involved are black carbon and polycyclic aromatic hydrocarbons (PAHs) [3]. Thus, the effect of air pollution is similar to that from smoking tobacco [3]. The present study [1] found that adverse effects of exercise on plasma fasting insulin, plasma HDL-C, plasma triglycerides, and resting systolic blood pressure were more pronounced in some of the seven studies than in others. A study of smokers in Japan found lower HDL-C concentrations and higher triglyceride concentrations associated with smoking more than 20 cigarettes per day compared to none [4]. A study in Poland found higher blood pressure for smokers than nonsmokers [5]. Those with consistently greater percentage of adverse responders in [1] were in studies conducted in Dallas, TX and Baton Rouge, LA. Ref. 2 found much higher rates of coronary heart disease (CHD) in the regions of the country including those cities, East South Central for Louisiana and West South Central for Texas compared to South Atlantic for Maryland and North Carolina. The odds ratios for CHD were approximately 1.5 (0.8-2.7), 1.45 (0.9-2.1), and 0.8 (0.85-1.15) respectively. See, also, Ref. 6 regarding the distribution of PM 2.5 in the United States in 2006.

Thus, when exercising out of doors, it would be worthwhile to do it when PM 2.5 concentrations are lowest. Evidently sunlight reduces PAH concentrations adsorbed on black carbon [7], suggesting that exercise would be better later in the day.

It would be worthwhile for the authors of Ref. 1 to examine the possible role of air pollution in explaining the findings of their study.

References

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