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U-shaped relationship between exercise intensity and cardiovascular health

Key clinical point: Higher-intensity exercise is not associated with proportionally greater reductions in the risk of coronary heart disease.

Major finding: Women who undertook daily strenuous activity had a 15% increase in their risk of coronary heart disease, a 25% increase in cerebrovascular disease risk, and 29% increase in venous thromboembolism risk, compared with those who exercised strenuously two to three times a week.

Data source: The Million Women longitudinal cohort study.

Disclosures: The study was supported by the UK Medical Research Council, Cancer Research UK, and the BHF Centre of Research Excellence. There were no other conflicts of interest declared.

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Food for thought?

The Million Women Study has many strengths. By its size and length of follow up, it has been able to overcome the notorious methodologic challenges of how to accurately measure not only the frequency of physical activity but also its type, intensity, and duration as well as variations in activity level over time. And, to its credit, the Million Women Study is one of the few large cohort studies to measure housework and to include it as a form of moderate physical activity.

Its findings may be subject to confounding, nonetheless. Such as data showing that the prevalence of current smokers was 25% in the group that exercised every day and in the group that reported no strenuous exercise – considerably higher than the prevalence estimates for women who did strenuous exercise one to six times per week. Even though the results were adjusted for smoking (and other risk factors), the authors acknowledged that residual confounding may have persisted and, thus, may have explained some of the association between physical activity and vascular risk in the most active women.

Further perplexing is the fact that even its sedentary participants were far from inactive. Current guidelines for the level of exercise needed to maintain cardiovascular health call for at least 30 minutes of moderate-intensity aerobic activity at least 5 days per week in bouts of 10 minutes of more moderate or 25 minutes of vigorous aerobic activity at least 3 days per week, or a combination thereof. In total, this amount of activity would equate to approximately 8-12 MET-hrs per week. Thus, it is puzzling that the women in the Million Women Study, even women who reported doing no physical activity at study baseline, still accrued over 15 excess MET-hrs per week (after excluding housework), predominantly through walking and gardening.

Dr. Rachel Huxley, D.Phil., is from the University of Queensland in Herston, Australia. She did not disclose whether she had any financial conflicts of interest. Her remarks were distilled from an editorial (Circulation 2015 Feb. 16 [doi: 10.1161/CIRCULATIONAHA.115.014721] accompanying the research report.


 

FROM CIRCULATION

References

Strenuous daily exercise actually increased the risk of coronary heart disease, venous thromboembolism, and cerebrovascular disease, compared with moderate physical activity, according to new data from the Million Women Study.

At baseline, the 1.1 million women who participated in the study were 55.9 years old on average, with a mean body mass index of 26 kg/m2. Over the next 9 years, those reporting moderate activity had significantly lower risks of all three conditions than did inactive women, according to a study published online Feb. 16 in Circulation [doi:10.1161/CIRCULATIONAHA.114.010296].

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However, women who undertook daily strenuous activity had a 15% increase in their risk of coronary heart disease, a 25% increase in cerebrovascular disease risk, and a 29% increase in venous thromboembolism (VTE) risk, compared with those who exercised strenuously two to three times a week, judging from the findings of Cox regression models that controlled for BMI, smoking, and alcohol consumption.

“Among active women, there was little evidence of progressive reductions in risk with more frequent activity, and even an increase in risk for CHD, cerebrovascular disease, and VTE in the most active group,” wrote Dr. Miranda E. G. Armstrong of the University of Oxford, England, and colleagues.

The study was supported by the UK Medical Research Council, Cancer Research UK, and the BHF Centre of Research Excellence. There were no other conflicts of interest declared.

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