In this large, prospective population study of men and women with metabolic syndrome, we found a gradual reduction in all-cause mortality associated with increasing levels of PA. Importantly, the greatest mortality difference was found between people who reported no activity and those who reported low levels of PA.
To our knowledge, this is the first study to evaluate the association of PA with mortality among people with metabolic syndrome. The main finding was in line with prospective studies in the general population [18
] in showing that PA is associated with a reduction in mortality from all causes, and specifically from CVD, among people with metabolic syndrome. The substantial mortality difference that we found between low levels of PA and no activity also corresponds to findings in the general population [21
]. Less than 50% of adults in the US meet the recommended level of PA [22
], but our results suggest that even PA at a level that most people are able to achieve is likely to reduce mortality in people with metabolic syndrome.
We cannot exclude the possibility that unknown factors other than PA might explain the high mortality related to physical inactivity. It is possible that being inactive reflects prevalent illness and that therefore the stronger association with mortality might be as expected. We attempted to take this possibility into account by excluding participants with a history of stroke, myocardial infarction or known angina pectoris.
We found no difference regarding risk in mortality due to CVD between those who reported being inactive and those who reported low levels of PA among people with metabolic syndrome in the age group younger than 65 years of age. However, that analysis was based on a small number of deaths, and thus the statistical power required to detect any effect was modest.
A number of studies have shown that metabolic syndrome is associated with increased risk of death from all causes, and specifically from CVD, compared to people without metabolic syndrome [1
]. In our study, there was a positive association of metabolic syndrome with all-cause and CVD mortality only among people younger than 65 years of age. Our finding is in line with that of Hildrum et al
], who reported higher all-cause mortality associated with metabolic syndrome in middle-aged people but not in older adults. A possible explanation for our observation is that people with low tolerance for cardiovascular risk factors die before they reach old age and that the risk factors associated with metabolic syndrome have less effect on survival in older individuals.
It has been shown that PA may improve the factors that constitute metabolic syndrome [12
]. Therefore, beneficial effects on weight, blood pressure, serum lipid levels and metabolism of carbohydrates may be important for the lower mortality associated with PA among people with metabolic syndrome. Also, improvement in cardiac function, especially in relation to the intensity of PA [27
], is likely to play a role.
The question about leisure time PA in the HUNT 2 questionnaire did not distinguish between strength and aerobic exercise, and therefore we cannot attribute the observed effects to a particular type of exercise. People who report being inactive are likely to be inactive; however, PA in questionnaire-based data is typically overreported [28
]. In this study, misclassification caused by overreporting of PA would place some people into a higher PA category than their true activity level would indicate. The possible effect of this misclassification would most likely underestimate the effect of PA in this study.
The use of nonfasting glucose values in our study could have been a potential source of bias. Glucose level was used as a diagnostic criterion for metabolic syndrome only if the levels were elevated four hours postprandial. Thus, there was a potential risk of placing some people with metabolic syndrome into the category of people without metabolic syndrome. However, in a subanalysis, glucose was used as a criterion in all subjects regardless of the time since their most recent meal, and the results were essentially the same. Validation of the PA questionnaire in the HUNT 2 study showed that the "light activity" question was not adequately correlated with other measures of PA, whereas the reliability of hard PA and occupational activity was found to be satisfactory [29
]. The validation was conducted among 108 men ages 20 to 39 years, however, and therefore we cannot be certain that the results can be generalized to the rest of our study population. In this study, an attempt was made to classify self-reported PA so that it would correspond to the currently recommended guidelines for PA. However, the classification was not exact, and caution should be taken when interpreting these data. Also, some participants might have changed their PA levels after data collection, and these changes might also have influenced the results. However, such changes most likely would have underestimated the differences in mortality between the inactive group and the groups with different activity levels.