There is overwhelming evidence from epidemiological prospective follow-up studies indicating that physical activity, assessed by questionnaire or interview, and physical fitness, measured on exercise testing, are inversely related to the incidence of cardiovascular disease and mortality [1-3]. The relative risks of physical inactivity and low fitness are approximately 2. The benefits of moderately vigorous activity  and of greater fitness [5,6] have also been shown in hypertensive patients. It is currently not sufficiently known whether the beneficial effect of physical activity derives from the influence of physical activity itself or results from its effects on other risk factors or pathophysiological mechanisms, or all of these. There is evidence from randomized controlled trials and/or observational studies that exercise has favourable effects on blood pressure, glucose homeostasis, blood lipids, body fatness, smoking behaviour and endothelial function, amongst other factors . With regard to blood pressure, many epidemiological studies have analysed the relationships between habitual physical activity or physical fitness and blood pressure. Although several studies did not observe significant independent relationships, others concluded that blood pressure is lower in fitter and more active subjects .
However, longitudinal intervention studies are more appropriate to assess the effects of physical activity and training on blood pressure. In two recent meta-analyses of randomized controlled trials, decreases of resting blood pressure in response to dynamic physical training were significant and amounted to 3.4/2.4 mmHg  and 3.8/2.8 mmHg , respectively. The weighted net decrease of blood pressure averaged 7.4/5.8 mmHg in study groups in which average baseline blood pressure was in the hypertensive range, and 2.6/1.8 mmHg in normotensive study groups . Aerobic training has also been shown to reduce ambulatory blood pressure and blood pressure measured during exercise . The influence of exercise on blood pressure may therefore contribute to the beneficial effect of physical activity or fitness on the incidence of cardiovascular morbidity and mortality.
In the current issue of the journal, Barengo et al.  report on another potential benefit of physical activity, namely the prevention of the development of hypertension, using data from the Finnish part of the multinational MONICA project. The purpose was to determine whether low levels of leisure-time physical activity, occupational physical activity and commuting activity independently increase the risk of hypertension after adjusting for most risk factors for hypertension and other forms of physical activity. In a previous publication, the authors combined the three types of activity and found that adjusted hazard ratios of hypertension associated with light, moderate and high physical activity, respectively, were 1.00, 0.60 and 0.59 in men and 1.00, 0.80 and 0.72 in women . When analysed separately, the independent preventive effect was most convincing for leisure-time physical activity, particularly in men. High occupational activity was only significant when men and women were combined, and commuting activity was not associated with the risk of hypertension in multivariate models . Most previous studies have analysed the relationship between leisure-time physical activity and future blood pressure and, in general, the results of Barengo et al.  concord with previous findings. Paffenbarger et al.  assessed the incidence of hypertension in 14 998 Harvard male alumni during a 6-10-year follow-up beginning 16-50 years after college entrance. The presence or absence of a background of collegiate sports did not influence the risk of hypertension in this study population, nor did stair-climbing, walking or light sports play by alumni, based on physical activity information obtained by mailed questionnaires in a post-college health survey. However, alumni who did not engage in vigorous sports play in post-college years were at a 35% greater risk of hypertension than those who did (P < 0.001), and this relationship held at all ages (35-74 years); lack of strenuous exercise independently predicted an increased risk of hypertension. In a similar study on 5463 University of Pennsylvania alumni, Paffenbarger et al.  confirmed that vigorous sports play reduced the incidence of hypertension and that this was not the case for collegiate sports play and contemporary walking, stair-climbing or light sports play. In a later analysis of the Harvard College alumni study, Paffenbarger and Lee  concluded that lack of moderately vigorous sports play, being overweight and a parental history of hypertension, independently increased the risk of developing hypertension. Hayashi et al.  investigated the association of the leisure-time physical activity with the 10-year risk of hypertension in 6017 Japanese men aged 35-60 years. The multivariate adjusted risk for hypertension was significantly reduced by 30% in men who engaged in regular physical activity at least once a week. Folsom et al.  examined the 2-year incidence of hypertension in a cohort of 41 837 women aged 55-69 years. High levels of leisure physical activity were associated with a significant 30% reduction in risk of hypertension, but physical activity no longer contributed to hypertension risk after adjustment for age, body mass index, waist-to-hip ratio and smoking history in this short-term, follow-up study. Haapanen et al.  studied the effect of the total amount and intensity of leisure-time physical activity on the 10-year incidence of hypertension in a Finnish cohort of 1340 men and 1500 women aged 35-63 years. The men's total amount of activity, as well as vigorous activity once or more often a week, were inversely associated with the age-adjusted risk of hypertension (P = 0.02). After further control for body mass index and diabetes, the associations were only suggestively statistically significant (P = 0.08). For women, neither of these leisure-time physical activity measures were significantly associated with risk of hypertension. In the Atherosclerosis Risk in Communities (ARIC) Study, Pereira et al.  related leisure-time physical activity with incident hypertension in 7459 Black and White adults aged 45-65 years. After multivariate adjustment, White men in the highest quartile of leisure activity, primarily cycling and walking, had a 34% lower risk of developing hypertension over 6 years compared to the least active. Baseline activity was not significantly associated with incident hypertension in White women or in Blacks. In 1034 19- to 69-year-old participants of the Canada Fitness Survey, physical activity level was a significant and independent predictor of future levels of blood pressure during 7 years of follow-up, but only in women and not in men . Similarly, among 2357 adults in a population sample of Almeda County, low leisure-time physical activity independently predicted the incidence of treated hypertension over the subsequent 20 years in women but not in men .
The current evidence therefore suggests that higher levels of leisure-time physical activity are associated with a reduced incidence of hypertension but the results are not always consistent. This could be due to there being no gold standard for the assessment of physical activity, as well as variation among studies with respect to the methodology, activities involved and their quantification. Furthermore, the confounding factors that were considered in the multivariate analyses differ from study to study. Unfortunately, baseline blood pressure, which is probably the most important predictor of future hypertension , has only been accounted for in a limited number of studies [11,16,19,20].
Few data have been published on the relationship between incident hypertension and, respectively, commuting activity and physical activity at work. Whereas the ‘work index’ was not independently predictive for future hypertension in the ARIC study , Hayashi et al.  investigated the association of the duration of the walk to work with the risk of hypertension in men and found that the multivariate adjusted risk for hypertension was significantly reduced by 12% when the duration of the walk to work was increased by 10 min. By contrast, Barengo et al.  observed that high occupational physical activity independently reduced the risk of hypertension, but only when men and women were combined, and commuting activity was not independently associated with the risk of hypertension. It is obvious that more data are needed with regard to occupational and commuting activity.
It is noteworthy that Barengo et al.  observed a significant effect of leisure-time physical activity on incident hypertension and not of commuting activity. It is likely that leisure-time and commuting activities are predominantly of the dynamic type. Unfortunately, total energy expenditure of each type of activity during the study week was not assessed and a detailed description of the actual activities involved was not given. Therefore, it cannot be excluded that the so-called high level of commuting activity was close to the low level of leisure-time physical activity.
Finally, three studies have assessed the relationship between measured physical fitness and the incidence of hypertension and found that individuals with low fitness had a relative risk of 1.5 , 1.9  and 2.2 , respectively, for the development of hypertension compared to highly fit individuals; baseline blood pressure was included in the multivariate models in two of these studies [22,23]. Although physical fitness is mainly determined by genetic factors , with a limited influence of actual physical activity, these findings appear to support the importance of physical activity in the prevention of hypertension.
In conclusion, the results of Barengo et al.  indicate that leisure-time physical activity reduces the incidence of hypertension, whereas the evidence is less convincing for occupational activity and not significant for commuting activity. However, calculation of total energy expenditure of the three types of activity during the week of observation, based on the actual activities involved and their respective duration and intensity, would have allowed a more justified and scientifically sound comparison between these types of activity.
The authors gratefully acknowledge the secretarial assistance of N. Ausseloos.
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