Fitness, Fatness, Importance in Systolic Blood Pressure

Modifying risk factors to delay or prevent hypertension is critical for subsequent cardiovascular risk reduction. Therefore, understanding the independent and joint associations between cardiorespiratory fitness, obesity, and systolic blood pressure (SBP) is of major significance. In this study, researchers assessed the relative contribution of body mass index (BMI) and cardiorespiratory fitness to SBP in a large, healthy population.

Blood pressure, BMI, and cardiorespiratory fitness were measured in 35,061 patients seen for a preventive health examination (1990 to present). BMI was treated as a continuous variable and categorized into sex-specific quartiles. Cardiorespiratory fitness was defined as time achieved during maximal exercise testing and categorized into age- and sex-adjusted quintiles. Generalized linear models were used to determine the independent contribution of fitness and BMI on systolic blood pressure estimates.

The study group was predominately white men (69%) with an average age of 46 years. Normal-weight subjects had a mean SBP 12 mm Hg lower than in the obese (115 vs. 127 mm Hg, P < .001), while being high, fit was associated 6 mm Hg difference in mean SBP comparing the highest and lowest fitness quintile (119 vs 125 mm Hg, P < .001). Normal-weight individuals with a cardiorespiratory fitness level greater than the first quintile (Q1) had the lowest mean SBP (P < .001). Both BMI and cardiorespiratory fitness were associated with SBP (P < .001 for both); however, when assessed simultaneously, BMI had a greater impact on SBP estimates than fitness.

When comparing lifestyle risk factors BMI and cardiorespiratory fitness, BMI was a more important factor in predicting SBP. Importantly, only modest fitness levels among normal-weight individuals were associated with the lowest systolic blood pressure estimates.

In this current study, being normal weight was associated with markedly lower systolic blood pressure. When comparing lifestyle risk factors BMI and cardiorespiratory fitness, BMI was a more important factor in predicting SBP. Importantly, only modest fitness levels among normal-weight individuals were associated with the lowest systolic blood pressure estimates.

Recent evidence suggests obesity is such a strong determinant of hypertension that the benefits of other lifestyle factors are not obtained until individuals are normal weight. The findings were consistent with the observation that the impact of fitness on systolic blood pressure is most apparent in individuals who were normal weight. These data are supported by a recent meta-analysis among exercise intervention trials demonstrating a greater impact of fitness on hypertension among normal-weight (−4 mm Hg) compared to overweight (−2 mm Hg) subjects.

The mechanism by which cardiorespiratory fitness provides differential benefit among lean versus obese subjects is not clear. Cardiorespiratory fitness, an individual's maximal capacity to transport and use oxygen during exercise, can be modified by exercise training. Repetitive exercise can lower resting blood pressure and heart rate by increasing arterial compliance and insulin sensitivity, decreasing in sympathetic activity, increasing release of endothelium-derived nitric oxide, and lowering systemic vascular resistance. Exercise intervention trials have not observed changes in systolic blood pressure among obese subjects who did not lose weight, despite improvements in cardiorespiratory fitness. It can be hypothesized that the beneficial effects of exercise on blood pressure are modest in obese subjects because of the competing effects of obesity on vascular and metabolic pathways. In addition, it should also be noted that being highly fit and obese is rare, and therefore may be difficult or impossible to achieve in practice.

Limitations and strengths to the current study require review. The CCLS cohort is predominately healthy, white men and women within higher socioeconomic strata. This point must be taken under consideration when generalizing to other population subgroups. However, because of the large sample size, we were able to assess a range of BMI and fitness estimates on systolic blood pressure (i.e., more than 6,000 obese subjects were analyzed in the current study). In addition, BMI, cardiorespiratory fitness, and systolic blood pressure were systemically captured in the current cohort, allowing for one of the first studies to our knowledge to compare BMI and cardiorespiratory fitness in predicting systolic blood pressure in a large population cohort. As with the US population overall, secular changes in risk factors such as BMI have occurred in the CCLS. However, time-dependent changes in any particular risk factor cannot be ascertained from this cross-sectional study. Secondly, although our data support that BMI is a more important predictor of SBP estimates than fitness, we acknowledge that BMI and fitness are interrelated. Therefore, we cannot rule out that independent estimates of BMI and fitness, when simultaneously assessed in any given model, may be less precise.

The residual lifetime risk for hypertension is 90%, representing a major public health burden. The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure stresses the importance of healthy lifestyle adherence as key steps in prevention. Despite these recommendations, most of the US population fails to adopt or achieve an optimal number of healthy risk factors to prevent hypertension. The findings suggest that achieving normal-weight status should be the primary goal for hypertension prevention, and only modest levels of cardiorespiratory fitness are needed to obtain optimal blood pressure among individuals who are normal weight.

American Heart Journal. 2010;160(1):166-170.