We observed several important findings in this study. First, the lifetime risks for CVD death according to traditional risk factors in the CCLS were similar to those obtained in a larger, more representative cohort. Thus, although the risk factor burden is lower in the CCLS compared to the general population, the association of these risk factors with lifetime risks for CVD mortality was quite similar, particularly for men at age 55- and 65- years. Second, low fitness obtained from a single fitness measurement was associated with marked differences in the lifetime risks for CVD death more than 30 years later. Finally, the combination of high fitness with a high traditional risk factor burden was associated with a lifetime risk for CVD death that was comparable to an individual with low risk factor burden.
Multiple prior studies have shown that a single measurement of cholesterol or blood pressure in mid-life was associated with marked differences in CVD risk across 30 or more years of follow-up (16
). The findings in the present study extend these observations to physical fitness. Despite the effects of interval changes in fitness over time, the presence of a single measurement of low physical fitness in mid-life translated into a 15–20% absolute difference in the lifetime risk for CVD death.
Furthermore, among individuals with elevated cholesterol, hypertension, diabetes, or current smoking status, the presence of a higher fitness level in midlife attenuated substantially the risk from traditional risk factors. These data have potential implications for public health and clinical practice, providing novel insight into the importance of fitness for long-term CVD risk, particularly among those with elevated traditional risk factor burden.
Current Study in Context
Several prior studies have described the association between levels of fitness and the risk for CVD mortality, including the Lipid Research Clinics Trial (5
), the Veteran Affairs study (7
), and previous reports from the Cooper Center Longitudinal Study (3
). These prior studies as well as a recent meta-analysis (6
) have demonstrated a consistent, inverse association between fitness and mortality in individuals with and without prevalent CVD at baseline and after adjustment for traditional risk factors.
In general, these studies have applied conventional statistical techniques to provide both absolute and relative risk estimates. Because these techniques censor data at the time of a competing (non-CVD) death, they cannot be extrapolated to estimate cumulative risk across the lifespan when competing risks can be substantial. In addition, relative and absolute risk estimates have well-recognized limitations for risk communication strategies (8
). In particular, relative risks require comparison to a basal rate that is rarely known in practice by patients or by physicians. Although absolute risks overcome this limitation, short-term absolute risks do not reflect risks across the remaining lifespan (28
The current study provides a clinically relevant and intuitive estimate of the association between fitness, traditional risk factors, and long-term risk. With the knowledge of a man’s age, fitness, and risk factor status, clinicians can provide an estimate of the lifetime risk for CVD death. For example, consider a 55-year old man able to walk one mile in 15 minutes, consistent with very low fitness (METS < 6). With these data, we would predict that his lifetime risk for CVD death was nearly 30%. In contrast, a man able to run a 10-minute mile (i.e. moderate fitness or 10 METS) would have an estimated lifetime risk of CVD mortality of just 10%.
The present study has several important limitations. First, participants in the CCLS represent a unique sample of well-educated individuals with a relatively high socioeconomic status compared to the general population. Therefore, these results may not generalize to individuals with a lower educational status or socioeconomic level. However, the association between traditional risk factor burden and lifetime risk observed in the CCLS at age 55- and 65-years were strikingly similar to those obtained from a large, pooled cohort of 16 unique studies(22
). For men at these ages, we believe these findings are generalizable to the broader population, extending prior work on lifetime risks for CVD to include fitness, an additional, modifiable lifestyle factor. However, the present findings may not generalize to men at age 45-years given the overall lower lifetime risks for CVD in this subgroup.
Second, the ability to adequately control for competing risks is reliable but somewhat limited using mortality from death certificates because of the presence of misclassifications of non-CVD death as CVD death at older ages (29
). Thus, we expect that adjudicated CVD deaths would have yielded lower estimates across all risk groups, particularly among the highest fit groups. Therefore, we expect that the actual lifetime risks for CVD death among the highest fit groups are actually lower than those reported in the present paper.
Finally, we assessed the association between a single measurement of fitness and the risk for CVD more than 30 years later. Although we acknowledge that fitness levels most certainly changed over the follow-up period, we feel that this actually represents a significant strength of our findings. In spite of these changes over time, a single measurement of fitness in midlife represents a strong determinant of long-term CVD risk. These data provide additional support for the importance of fitness in midlife and could be useful in efforts to promote risk communication.
In summary, using a large, well-characterized cohort with long-term follow-up, we report the association between fitness levels and lifetime risks for CVD. In this study, a single measurement of fitness in midlife was associated with marked differences in the lifetime risk for CVD mortality more than 30 years later—particularly among those with at least on risk factor in midlife. These data emphasize the importance of fitness on long-term cardiovascular health and could be useful for practicing clinicians to facilitate more effective risk communication regarding the health benefits of fitness.