Mortality Risk Among Men With Low Cardiorespiratory Fitness
Purpose: A low level of cardiorespiratory fitness (CRF) is a strong and independent predictor of all-cause mortality in men; however, it is unknown whether a gradient of mortality risk exists within the lowest CRF category.
Methods: A total of 6251 apparently healthy men (mean age, 48.7 ± 6.3 yr) completed a comprehensive baseline clinical examination, including a maximal treadmill exercise test at Cooper Clinic between 1971 and 2006. In accord with previous studies using this cohort, low CRF was defined as a treadmill time in the first quintile within each age category of 40–49, 50–59, and 60–69 yr. The low CRF cohort was then grouped by tertiles (low/low, LL; mid/low, ML; and high/low, HL) using the same age categories.
Results: After a mean follow-up period of 19.1 ± 10.4 yr, 1259 deaths occurred. Adjusted all-cause mortality rates were 57.0, 31.1, and 34.4 deaths per 10,000 man-years across LL, ML, and HL CRF categories for the 40- to 49-yr-old age group (P trend = 0.007). Similar trends were seen across low CRF categories for the 50- to 59-yr-old and 60- to 69-yr-old age groups (P trend = 0.02 and 0.09, respectively). When using treadmill time as a continuous variable, each 1-min increment in treadmill time was associated with a 9%, 11%, and 15% reduction in risk of all-cause mortality among low-CRF men in the 40–49, 50–59, and 60–69 age groups, respectively.
Conclusions: An inverse trend in all-cause mortality exists among men across LL, ML, and HL CRF groups. Although all low-fit men should be targeted for physical activity intervention, it is especially important to target the LL CRF group.
Cardiorespiratory fitness (CRF) is defined as the body's maximal ability to transport and utilize oxygen at the tissue level and is dependent primarily on maximal cardiac output and maximal arterial–venous oxygen difference as well as efficient shunting of blood to working skeletal muscles. CRF is influenced by genetic factors and by habitual physical activity and can be objectively measured via maximal exercise testing. Over the past three decades, a low level of CRF has been shown to be a significant predictor of various adverse health outcomes in men including all-cause cardiovascular and cancer mortality as well as incidence of type 2 diabetes, hypertension, metabolic syndrome, and cardiovascular morbidity. Furthermore, Gupta et al. recently showed that a single measurement of CRF significantly improved classification of both short-term and long-term risk for cardiovascular disease (CVD) mortality when added to traditional risk factors. The Cooper Center Longitudinal Study (CCLS) has made significant contributions to the literature in this regard. In a landmark article using age-adjusted quintiles to classify CRF categories, Blair et al. demonstrated that CRF was strongly and inversely related to all-cause mortality in both men and women. Because the largest decrement in all-cause mortality was seen when comparing the lowest CRF quintile with the next higher quintile, the lowest quintile has been designated as "unfit" or "low CRF" in many CCLS articles published since then.
The notion that low CRF is a strong predictor of morbidity and mortality is not limited to the CCLS. For example, the Lipid Research Clinics Mortality Follow-up Study, Seattle Heart Watch, Baltimore Longitudinal Study of Aging, the Palo Alto Veterans Study, and the U.S. Railroad Study have all shown that low CRF is significantly associated with CVD. In a recent meta-analysis by Kodama et al. using 33 observational cohort studies, those with low CRF (<7.9 METs) had relative risk values of 1.70 and 1.56 for all-cause mortality and CVD events, respectively, when compared with those with high CRF (≥10.9 METs). When compared with individuals with intermediate CRF (7.9–10.8 METs), those with low CRF had relative risk values of 1.40 and 1.57 for all-cause mortality and CVD events, respectively (P < 0.001 for all comparisons).
Although it is evident that low CRF is a strong and independent risk factor for morbidity and mortality, very little is known regarding the gradient of risk within the low CRF category. For example, it is not known whether a risk gradient exists, whether it is linear, or whether there is a threshold for significantly increased risk within the low CRF category. Physical activity pattern and other coronary risk factor comparisons between generally healthy individuals at the low, intermediate, and high spectrum of the low CRF category have not been examined. In addition, studies concerning hemodynamic responses to maximal exercise testing; for example, double product reserve, are sparse.
Therefore, the primary purpose of the current study was to examine the risk gradient for all-cause mortality in otherwise apparently healthy men with low CRF. We will also compare physical activity patterns, other coronary risk factors, and hemodynamic variables across the spectrum of the low CRF category in this cohort.
Abstract and Introduction
Abstract
Purpose: A low level of cardiorespiratory fitness (CRF) is a strong and independent predictor of all-cause mortality in men; however, it is unknown whether a gradient of mortality risk exists within the lowest CRF category.
Methods: A total of 6251 apparently healthy men (mean age, 48.7 ± 6.3 yr) completed a comprehensive baseline clinical examination, including a maximal treadmill exercise test at Cooper Clinic between 1971 and 2006. In accord with previous studies using this cohort, low CRF was defined as a treadmill time in the first quintile within each age category of 40–49, 50–59, and 60–69 yr. The low CRF cohort was then grouped by tertiles (low/low, LL; mid/low, ML; and high/low, HL) using the same age categories.
Results: After a mean follow-up period of 19.1 ± 10.4 yr, 1259 deaths occurred. Adjusted all-cause mortality rates were 57.0, 31.1, and 34.4 deaths per 10,000 man-years across LL, ML, and HL CRF categories for the 40- to 49-yr-old age group (P trend = 0.007). Similar trends were seen across low CRF categories for the 50- to 59-yr-old and 60- to 69-yr-old age groups (P trend = 0.02 and 0.09, respectively). When using treadmill time as a continuous variable, each 1-min increment in treadmill time was associated with a 9%, 11%, and 15% reduction in risk of all-cause mortality among low-CRF men in the 40–49, 50–59, and 60–69 age groups, respectively.
Conclusions: An inverse trend in all-cause mortality exists among men across LL, ML, and HL CRF groups. Although all low-fit men should be targeted for physical activity intervention, it is especially important to target the LL CRF group.
Introduction
Cardiorespiratory fitness (CRF) is defined as the body's maximal ability to transport and utilize oxygen at the tissue level and is dependent primarily on maximal cardiac output and maximal arterial–venous oxygen difference as well as efficient shunting of blood to working skeletal muscles. CRF is influenced by genetic factors and by habitual physical activity and can be objectively measured via maximal exercise testing. Over the past three decades, a low level of CRF has been shown to be a significant predictor of various adverse health outcomes in men including all-cause cardiovascular and cancer mortality as well as incidence of type 2 diabetes, hypertension, metabolic syndrome, and cardiovascular morbidity. Furthermore, Gupta et al. recently showed that a single measurement of CRF significantly improved classification of both short-term and long-term risk for cardiovascular disease (CVD) mortality when added to traditional risk factors. The Cooper Center Longitudinal Study (CCLS) has made significant contributions to the literature in this regard. In a landmark article using age-adjusted quintiles to classify CRF categories, Blair et al. demonstrated that CRF was strongly and inversely related to all-cause mortality in both men and women. Because the largest decrement in all-cause mortality was seen when comparing the lowest CRF quintile with the next higher quintile, the lowest quintile has been designated as "unfit" or "low CRF" in many CCLS articles published since then.
The notion that low CRF is a strong predictor of morbidity and mortality is not limited to the CCLS. For example, the Lipid Research Clinics Mortality Follow-up Study, Seattle Heart Watch, Baltimore Longitudinal Study of Aging, the Palo Alto Veterans Study, and the U.S. Railroad Study have all shown that low CRF is significantly associated with CVD. In a recent meta-analysis by Kodama et al. using 33 observational cohort studies, those with low CRF (<7.9 METs) had relative risk values of 1.70 and 1.56 for all-cause mortality and CVD events, respectively, when compared with those with high CRF (≥10.9 METs). When compared with individuals with intermediate CRF (7.9–10.8 METs), those with low CRF had relative risk values of 1.40 and 1.57 for all-cause mortality and CVD events, respectively (P < 0.001 for all comparisons).
Although it is evident that low CRF is a strong and independent risk factor for morbidity and mortality, very little is known regarding the gradient of risk within the low CRF category. For example, it is not known whether a risk gradient exists, whether it is linear, or whether there is a threshold for significantly increased risk within the low CRF category. Physical activity pattern and other coronary risk factor comparisons between generally healthy individuals at the low, intermediate, and high spectrum of the low CRF category have not been examined. In addition, studies concerning hemodynamic responses to maximal exercise testing; for example, double product reserve, are sparse.
Therefore, the primary purpose of the current study was to examine the risk gradient for all-cause mortality in otherwise apparently healthy men with low CRF. We will also compare physical activity patterns, other coronary risk factors, and hemodynamic variables across the spectrum of the low CRF category in this cohort.
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