Importance of Optimizing Cardiorespiratory Fitness in Early Adulthood and Through Midlife

Cardiovascular disease (CVD) remains the leading cause of death in the United States, contributing to poor quality of life and disability.¹ Sedentary lifestyle, a reversible CVD risk factor, has been found to be associated with increase CVD morbidity and mortality. Regular physical activity increases the ability of the circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production (ie, cardiorespiratory fitness). A 1989 study² found that individuals with higher levels of cardiorespiratory fitness had lower death rates, independently of their age, smoking status, blood pressure, lipids, diabetes, and personal and parental history of CVD. Therefore, interventions to help people be active, avoid nicotine exposure, eat a heart-healthy diet, get appropriate sleep, and control glucose, blood pressure, and cholesterol may improve and maintain their cardiovascular health.

Disparities in cardiovascular fitness may lead to greater challenges in the efforts to prevent and control CVD. In the US, Black individuals are more likely to die from heart disease than non-Hispanic White individuals, and disparities have also been reported in the prevalence and control of CVD risk factors.^3,4 Enhancing cardiorespiratory fitness during early adulthood and keeping it through midlife may lead to better clinical outcomes and serve as a primary strategy to reduce disparities. In the early 2000s, the Dallas Heart Study,⁵ a longitudinal multicenter population-based study, examined the factors associated with racial and ethnic differences in cardiorespiratory fitness. The Dallas Heart Study⁵ found that cardiorespiratory fitness among Black individuals was significantly lower than their non-Hispanic White counterparts after adjusting for age and sex. Similarly, this study by Pettee Gabriel and colleagues⁶ provides an analysis of trends for all-cause mortality and fatal and nonfatal CVD rates across different levels of cardiorespiratory fitness using data from the ongoing Coronary Artery Risk and Development in Young Adults (CARDIA) study. Pettee Gabriel et al⁶ examined the association of early adult fitness and the percentage of early adult fitness retained in midlife with subsequent risk of all-cause mortality and CVD events, overall and stratified by sex and race groups. In contrast to the 1989 observational study by Blair et al² the CARDIA study⁶ included younger participants (mean age 24.3 years). Since evidence on the importance of fitness has largely been established with older populations, this study provides additional insights in earlier adulthood.

Pettee Gabriel and colleagues⁶ report that better cardiorespiratory fitness at baseline, measured using 1-minute increments in a maximal graded exercise test, was significantly associated with an 18% lower risk of premature death (hazard ratio [HR], 0.82 [95% CI, 0.76–0.88]; men: HR,0.87 [95% CI, 0.80–0.96]; women: HR, 0.73 [95% CI, 0.64–0.82]), as well as an 11% lower risk of fatal and nonfatal cardiovascular events (HR, 0.89 [95% CI, 0.82–0.96]; men: HR, 0.90 [95% CI,0.82–0.98]; women: HR, 0.86 [95% CI, 0.75–0.99]). In addition, retention of cardiorespiratory fitness through midlife (measured in 5% increments in the level of fitness after 20 years of follow-up) was significantly associated with a 11% lower risk of all-cause mortality (HR, 0.89 [95% CI,0.79–0.99]) but not significantly associated with a reduction in fatal and nonfatal cardiovascular events (HR, 0.89 [95% CI, 0.78–1.00]).⁶ These results support previous studies showing an inverse association of fitness with all-cause mortality and CVD events,^2,7 but the evidence in the CARDIA study⁶ adds the value of assessing cardiorespiratory fitness in early adulthood.

It is also relevant to evaluate the impact that fitness training across the lifespan may have on modifiable risk factors, relevant clinical outcomes, and the reduction or elimination of race and sex disparities in high-risk populations. Specifically, the importance of cardiorespiratory fitness in women requires particular attention. Even though the CARDIA study found that most Black women did not meet physical activity guidelines (63% at baseline and 71% after 20 years of follow-up), those who had better cardiorespiratory fitness at baseline had a 25% lower overall mortality risk. Several clinical studies have examined the value of physical activity in premenopausal and postmenopausal women,⁸ but additional trials are needed to assess the value of individualized interventions to improve cardiorespiratory fitness in women with the appropriate length, frequency, type, and time-volume of exercise.

It is well known that physical activity leads to improved cardiorespiratory fitness through improved endothelial function, improved insulin sensitivity, increased capillary density, higher lactate threshold, reduced resting heart rate and blood pressure, improved lipid profiles, and reductions in visceral adiposity.⁹ The results from the study by Pettee Gabriel et al⁶ may offer a perspective for studies targeting cardiorespiratory fitness, CVD, and overall mortality. The CARDIA study⁶ provides evidence that when an individual starts efforts at cardiorespiratory fitness matters and that maintaining higher levels of cardiorespiratory fitness may have a protective association against and reduce risk of CVD.^6,10 In addition, improved cardiorespiratory fitness may be one of the key attributes explaining what Ortega et al³ called the “fat but fit paradox” in their 2018 study reporting that all-cause and CVD mortality risk were not different between individuals with obesity and those with normal weight if they were both in the top 20% for cardiorespiratory fitness based on age and gender.³ Further studies could focus on understanding the current disparity gaps in cardiovascular fitness and provide insights to develop and implement strategies for prevention and control of CVD among Black women and racial and ethnic groups with higher prevalence of physical inactivity, such as American Indian or Alaska Native, non-Hispanic Black, and Hispanic adults.