Cardiorespiratory fitness reflects integration of several body systems, including the cardiovascular, respiratory and musculoskeletal systems. It helps optimize heart health and is an important predictor of adverse health outcomes, including premature death, heart attack and heart disease. Indeed, the American Heart Association suggests that cardiorespiratory fitness should be measured as a vital sign during a doctor’s visit. Yet, cardiorespiratory fitness declines with age, which is thought to result from changes to heart structure and function that are further influenced by factors we can (e.g., physical activity) and cannot (e.g., heredity) change.
Interestingly, much of what we know about changes in cardiorespiratory fitness with increasing age comes from cross-sectional studies. While cross-sectional studies provide important preliminary evidence to support further research, they cannot tell us how cardiorespiratory fitness changes in the same person over time, or if the rate of decline increases as we get older. While a handful of longitudinal studies have been conducted, few young adults and people of color were included.
Understanding changes in cardiorespiratory fitness from early adulthood to midlife may be particularly important. Young adults experience several important life events, such as starting a career and/or family that might temporarily or permanently change behaviors that influence cardiorespiratory fitness. Young adults may discount the long-term benefits of regular physical activity to optimize cardiorespiratory fitness in exchange for immediate rewards, like extra time in the day. Also, there is a lack of cardiorespiratory fitness data in population-based samples that include early adults. This may be due to prioritizing study questions that specifically examine the importance of cardiorespiratory fitness to reduce risk of adverse events, which typically occur later in life.
Our study, published in the July 2022 issue of Medicine & Science in Sports & Exercise®, sought to address this important research gap using data from the Coronary Artery Risk Development in Young Adults (CARDIA) study. In addition to describing age-related changes in cardiorespiratory fitness from early adulthood to midlife, we also examined if there were differences in these changes by factors present in early adulthood. The CARDIA cohort, which began in 1985, includes 5,114 Black and white men and women aged 18-30 years recruited at baseline from four clinical centers in the U.S. The cohort has been re-examined every 2-5 years and a symptom-limited maximal graded exercise test protocol was included at baseline as well as the year 7 and year 20 follow-up exams. Data collection for the year 35 follow-up exam is ongoing and includes a fourth symptom-limited maximal graded exercise test.
The most important takeaway from this study was that declines in cardiorespiratory fitness occurred within the first few years of early adulthood. The rate of decline through midlife was more accelerated than previously reported in the Baltimore Longitudinal Study of Aging, ranging from 4.6% from ages 20-25 to about 10% every 5 years from ages 30 to 50. Also, these declines varied based on sex, race and other factors present in early adulthood. For example, physically active participants and those who consumed alcohol but never smoked had higher cardiorespiratory fitness in early adulthood and less of a decline in fitness through midlife. In contrast, participants who experienced overweight/obesity or reported fair or poor reported health status had lower cardiorespiratory fitness during early adulthood which persisted through midlife.
Together, these findings suggest that early adulthood is a critical life-course stage to intervene in unhealthy behaviors to optimize cardiorespiratory fitness. We simply cannot put off maintaining a healthy lifestyle for later in life. With routine follow-up of adverse events, CARDIA is well-positioned to add to what we know regarding the importance of optimizing cardiorespiratory fitness across adulthood.
Kelley Pettee Gabriel, PhD, MS, FACSM, FAHA, is a professor of epidemiology at the University of Birmingham at Alabama with undergraduate and graduate training in exercise science, clinical exercise physiology and epidemiology. Dr. Gabriel actively collaborates with several large observational studies, including CARDIA, to examine the timing of physical activity and cardiorespiratory fitness on subsequent risk of disability and disease using a life-course framework. She currently serves as a principal investigator on four National Institutes of Health-funded studies.
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