Increased mortality associated with low cardiorespiratory fitness has shown to take effect during late adulthood in previous generations. A recent rise in early death was observed in the United States. We investigated the impact of low cardiorespiratory fitness during young and middle adulthood on premature death in healthy adults from recent generations.

A prospective cohort study of a nationally representative sample of US Baby Boomers and Generation Xers (born 1945-1980). Between 1999 and 2004, 3242 adults ages 20 to 49 years (weighted N = 59,888,450; mean age, 33.8 ± 0.2 years) underwent submaximal treadmill exercise test in the National Health and Nutrition Examination Survey study. Weighted Cox proportional hazards regression were used to evaluate the association of cardiorespiratory fitness with premature death at 65 years or younger.

During a mean follow-up of 13.8 years, 104 deaths (weighted deaths N =1,326,808) occurred. Low cardiorespiratory fitness was associated with an increased risk of premature death as a result of all-cause (hazard ratio [HR], low vs high: 2.26; 95% confidence interval [CI], 1.10 to 4.64, P for trend = 0.036) and cancer mortality (HR low vs moderate/high: 6.53; 95% CI, 2.38 to 17.9). Further, this association was stronger in adults ages 35 to 49 years at baseline (HR, 4.17 [95% CI, 1.19 to 9.11]).

We observed an inverse association between cardiorespiratory fitness during middle adulthood and premature death, which was not detected in preceding generations. These findings suggested that low cardiorespiratory fitness might be emerging as a new risk factor for early death among US Baby Boomers and Generation Xers.