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Being overweight or unfit in adolescence was linked with physical disability in adulthood, but better fitness mitigated that risk regardless of body weight, a national cohort study of Swedish men reported.

Among over 1 million men followed for nearly 3 decades, those in the lowest decile of cardiorespiratory fitness as adolescents were nearly four times more likely than those in the highest to receive disability pensions as adults (HR 3.74, 95% CI 3.55-3.95), after adjusting for age and socioeconomic status, said researchers led by Pontus Henriksson, PhD, of the Karolinska Institute in Huddinge, Sweden.

And compared with men of normal weight, those who as adolescents were severely obese -- defined as a body-mass index (BMI) of 40 or higher -- were more than three times as likely to receive disability pensions (HR 3.21, 95% CI 2.49-4.15), they wrote in Annals of Internal Medicine.

However, better cardiorespiratory fitness attenuated the risk for disability regardless of BMI category. For example, in adolescents classed as obese (BMI 30 or higher), those who were nonetheless "highly fit" had significantly lower risk (HR 2.27, 95% CI 1.94-2.66) compared with those who were unfit (HR 4.67, 95% CI 4.21-5.17), the study found.

"Although obesity in youth and adulthood is recognized to be associated with impaired health, evidence suggests that the negative effects of obesity can be attenuated by being fit (the 'fat but fit' paradox)," the researchers wrote. "This study also showed that being moderately or highly fit attenuated the risk for receipt of a disability pension in all BMI categories. Therefore, our results indicate that cardiorespiratory fitness is an important marker for later health, regardless of BMI."

In an accompanying video about the study, Christina Wee, MD, deputy editor of Annals, said: "My take-home message from this study is that our health habits in youth can affect the rest of our lives, and low fitness, in particular, is a major risk factor for becoming disabled later in life. What physicians should know is that, while helping patients lose and control weight is important, what may be just as important is helping them become physically fit."

The study included 1,079,128 Swedish males ages 16 to 19 who were conscripted into the military from 1972 to 1994. Their height, weight, and cardiorespiratory fitness were measured at baseline. Fitness was measured with an electrocardiogram and ergometer cycle test. Disability pension data were obtained from the Swedish Social Insurance Agency. Over a median follow-up of 28.3 years, 54,304 men were granted disability pensions.

The researchers had data on the reason for the pensions, and found that the least fit adolescents had significantly increased risk for disability due to specific causes, including psychiatric (HR 4.01, 95% CI 3.72-4.32), musculoskeletal (HR 3.72, 95% CI 3.29-4.20), injuries (HR 2.74, 95% CI 2.35-3.18), nervous system (HR 2.86, 95% CI 2.35-3.48), circulatory (HR 4.87, 95% CI 3.58-6.61), and tumors (HR 1.89, 95% CI 1.31-2.73).

"Although the mechanisms by which cardiorespiratory fitness may influence health are not fully understood, higher cardiorespiratory fitness has been linked to more favorable cardiometabolic risk profiles (lower blood pressure, insulin resistance, and healthier lipid profile); lower concentrations of inflammatory markers; and better cognitive function, which may influence both physical and mental health," the researchers said.

One limitation of the study was that data on smoking and alcohol consumption were not available for all men, the authors said. However, they conducted a subanalysis in 34,966 men for whom such data was available. "Low cardiorespiratory fitness and obesity remained strong risk factors for receipt of a disability pension due to all causes, even after adjustment for smoking and alcohol consumption at conscription," they said. For men in the lowest fitness category compared with the highest, the adjusted risk was 4.70 (95% CI 2.22-9.93). For severely obese men compared with those of normal weight, the adjusted risk was 2.75 (95% CI 1.37-5.50).

Another limitation was that the study measured BMI and physical fitness only at baseline and could not analyze changes in these factors over time. In addition, the study included only men, so the results might not be generalizable to women, Henriksson and colleagues said.