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Poor fitness common in teens and adults, with associated rise in cardiovascular disease risk factors

December 21, 2005

Approximately one-third of adolescents and 14 percent of adults (aged 20 to 49 years) in the U.S. have poor cardiorespiratory fitness, with an associated increased prevalence of cardiovascular disease risk factors such as higher total cholesterol and blood pressure levels, according to a study in the December 21 issue of JAMA.

There is strong and consistent evidence from observational studies that physical inactivity and poor cardiorespiratory fitness (i.e., fitness) are associated with higher illness and death from all causes, including cardiovascular disease (CVD) and cancer, according to background information in the article. United States population reports describe an increasingly less physically active society, with marked downturns in reported physical activity during adolescence and young adulthood. Prior to the current National Health and Nutrition Examination Survey (NHANES), data were not available to quantify objectively determined cardiorespiratory fitness in the U.S. population. The extent to which physical inactivity affects the risk of heart disease through its negative impact on cardiorespiratory fitness, which is associated with a high prevalence of other CVD risk factors, is not known at the population level.

Mercedes R. Carnethon, Ph.D., and colleagues from the Feinberg School of Medicine, Northwestern University, Chicago, examined the prevalence of low fitness in the U.S. population of adolescents and adults younger than 50 years and determined the relation between low fitness and CVD risk factors in this population. Using data from NHANES 1999-2002, the researchers analyzed data for adolescents (aged 12-19 years; n = 3,110) and adults (aged 20-49 years; n = 2,205) who were free from previously diagnosed CVD. The participants underwent submaximal graded exercise treadmill testing to achieve at least 75 percent to 90 percent of their age-predicted maximum heart rate. Maximal oxygen consumption (Vo2max) was estimated by measuring the heart rate response to reference levels of submaximal work.

The researchers found that 19.2 percent of the surveyed population-an estimated 16 million U.S. adolescents and adults younger than 50 years-were in the low fitness category, and 33.6 percent of adolescents (approximately 7.5 million) and 13.9 percent of adults (approximately 8.5 million) had low fitness. Among adolescents, the prevalence of low fitness was similar between females (34.4 percent) and males (32.9 percent), but among adults the prevalence of low fitness was significantly higher in females (16.2 percent) compared with males (11.8 percent). Non-Hispanic blacks and Mexican Americans were less fit than non-Hispanic whites. Body mass index and waist circumference demonstrated the most consistent inverse associations with fitness. Total cholesterol levels and systolic blood pressure were higher and levels of high-density lipoprotein cholesterol were lower among participants with low vs. high fitness.

The researchers add that because older adults and individuals with existing risk factors for CVD were not tested on the treadmill because of possible health risks, the results of this study likely underestimate the true prevalence of low fitness in the population.

" this report indicates that low fitness is a prevalent and important public health problem in the U.S. population. The consequences of declines in physical activity over time are now evident by the large proportion of society with low levels of fitness. The correlations we report between low fitness and CVD risk factors suggest a potential trend of increasing morbidity and mortality from chronic diseases-the first sign of which is the burgeoning obesity epidemic. Historical evidence from the campaign to educate about the dangers of cigarette smoking indicates that education efforts, particularly among youth, can retard and reverse these negative health behaviors. Thus, it is plausible that a similar education campaign about the health benefits of physical activity to improve cardiorespiratory fitness, in combination with changes in health care policy to make environments more conducive to physical activity, could begin to reverse this serious public health problem," the authors conclude.

JAMA and Archives Journals