difficulty in measuring changes in caloric consumption or physical activity over the past decades, it is clear that physical inactivity is a major culprit in the obesity epidemic. They go on to suggest that perhaps physical inactivity is the major culprit.
This concept is supported by studies from the 50s in carefully performed studies in animals86 and humans.87 Meyers et al. observed that at significant physical activity levels, increases or decreases in physical activity were matched with increases or decreases in food intake. However, below certain minimal levels of physical activity, further decreases in physical activity were not met by further decreases in food intake, but rather by increases in food intake and consequent body weight. They interpreted the data to suggest that a minimal level of physical activity might be necessary for appropriate appetite control. Recent data from our group provides support for this theory. In our study,74 inactive controls gained weight over a six-month period, whereas two different low-dose exercise-training groups (equivalent to ~ 12 miles/wk of walking or jogging) lost weight and a higher dose (equivalent to ~ 17 mile/wk) lost even more body mass. The data suggest that below a certain level, appetite control is not appropriately balanced, and weight gain occurs. If these suggestions are correct, then the extreme levels of physical inactivity present in today's society may be at the root of the obesity-and-diabetes epidemic in that these levels of inactivity may directly lead to the inability to balance food intake with decreasing physical-activity levels, resulting in continuous weight gain and progression from metabolic syndrome to diabetes.
As reviewed previously, physical-activity levels are poorly measured, whereas cardiorespiratory fitness is a relatively easily measured, reliable, and accurate clinical assessment that is a good surrogate for physical activity. Studies show that both moderate and vigorous-intensity activity can lead to increases in cardiorespiratory fitness.38,88
Finally, in a particularly compelling article entitled "Exercise capacity: The prognostic variable that doesn't get enough respect," Mark and Lauer discuss what they refer to as "one of the most potent prognostic variables,"89 i.e., exercise capac-ity/cardiorespiratory fitness. The article is precipitated by the Gulati et al.83 study (see discussion above) reported in the same issue, along with numerous studies that have come before this. Mark and Lauer argue that the overwhelming amount of data support the role of exercise capacity as a potent prognostic indicator of future health for both men and women and for both symptomatic and asymptomatic individuals.
We believe that the accumulated evidence provides strong rationale for including cardiorespiratory fitness as one of the major defining diagnostic components of the metabolic syndrome. Furthermore, by including cardiorespiratory fitness in the definition of metabolic syndrome, individuals and physicians would more likely focus directly on physical activity and fitness as measures of health. This would also demand attention to methods for increasing physical activity and cardiorespiratory fitness as an effective, therapeutic intervention for metabolic syndrome and the prevention of progression to type 2 diabetes. A direct emphasis on physical activity and cardiorespiratory fitness certainly will have significant consequences on the prevalence of metabolic syndrome, obesity, progression to type 2 diabetes, and cardiovascular disease in the U.S. population.
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