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Adiposity, not cardiorespiratory fitness, associated with metabolic health
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Adiposity was strongly associated with components of metabolic syndrome, including insulin resistance, visceral fat volume and liver fat percentage, whereas cardiorespiratory fitness and free fat mass index were only weakly or nonsignificantly associated with the same parameters, according to an analysis of two large twin samples.
“There has been extensive discussion on whether adiposity or [cardiorespiratory fitness] matters more for metabolic health,” Sakari Jukarainen, a PhD student in the obesity research unit at the University of Helsinki, and colleagues wrote. “Our study clearly shows that while adiposity is strongly, negatively associated with metabolic health, both [free fat mass index] and VO2max/[fat free mass] have much smaller (or nonsignificant) effects.”
Jukarainen and colleagues analyzed data from two cross-sectional samples of healthy adult monozygotic and dizygotic twins: the GEMINAKAR twin study, recruited through the population-based Danish Twin Registry (n = 996), and TwinFat, based on two Finnish, population-based studies of five consecutive birth cohorts of twins (n = 309).
Researchers assessed fat mass and free fat mass by bioelectrical impedance analysis and DXA. All participants completed a maximal exercise test using an electronically braked bicycle ergometer. Researchers defined cardiorespiratory fitness as VO2max divided by free fat mass (vs. dividing by body weight) to avoid correlation with adiposity and subsequently underestimating cardiorespiratory fitness in obesity. Participants provided fasting blood samples to measure total cholesterol, HDL cholesterol and triglycerides, and underwent a 75-g oral glucose tolerance test to assess insulin sensitivity. Visceral fat volume and liver fat percentage were measured in 83 individual twins in TwinFat by MRI and spectroscopy, respectively.
Researchers found that fat mass index was associated with all metabolic outcome variables. The strongest associations were observed with homeostatic model of insulin resistance (beta = 0.67; P < .001), metabolic syndrome score (beta = 0.69; P < .001), waist-to-height ration (beta = 0.85; P < .001), visceral fat volume (beta = 0.75; P < .001) and liver fat percentage (beta = 0.53; P < .001). Free fat mass index, however, was associated only with waist-to-height ratio (beta = 0.29; P < .001); any associations between VO2max/free fat mass and metabolic parameters were weak or nonsignificant. Results persisted in monozygotic intrapair difference analyses, according to researchers.
“It is not surprising that VO2max is not necessarily correlated with metabolic health independently of adiposity, since differences in maximal cardiac output during exercise result mainly from central cardiovascular factors that are not directly connected to peripheral tissues relevant to metabolism (skeletal muscle, adipose tissue, the liver),” the researchers wrote. “This might explain our results and the results of previous studies’ results finding weak or no associations between adequately scaled [cardiorespiratory fitness] and metabolic health. However, these results do not undermine the message from studies showing beneficial effects of physical activity on metabolic health.” – by Regina Schaffer
Disclosure: The researchers report no relevant financial disclosures.
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