Concurrent Relationship of Objectively Measured Physical Activity and Cardiorespiratory Fitness on Two Different Measures of Obesity in U.S. Adults

Background: Few population-based studies have examined the relationship of both physical activity (PA) and cardiorespiratory fitness (CRF) on the growing health problem of obesity. The purpose of this study was to examine the concurrent relationship of PA and CRF on two different measures of obesity. Methods: This study used data from adults 20-49 years of age participating in the 2003-2004 National Health and Nutrition Examination Survey (NHANES). Moderate-to-vigorous PA (MVPA, min/day) was objectively determined by use of accelerometer and participants were categorized into low or high groups using the median. CRF (ml/kg/min) was assessed using a submaximal treadmill test and measures were categorized into low or high values by applying ageand sex-specific standards. Using body mass index (BMI), participants were categorized as obese if their values were 30 kg/m or greater. Using waist circumference (WC), participants were categorized as obese if their values were greater than 88 (females) or 102 cm (males). Linear regression was used to test for mean CRF differences in study variables. Logistic regression was used to model the relationship between MVPA, CRF and obesity. Results: Participants in the high CRF group had significantly (ps<.05) greater MVPA across all obesity groupings. In BMI obese, neither BMI nor WC were significantly different between CRF groups. However, in WC obese, both BMI and WC were significantly (ps<.05) lower in the high CRF group. Among adults with low CRF, those with low MVPA were more than twice as likely to be obese than those with high MVPA using BMI (OR=2.48; 95% CI: 1.23-5.01) and WC (OR=2.06; 95% CI: 1.29-3.29). Among adults with high CRF, no MVPA and obesity relationship was seen. Conclusion: Results from this study indicate that PA is only related to obesity when CRF is low. Furthermore, high CRF may protect less physically active adults from both overall and abdominal obesity.


Introduction
Physical activity (PA) is a health behavior promoted to all individuals because of its preventive relationship with disease [1,2,3,4,5], its positive relationship with longevity [6,7,8,9,10], and its ability to enhance health-related quality of life (HRQOL) [11,12,13,14]. Cardiorespiratory fitness (CRF) is a trait possessed by individuals, characterized by the ability to engage in dynamic exercise of at least moderate intensity for prolonged periods [15], and is also related to positive health outcomes [16,17,18,19,20]. Obesity is a growing concern world-wide [21] with health promotion efforts directed toward increasing PA and modifying diet [22,23,24]. However, few efforts have been directed toward improving both PA along with CRF in the primary and tertiary prevention of obesity. Furthermore, data examining the obesity relationship with both PA and CRP are sparse. Therefore, the purpose of this study was to examine the concurrent relationship of PA and CRF on two different measures of obesity.

Study Design
Data for this research came from the 2003-04 National Health and Nutrition Examination Survey (NHANES) [25]. The sample consisted of participants who were 20-49 years of age and had complete PA, CRF, and body composition data.

Variables Utilized
The two main independent variables were moderate-tovigorous PA (MVPA) and CRF. The two main dependent variables were obesity status measured by body mass index (BMI) and obesity status measured by waist circumference (WC). Covariates were age, sex, race, and income.

Assessment of MVPA and CRF
MVPA (min/day) was objectively determined by use of accelerometer and participants were categorized into low or high groups using the median. CRF (ml/kg/min) was assessed using a submaximal treadmill test and measures were categorized into low or high values by applying ageand sex-specific standards [26,27].

Assessment of Obesity
Using BMI, participants were categorized as obese if their values were 30 kg/m 2 or greater. Using WC, participants were categorized as obese if their values were greater than 88 (females) or 102 cm (males) [28].

Statistical Analyses
Participant characteristics were described using means, percentages, standard errors (SEs) and 95% confidence intervals (CIs) ( Table 1). Linear regression was used to test for mean CRF differences in study variables ( Table 2). Multiple logistic regression was used to model the relationship between MVPA, CRF and obesity by computing adjusted odds ratios (ORs) and 95% CIs ( Table 3 & Table 4). SAS version 9.4 was used for all analyses [29,30,31]. Note. *indicates percentages (%) were reported. SE is standard error. BMI is body mass index (kg/m 2 ). WC is waist circumference (cm). MVPA is objectively measured moderate-to-vigorous physical activity (average min/day). ST is sedentary time (average min/day). CRF is cardiorespiratory fitness as measured by VO2 max (ml/kg/min).   Note. Obese status in this table was defined as a WC > 102 cm for males or > 88 cm for females. MVPA is objectively measured moderate-to-vigorous physical activity. Model I is unadjusted. Model II is age-adjusted only. Model III is fully adjusted for age, sex, race, and income. OR is odds ratio. CI is confidence interval. Low MVPA represents those with objectively measured MVPA below the median. High MVPA represents those with objectively measured MVPA above the median. Values in bold are significantly different from 1.00, with 95% confidence. CRF is cardiorespiratory fitness. Lower CRF represents those with measured VO2max rated at age-sex-specific low to moderate fitness. Higher CRF represents those with measured VO2max rated at age-sex-specific high fitness.

Results
Participants in the high CRF group had significantly (ps<.05) greater MVPA across all obesity groupings. In BMI obese, neither BMI nor WC were significantly different between CRF groups. However, in WC obese, both BMI and WC were significantly (ps<.05) lower in the high CRF group. Among adults with low CRF, those with low MVPA were more than twice as likely to be obese than those with high MVPA using BMI (OR=2.48; 95% CI: 1.23-5.01) and WC (OR=2.06; 95% CI: 1.29-3.29). Among adults with high CRF, no MVPA and obesity relationship was seen.

Discussion
The purpose of this study was to examine the relationship between concurrent PA and CRF on two different measures of obesity in middle-aged adults. Results showed, as expected, that adults with higher CRF also had greater amounts of MVPA. Additionally, this same trend was seen in both obese and non-obese adults using both BMI and WC. However, a not expected finding, was that non-BMI-obese adults with higher CRF had significantly smaller mean BMI and significantly smaller mean WC than their lower CRF counterparts. This same trend was also seen in the non-WC-obese adults. These finding suggest that CRF may be related to body weight among adults who are not classified as obese. Although not analyzed in this study, it is possible that CRF may differentiate between normal (BMI: < 25) and overweight (BMI: 25 to 29.99) adults.
Another interesting finding in this study was that among adults with lower CRF, those with low MVPA were approximately twice as likely to be obese (by both BMI and WC), as compared to their high MVPA counterparts. Whereas, among adults with higher CRF, no MVPA and obesity relationship was seen. These finding suggest that CRF may protect adults from both types of obesity, regardless of MVPA behavior. Finally, these findings should be considered in light of the limitations associated with NHANES data [32].

Conclusions
Results from this study indicate that PA is only related to obesity when CRF is low. Furthermore, high CRF may protect less physically active adults from both overall and abdominal obesity. Health promotion programs should advocate for the improvement of CRF as well as increased PA in combating obesity.