Abstract

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 age- and sex-specific standards. Using body mass index (BMI), participants were categorized as obese if their values were 30 kg/m² 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.

1. 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.

2. Methods

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.

The two main independent variables were moderate-to-vigorous 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.

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 age- and sex-specific standards ^{26, 27}.

Using BMI, participants were categorized as obese if their values were 30 kg/m² or greater. Using WC, participants were categorized as obese if their values were greater than 88 (females) or 102 cm (males) ²⁸.

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}.

3. 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.

4. 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.

5. 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.

Acknowledgements

No financial assistance was used to assist with this project.

References