Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Exercise Science

First Advisor

Steven N Blair

Abstract

At the most basic level obesity is the result of a chronic imbalance between energy intake and energy expenditure. However, the exact etiology is considerably more complex and may involve a variety of physiological and behavioral factors. Metabolic disturbances, including reduced fat oxidation as measured by the respiratory quotient (RQ) and reduced resting metabolic rate (RMR), have been identified as possible predictors of changes in body weight and body composition. RMR represents the largest component (60-80%) of caloric expenditure that contributes to total daily energy expenditure in humans and has high inter-person variability (±25%) but not within individuals (<±5%). The cause of this variability between individuals and the exact role of RMR and RQ in determining body weight and body composition are uncertain.

This dissertation consists of three studies that were designed to 1) Identify correlates of RMR among behavioral and physiological variables in a cohort of young adult men and women; 2) Examine racial differences in RMR, body weight, and body composition among young adult women; and 3) Explore the longitudinal effects of RMR, RQ, physical activity, and dietary intake on subsequent changes in body weight and body composition in young adults followed for nine months.

Three manuscripts were composed by analyzing data collected from the Energy Balance Study, an observational research study involving young adults (N=430). We measured RQ and RMR using indirect calorimetry, along with body weight and body composition using dual energy X-ray absorptiometery, energy expenditure and time spent in physical activity using an arm-based activity monitor, and energy intake using interviewer-administered dietary recalls.

The results of study 1 found fit individuals had a higher RMR compared to unfit individuals after controlling for differences in body composition between the groups. However, the decrease in RMR from low levels fitness compared to moderate or high levels of fitness was modest and represented approximately 3% of RMR or 47 kcal/day. Time spent in moderate to vigorous physical activity was also significantly related to RMR, but this influence was also small and had little predictive value over adjustments for body composition.

Study 2 confirmed previous research study finding young adult African-American women have a lower RMR compared to their white peers after statistical adjustments for differences in body composition (1400.3±9.1 kcal/day vs. 1299.8±18.9 kcal/day, P<0.0001). African-American women had higher levels of fat mass compared to white women which resulted in elevated RMR beyond the differences in fat free mass prior to statistical adjustment. Additionally, cardiorespiratory fitness was significantly positively associated with RMR, but time spent in moderate to very vigorous physical activity was not.

Finally, individuals with a high RQ gained significantly more body weight (1.55±0.23 vs. 0.83±0.18 kg, P=0.0040) and fat mass (1.19±0.23 vs. 0.60±0.18 kg, P=0.0150) over a 9 month period compared to those with a low or moderate RQ, independent of changes in energy intake, energy expenditure, macronutrient composition of the diet, and physical activity. Additionally, a low RMR was not associated with gains in body weight or fat mass over the same period.

Rights

© 2013, Robin Shook

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