Associations Between Resting Metabolic Rate and Energy Storage in Young Adults

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 (\u3c±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\u3c0.0001). 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

Effect of chronic strength training on the baroreflex response during acute exercise

Chronic strength training at high intensities produces large increases in arterial pressure and heart rate, similar to the increased loading on baroreceptors that result in reductions of the baroreflex experienced in endurance subjects. To determine the effect of high intensity weight training on the baroreflex response, incremental intensities of neck pressure and suction were applied to 10 untrained and 10 strength-trained individuals during 1) rest, 2) isometric biceps contraction at 30% of maximal voluntary contraction (MVC), and 3) isometric quadriceps contraction at 30% MVC. Heart rate (HR) saturation, threshold, and operating ranges increased similarly during isometric biceps and quadriceps contraction, indicating baroreceptor resetting under each condition. The maximum slope of the stimulus-response curves plotting HR versus carotid distending pressure of the chronically strength trained subjects did not differ from the untrained subjects. There was no difference between groups in terms of HR saturation, threshold, or operating point. The data suggest there is not a difference in central command-mediated baroreceptor resetting between chronically strength-trained and untrained individuals performing isometric exercise at the same intensities

Subjective Estimation of Physical Activity Using the IPAQ Varies by Fitness Level

Background: Subjective measures of moderate and vigorous physical activity (MVPA) rely on ‘relative’ intensity while objective measures capture ‘absolute’ intensity, thus fit individuals may perceive the same activity differently than unfit individuals. Methods: Adults (N=211) wore the SenseWear Armband (SWA) for ten consecutive days to objectively assess sedentary time and MVPA. On day eight participants completed the International Physical Activity Questionnaire (IPAQ) to subjectively assess sitting time and MVPA. Fitness was assessed via a maximal treadmill test, and participants were classified as ’unfit’ if the result was in the bottom tertile of the study population by sex or ‘fit’ if in the upper two tertiles. Results: Overall, estimates of MVPA between the IPAQ and SWA were not significantly different (IPAQ minus SWA, 67.4±919.1 MVPA minutes/week, P=.29). However, unfit participants overestimated MVPA using the IPAQ by 37.3% (P=.02), but fit participants did not (P=.99). This between-group difference was due to overestimation using the IPAQ of moderate activity by 93.8 minutes/week among the unfit individuals, but underestimation of moderate activity among the fit participants by 149.4 minutes/week. Conclusion: Subjective measures of MVPA using the IPAQ varied by fitness category, with unfit participants overestimating their MVPA and fit participants accurately estimating their MVPA

The Association Between Sedentary Behaviors During Weekdays and Weekend with Change in Body Composition in Young Adults

Background: High sedentary time has been considered an important chronic disease risk factor but there is only limited information on the association of specific sedentary behaviors on weekdays and weekend-days with body composition. The present study examines the prospective association of total sedentary time and specific sedentary behaviors during weekdays and the weekend with body composition in young adults. Methods: A total of 332 adults (50% male; 27.7 ±3.7 years) were followed over a period of 1 year. Time spent sedentary, excluding sleep (SED), and in physical activity (PA) during weekdays and weekend-days was objectively assessed every 3 months with a multisensor device over a period of at least 8 days. In addition, participants reported sitting time, TV time and non-work related time spent at the computer separately for weekdays and the weekend. Fat mass and fat free mass were assessed via dual x-ray absorptiometry and used to calculate percent body fat (%BF). Energy intake was estimated based on TDEE and change in body composition. Results: Cross-sectional analyses showed a significant correlation between SED and body composition (0.18 ≤ r ≤ 0.34). Associations between body weight and specific sedentary behaviors were less pronounced and significant during weekdays only (r ≤ 0.16). Nevertheless, decrease in SED during weekends, rather than during weekdays, was significantly associated with subsequent decrease in %BF (β = 0.06, p \u3c0.01). After adjusting for PA and energy intake, results for SED were no longer significant. Only the association between change in sitting time during weekends and subsequent %BF was independent from change in PA or energy intake (β%BF = 0.04, p = 0.01), while there was no significant association between TV or computer time and subsequent body composition. Conclusions: The stronger AIMS Public Health Volume 3, Issue 2, 375-388. prospective association between sedentary behavior during weekends with subsequent body composition emphasizes the importance of leisure time behavior in weight management

The Association Between Sedentary Behaviors During Weekdays and Weekend with Change in Body Composition in Young Adults

Background: High sedentary time has been considered an important chronic disease risk factor but there is only limited information on the association of specific sedentary behaviors on weekdays and weekend-days with body composition. The present study examines the prospective association of total sedentary time and specific sedentary behaviors during weekdays and the weekend with body composition in young adults. Methods: A total of 332 adults (50% male; 27.7 ±3.7 years) were followed over a period of 1 year. Time spent sedentary, excluding sleep (SED), and in physical activity (PA) during weekdays and weekend-days was objectively assessed every 3 months with a multisensor device over a period of at least 8 days. In addition, participants reported sitting time, TV time and non-work related time spent at the computer separately for weekdays and the weekend. Fat mass and fat free mass were assessed via dual x-ray absorptiometry and used to calculate percent body fat (%BF). Energy intake was estimated based on TDEE and change in body composition. Results: Cross-sectional analyses showed a significant correlation between SED and body composition (0.18 ≤ r ≤ 0.34). Associations between body weight and specific sedentary behaviors were less pronounced and significant during weekdays only (r ≤ 0.16). Nevertheless, decrease in SED during weekends, rather than during weekdays, was significantly associated with subsequent decrease in %BF (β = 0.06, p \u3c0.01). After adjusting for PA and energy intake, results for SED were no longer significant. Only the association between change in sitting time during weekends and subsequent %BF was independent from change in PA or energy intake (β%BF = 0.04, p = 0.01), while there was no significant association between TV or computer time and subsequent body composition. Conclusions: The stronger AIMS Public Health Volume 3, Issue 2, 375-388. prospective association between sedentary behavior during weekends with subsequent body composition emphasizes the importance of leisure time behavior in weight management

The Association of Physical Activity during Weekdays and Weekend with Body Composition in Young Adults

Physical activity (PA) is a key contributor in long-term weight management but there remains limited research on the association between weekly PA patterns and weight change. The purpose of the present study was to examine the prospective association between weekly PA patterns and weight change in generally healthy young adults. Anthropometric measurements, including dual X-ray absorptiometry, were obtained every 3 months over a period of one year in 338 adults (53% male). At each measurement time, participants wore a multisensor device for a minimum of 10 days to determine total daily energy expenditure and time spent sleeping, sedentary, in light PA (LPA), in moderate PA (MPA), and in vigorous PA (VPA). PA did not differ between weekdays and the weekend at baseline. Twenty-four-hour sleep time, however, was significantly longer during weekends compared to weekdays, which was associated with less time spent sedentary. Weight loss was associated with a significant increase in LPA at the expense of sedentary time during the weekend but not during weekdays. Regression analyses further revealed an inverse association between change in VPA during the weekend and body composition at 12-month follow-up. Taken together, these results suggest that weekend PA plays an important role in long-term weight management

The Association of Physical Activity during Weekdays and Weekend with Body Composition in Young Adults

Physical activity (PA) is a key contributor in long-term weight management but there remains limited research on the association between weekly PA patterns and weight change. The purpose of the present study was to examine the prospective association between weekly PA patterns and weight change in generally healthy young adults. Anthropometric measurements, including dual X-ray absorptiometry, were obtained every 3 months over a period of one year in 338 adults (53% male). At each measurement time, participants wore a multisensor device for a minimum of 10 days to determine total daily energy expenditure and time spent sleeping, sedentary, in light PA (LPA), in moderate PA (MPA), and in vigorous PA (VPA). PA did not differ between weekdays and the weekend at baseline. Twenty-four-hour sleep time, however, was significantly longer during weekends compared to weekdays, which was associated with less time spent sedentary. Weight loss was associated with a significant increase in LPA at the expense of sedentary time during the weekend but not during weekdays. Regression analyses further revealed an inverse association between change in VPA during the weekend and body composition at 12-month follow-up. Taken together, these results suggest that weekend PA plays an important role in long-term weight management

Low fitness partially explains resting metabolic rate differences between African American and white women

Background High levels of obesity among African American women have been hypothesized to be partially resultant from a lower resting metabolic rate compared with white women. The aim of the current study was to determine if differences in cardiorespiratory fitness and moderate-to-vigorous physical activity are associated with differences in resting metabolic rate among free-living young adult African American women and white women. Methods Participants were 179 women (white women n = 141, African American women n = 38, mean age = 27.7 years). Resting metabolic rate was measured using indirect calorimetry, body composition using dual energy x-ray absorptiometry, cardiorespiratory fitness via maximal treadmill test, and moderate-to-vigorous physical activity using an activity monitor. Results African American women had higher body mass index, fat mass, and fat-free mass compared with white women but lower levels of cardiorespiratory fitness. No differences were observed between African American and white women in resting metabolic rate when expressed as kcal/day (1390.8 ± 197.5 vs 1375.7 ± 173.6 kcal/day, P =.64), but African American women had a lower resting metabolic rate when expressed relative to body weight (2.56 ± 0.30 vs 2.95 ± 0.33 mL/kg/min,

Cardiorespiratory Fitness Reduces the Risk of Incident Hypertension Associated With a Parental History of Hypertension

Family history of hypertension increases the risk of an individual to develop hypertension, whereas moderate-to-high cardiorespiratory fitness has the opposite effect. However, the joint association of each on the development of hypertension is not well understood. We studied fitness and incident hypertension in 6278 participants who were given a preventative medical examination. Thirty-three percent reported a parent with hypertension, and there were 1545 cases of incident hypertension after a mean of 4.7 years. The presence of parental hypertension was associated with a 28% higher risk of developing hypertension after adjustments for age, sex, and examination year. After further adjustments for smoking, alcohol intake, resting systolic and diastolic blood pressures, hypercholestrolemia, body mass index, physical inactivity, and fitness, there was a 20% higher risk associated with parental hypertension. After adjusting for age, sex, and examination year, both moderate and high levels of fitness were associated with lower risk for developing hypertension by 26% and 42%, respectively. In the joint analysis, individuals with both a low level of fitness and a parent with hypertension exhibited a 70% higher risk for developing hypertension compared with high fit individuals with no parental history (P=0.004). However, individuals with a high level of fitness and a parent with hypertension only experienced a 16% higher risk of developing hypertension compared with fit individuals with no parental history (P=0.03). The significantly lower risk of developing hypertension when progressing from low- to high-fit groups among those with a parental history of hypertension has important clinical implications