Adaptation to weight-loss in the obese

Despite the increasing prevalence of obesity and the array of treatment modalities available, maintenance of reduced-weight in the long-term is uncommon. The reason(s) why weightregain is so commonplace remains contentious. The primary purpose of this dissertation was to determine the extent to which weight-loss in obese adults, when the intervention included an optimal exercise prescription, resulted in adaptations that may be considered aberrant and reflect risk of weight-regain. Unique to this investigation was the multifactorial approach to addressing the issue of adaptation through consideration of metabolic, physiological, and psychosocial factors. Accurate body composition assessment is a fundamental requirement upon which evaluation of adaptation to weight-loss is based. An additional purpose of the dissertation was to evaluate multi-frequency bioelectrical impedance analysis (MFBIA) as a technique for use in assessing body composition change of obese adults during weight-loss. Further, optimal weight management requires an incorporation of effective exercise prescription. Therefore, a final aim of the dissertation was to determine the optimal exercise intensity for weight management. Together the goals of this thesis were addressed through a series of studies, the first three relate to use of bioelectrical impedance analysis; the fourth and fifth studies were associated with exercise prescription for obese adults; and the final study, incorporating three components, dealt with adaptation to weight-loss in the obese.\ud \ud In the first study, twenty-nine young healthy adults, 17 males and 12 females (mean± SEM; 21.8 ± 0. 7 yrs and 20.3 ± 0.3 yrs) were recruited to assess the technical reliability (i.e., interinstrument and inter-operator reliability) of three MFBIA monitors. Technical reliability was assessed for both errors of measurement and associated analyses. In addition, intra-operator and intra-instrument variability was evaluated for repeat measures over a 4-hour period. The measured impedance values from a range of resistance-capacitance circuits were accurate to within 3 percent of theoretical values over a range of 50- 800 ohms. Similarly, phase was measured over the range 1 degrees-19 degrees with a maximum deviation of 1.3 degrees from the theoretical value. The extrapolated impedance at zero frequency was equally well determined (+/-3 percent). However, the accuracy of the extrapolated value at infinite frequency was decreased, particularly at impedances below 50 ohms (approaching the lower limit of the measurement range of the instrument). The inter-instrument/operator variation for whole body measurements were recorded on human volunteers with biases of less than +/-1 percent for measured impedance values and less than 3 percent for phase. The variation in the extrapolated values of impedance at zero and infinite frequencies included variations due to operator choice of the analysis parameters but was still less than +/-0.5 percent.\ud \ud The second study investigated the effect of moving from standing to lying supine on blood and plasma volumes, serum electrolyte concentrations, and measurements of whole-body impedance using MFBIA over a 60 minute period in 12 subjects (8 males, 4 females) 26.7 ± 5.5 yrs, 172.3 ± 6.9 em, and 71.0 ± 7.7 kg. While no significant differences were found in serum sodium, chloride or potassium concentrations as a function of the posture change, estimates of both blood and plasma volumes increased during the 60 minutes spent supine. Blood volume increased by 10.5 (4.2 to 16.4)percent and plasma volume increased 10.5 (5.0 to 17.6)percent. Similarly, whole-body resistivity measured at 50 kHz increased significantly by 24.1 (6.3 to 36.5) Q in absolute values across the hour supine, with the measurement at 60 minutes reflecting a 4. 7 ( 1.3 to 6. 7)percent increase in resistivity from the baseline measure. Although the weight lost across the hour was only 0.11 (0.05 to 0.15) kg, the change in total body water estimated from resistivity measures is likely to be significantly larger. The results from this study reflect the need for implementation of a standardised preparatory rest time prior to taking BIA measurements, and demonstrate the limitations associated with a single frequency, whole-body approach for assessing body-fluid distribution.\ud \ud The purpose of the third study was to validate the use of MFBIA to determine change in body composition of obese adults during weight reduction by direct comparison with total body water as measured by stable-isotope dilution. Specifically, the study had three parts. Firstly, from the data obtained on the total sample at baseline, the aim was to develop a prediction equation for the estimation ofTBW from MFBIA. The second aim was to validate the prediction equation in a subsample during (wk-5) and after the completion of a 12-wk weight-loss intervention. The final aim of the study was to determine whether intervention type influenced the accuracy of TBW predicted by MFBIA. Multiple regression analysis revealed that the MFBIA-derived prediction equation prior to weight-loss was able to determine TBW with a SEE of 1.9L by combining the impedance index (Ht2/R10o), age, gender, and body weight. However, during weight-loss, the comparison of TBW predicted by MFBIA and measured with deuterium dilution had a much larger bias than after completion of the weight-loss intervention. It was further revealed that the magnitude of the bias was influenced by the intervention mode, with those individuals on a very-low-energy diet recording greater measurement bias. These results raise concern for the accuracy of MFBIA during periods of weight-loss in obese adults. In particular, weight-loss elicited through severe restriction in dietary intake appears most problematic. In contrast, the post-intervention TBW measures determined from the MFBIAequation derived at baseline had a mean difference of only 0.9L relative to the group mean derived by deuterium dilution. Further, using the impedance index alone post-intervention, the mean TBW measures were within 0.5L of the mean reference measure.\ud \ud In the fourth study the relative and absolute levels of exercise intensity corresponding with the lactate threshold (LT) and ventilatory threshold (Tvent) were determined in forty-two sedentary obese adults, 23 females and 19 males (42.9±1.8 yrs; 36.8±0.9 kg.m-2 ). Subjects were obese but otherwise healthy and taking no medication known to modifY heart rate. Subjects were required to attend a testing session each week for three weeks scheduled for the same time of day, and day of the week. The three sessions involved: (1) a treadmill test to assist in subject familiarisation. The test enabled researchers to gauge working capacity and enable subjects to become accustomed with the treadmill and gas analysis apparatus; (2) a discontinuous graded treadmill test to assess cardiorespiratory function; and (3) assessment of body composition and resting metabolic rate. On two separate occasions subjects undertook a discontinuous graded treadmill walking test consisting of 4-minute work stages separated by 2-minute rest periods. During the last 30 seconds of each work stage, subjects rated perceived exertion (RPE) using the Borg 6-20 scale. Immediately on completion of each work stage, duplicate blood samples were collected by finger prick, haemolysed in 1 OO)ll of chilled perchloric acid, and stored for later blood lactate analysis. Throughout both tests HR was recorded and respiratory gases were collected and analysed. Resting metabolic rate was assessed by indirect calorimetry using a ventilated hood system. The major finding of this study was that corresponding with LT and Tvent respectively, the relative intensities defined as percentV02R (63.4±1.4 percent; 63.8±1.8 percent), percent HRR (65.2±1.6 percent; 66.6±1.5 percent) and percent HRpeak (78.2±1.0 percent; 79.2±1.0 percent), and the absolute intensity defined by METs calculated physiologically would be categorised as "hard" by the current guidelines. In contrast, at L T and T vent the relative intensity defined by RPE (12.6±0 .2; 12.8±0.3), and absolute intensity defined by METs calculated from mechanical parameters (4.8±0.2; 5.0±0.2), fall into the "moderate" intensity category. Therefore, there is a need to redefine the descriptors of exercise intensity for the obese population. Further, linear regression analysis revealed a relatively strong negative relationship (r=-0.58, P<0.001) between HRLT and age , and the addition of gender to the regression analysis improved the strength of the prediction equation (HRLr = 145.6- 0.623* Age(yrs) + 9.824*Gender (M=l, F=2) r=-0.70, P