BACKGROUND: The prevalence and severity of obesity across all age ranges is a continuing problem not only in the United States, but worldwide. While a simple explanation for obesity is a greater number of calories consumed versus the number of calories expended, there are numerous intrinsic physiological mechanisms in place that are responsible for the maintenance of body weight. Previous literature has determined that an individual may be more susceptible to body weight fluctuations based on their metabolic phenotype classification, denoted as thrifty and spendthrift. Individuals who are thrifty largely restrict energy expenditure (EE) during times of famine or fasting, and have smaller increases in EE when overfed, while individuals who are spendthrift are less metabolically efficient and exhibit smaller decreases in EE during fasting and greater EE when overfed. Consequently, thrifty phenotypes are more susceptible to obesity, and the determination of these phenotypes may have future implications for exercise prescription. To date, these phenotypes have been studied through precise, multiple week studies in which a participant lives in a metabolic chamber, which uses indirect calorimetry to measure EE. Building on previous nutritional and metabolic studies used to elucidate these phenotypes, the purpose of this novel and prospective study is to classify thrifty and spendthrift phenotypes through two acute 24-hour dietary conditions, utilizing indirect calorimetry to assess changes in resting energy expenditure (REE). METHODS: I plan to recruit 10 subjects (5 overweight/obese) between the ages of 18-45 for this pilot study. Each participant will complete a baseline REE, a 24h-fast, and 2x daily caloric needs of low-protein overfeeding (LPO), each interspersed with an eucaloric washout day. An MGC Diagnostic metabolic cart and face tent will be used for REE. Results will be analyzed using repeated measures ANOVA to determine the impact of fasting and LPO on REE. Additionally, multivariate linear regression will be used to model REE as a function of the covariates in this study such as sex, age, ethnicity, and body composition. ANTICIPATED RESULTS: It is hypothesized that the acute dietary interventions will reveal the different metabolic phenotypes, in which thrifty individuals will exhibit a larger decrease in REE during fasting, and a smaller increase during LPO.

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