Room Calorimetry as a Method of Measuring Metabolic Flexibility

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Metabolic flexibility is commonly measured as the change in respiratory quotient (RQ) from the fasted state to the insulin-stimulated state. This measurement is performed using a hyperinsulinemic-euglycemic clamp and hood calorimetry. A study was performed to evaluate if room calorimetry alone could provide the same results and to examine how diet composition and exercise affect metabolic flexibility. Sixteen healthy males and females stayed overnight in a room calorimeter on four separate occasions. While in the calorimeter, they participated in meal and exercise challenges. The subjects took part in one of two exercise sessions on the treadmill (high intensity – short duration or low intensity – long duration). They were also provided one of two treatment beverages for lunch (high carbohydrate or high fat shake). Blood samples were collected before and 30, 60, 90, and 120 minutes after breakfast and lunch to measure glucose, insulin, and non-esterified fatty acids (NEFAs). Metabolic flexibility was calculated in a manner similar to the literature, and new metabolic flexibility variables were also considered including the rate of change of RQ (slope) following exercise or a meal and the lag in RQ following exercise. Delta RQ - calculated as the difference between a two hour average night RQ and a two hour average morning RQ - was negatively correlated to % body fat, positively correlated to VO2peak, and trending negatively with age. The post-lunch slope of RQ was significantly larger following the high carbohydrate shake compared to the high fat shake. The slope of the increase in RQ following the start of exercise was negatively correlated with age and % body fat and positively correlated with VO2peak. This study shows that the non-invasive room calorimetry method can be used to measure many metabolic flexibility variables.