Fatty Acid Uptake Is Enhanced after Modest Energy Deficit via Acute Exercise, Not Caloric Restriction

PURPOSE: To compare changes in fatty acid metabolism in response to a very modest energy deficit induced by either one-day of dietary caloric restriction or exercise. METHODS: Seven obese (BMI: 35±2 kg/m2) but otherwise healthy young women performed 3 separate, two-day trials. On the first day they consumed: 1) a control, weight-maintaining diet [CON; 2470±111 kcal/day] while remaining sedentary, 2) a hypocaloric diet [LO-CAL; 2120±135 kcal/day] while remaining sedentary, and 3) the same diet as CON, but they exercised for 90 min in the morning [EX], resulting in the same energy deficit on day 1 as LO-CAL. The next morning we measured whole-body fatty acid uptake (isotope dilution methods), fatty acid oxidation [FAO] (indirect calorimetry), and a skeletal muscle sample was obtained to measure intramyocellular triglyceride [IMTG] concentration, as well as the activity of two key enzymes that regulate IMTG synthesis (glycerol-3-phosphate acyltransferase [GPAT] and diacylglycerol acyltransferase [DGAT]). RESULTS: The morning after the diet and/or exercise treatments (>20h after exercise during EX), fatty acid uptake was greater during EX compared with CON (P<0.03) and LO-CAL, but the difference between EX and LO-CAL was not quite statistically significant (P=0.055). In contrast, fatty acid uptake during LO-CAL was nearly identical to CON. The higher fatty acid uptake during EX was accompanied by a higher resting FAO (255±34 umol/min) compared with both CON and LO-CAL (198±26 and 156±31 umol/min, respectively; P<0.02). Interestingly, IMTG concentration the morning after exercise (48.5±10 mmol/kg dry weight (dw)) was not reduced compared with CON and LO-CAL (29.4±3 and 55.4±17 mmol/kg dw, respectively; P=NS), suggesting a portion of the elevated fatty acid uptake during EX may have been directed toward IMTG synthesis in the hours after exercise. However, the enzyme activities of muscle GPAT and DGAT were not different among the trials when measured the next morning. CONCLUSIONS: Inducing a very modest energy deficit in obese women by exercising augmented fatty acid uptake, whereas the same magnitude of energy deficit via dietary caloric restriction did not. Therefore, the increase in fatty acid uptake the day after exercise was not directly due to the resultant energy deficit.