The Role of Acute Fasting on Substrate Utilization During Submaximal Exercise: 1407 Board #169 May 30 10:30 AM - 12:00 PM

Purportedly, performing 20 minutes of aerobic exercise following an overnight fast has a greater effect on fat loss than in the postprandial state. Theoretically, a fast may attenuate glycogen stores, thus shifting to the utilization of increased free fatty acids, and resulting in greater fat burn throughout exercise. PURPOSE: To determine if exercising at a titrated intensity, as determined by equal fat and carbohydrate fuel utilization, will elicit more fat utilization in a fasted state when compared to a fed state. METHODS: 13 (12♂) asymptomatic (ht. 176.6 + 2.9 cm; wt. 79.1 +17.2 kg), healthy college students (age range 19-24) who regularly engage in aerobic exercise a minimum of twice per week volunteered for the study. During familiarization, individual treadmill workloads were titrated by using a ramp protocol (speed increase of 13.4 m/min) at 1% grade to achieve a RER of .85. All trials were preceded by a 12 hr fast w 24 hr abstinence from alcohol & caffeine, and 30 minutes of rest immediately prior to the trial. The fasted trial (Fa) required 25 minutes of rest and 5 min of measured resting metabolism (RMR) prior to the treadmill exercise, while the fed trial (Fe) required ingestion of 450 kcal (74% CHO, 14% pro, & 12% fat) in 10 minutes with 15 minutes of rest immediately prior to 5 min of RMR. Single blind randomization determined trial cross over order. Blood glucose (BG) was obtained by finger stick. Statistical analysis by paired samples T-test were applied to these data (p.05). BG pre and post exercise of 81.3 ± 9.2 & 61.8 ± 9.1 for Fe and 80.9 ±5.5 & 85.5 ±10.1 mg/dL for Fa was NSD for Fa. CONCLUSION: A 12 hr fast can alter resting fuel substrate, but exercise fuel sources are unaffected. Reductions in BG from exercise during the fed state may be attributed to fuel use, however the maintenance of BG in the fasted state may be related to glucagon release. Relative to fat expenditure, there is no advantage to exercising in an acute fasted state (12 hr) or in a fed state, however an extended fast may provide a different outcome.