An Invictus Research Study on Fasted and Fed Exercise States
Written by Calvin Sun
Sports nutrition is one of the many topics your coaches at Invictus love to debate. Pre-, peri-, and post-workout nutrition protocols are continuously studied and researched at our facility. Training on an empty stomach has been discussed several times on this blog and at our various nutrition seminars. I wrote a blog covering my thoughts on the subject several months ago here: http://www.crossfitinvictus.com/2011/02/14/tuesday-february-15-2011/.
This past semester, as part of my exercise physiology course, I had to conduct a research study on a topic of my choice. It seemed like a logical step to study fasted and fed exercise states. I had originally intended on using a graded-exercise test to assess the effect of pre-workout nutrition, or lack thereof, on various biometrics such as respiratory exchange ratio, blood glucose, and blood lactate. My thought was that this might allow us to replicate the higher intensity of training we use in our conditioning workouts. The department director at San Diego State University modified our original experimental design due to concerns with logistics and equipment. Instead, we were required to perform a steady-state exercise test where subjects pedaled on a bike at approximately 65% of their max heart rate for 30 minutes.
In our study, we had test subjects come in on the same day, two weeks in a row with one day in a fasted state and one day in a fed state. The order was randomized with some subjects performing their first trial fasted and others fed. For fasted-state exercise, we had our subjects fast for 12 hours prior to exercise with no caffeine consumption prior to exercise. For fed-state exercise, subjects were fed a meal consisting of a banana, a store-bought “energy” bar, and orange juice (again, not my choice).
Respiratory Exchange Ratio (RER)
RER is a means to measure what fuel sources are being consumed during exercise to produce ATP energy. It’s simply a ratio of carbon dioxide output to oxygen uptake, with normal values for RER ranging between 0.7 and 1.0. An RER value of 0.7 suggests you are burning predominantly fats whereas an RER value of 1.0 suggests you are using mostly carbohydrates. A value of 0.85 suggests you are burning both fats and carbohydrates as fuels. In our study, we found that average RER value in the fasted state was approximately 0.85 ± 0.02. Average RER in the fed state was approximately 0.88 ± 0.05. While we expected RER values to be higher in the fed state, we expected RER values in the fasted state to be closer to 0.7.
Lactic acid is a byproduct of exercise. It produced as result of glycolysis, a pathway for metabolizing carbohydrates. In our study, we found that subjects tended to have higher levels of blood lactate in the fasted state. This was not entirely expected as we assumed the greater level of carbohydrate metabolism in the fed subjects to result in a greater amount of glycolysis and therefore lactic acid.
Keep in mind our study was conducted with a sample size of only eight and the test subjects were mixed in age, gender, and athletic ability. Nichole and I both served as test subjects while others in my research group were not quite as well trained. Due to these various issues, our results were not totally conclusive or definitive by any means but, then again, no single study ever is. Having said that, the idea that exercise on an empty stomach burns more fat seems be true though only by a marginal amount. If you are a casual exerciser who is more concerned with aesthetics than performance, training on an empty stomach is probably less likely to affect your results, either positively or negatively. However, if you are an athlete that is focused on performance, I would still advise you to have a pre-training meal. Keep in mind that pre-workout nutrition can be highly variable between people so experiment to see what works best for you. Of course, if you are in doubt, you can consult one of your coaches.