This post explores the article “Metabolism and Whole-Body Fat Oxidation Following Postexercise Carbohydrate or Protein Intake” by S. Petterson et al.
For this study, 12 women with a VO2max of 45 ± 6 ml/min/kg (not pros, but good) were given one of three post-exercise drinks (placebo, protein, and carbohydrates) and their fat oxidation rates were measured. The protocol was a 23 minute fat oxidation test, followed by an hour workout at approximately 75% of VO2max, followed by the post-exercise drink, followed by a second fat oxidation test 2 hours later. The goal was to evaluate changes in the second fat oxidation test depending on which post-exercise drink was given to the athletes.
The biggest takeaway was that for all groups, the intensity at which maximal fat oxidation occurred increased at the second test compared to the first. Also, resting fat oxidation and maximal fat oxidation were the same for the placebo and protein groups, but lower for the carbohydrate group.
Let us start with a little background. Fat oxidation occurs at aerobic levels in endurance athletes. Specifically, there is a curve of fat utilization where very low intensities have very little fat oxidation because total energy use is low. As energy use increases, a large amount of the energy comes from converting fat into energy. As the intensity increases though, so does carbohydrate utilization. At some point, the intensity gets so high, that fat can no longer by oxidized fast enough to provide energy to the muscles and only carbohydrates are used as an energy source. This decline in fat oxidation occurs between tempo and sweetspot. By the time you reach threshold, almost no fat is used to produce energy.
The fat oxidation test slowly ramped up the intensity and the researchers measured the fat oxidation throughout the ramp. There is some peak where the fat oxidation is at its highest, usually around tempo intensity. For all of the athletes tested, they had a maximal fat oxidation at a higher intensity. Basically, they were able to do harder work and still use fat as their energy source, BUT the athletes who consumed carbohydrates after their first workout, had lower fat oxidation that the other two groups.
It appears that carbohydrates post-workout stunt our post-workout fat oxidation, but protein does not. This phenomenon is actually quite common in cosmetic weight loss recommendations (as opposed to performance training). A lot of weight loss programs will say ‘don’t eat for one hour following your workout’ and this is to maximize the post-workout fat oxidation to maximize utilization of adipose tissue for energy.
What does this mean for endurance athletes? Well, if you are interested in losing a bit of adipose tissue, it appears that shirking on the carbs post-workout might not be a bad idea. As long as you refuel with carbohydrates later, at least a few meals of carbohydrates before your next workout, you should be fine. With that being said, if only taking protein after your workouts inhibits your energy levels post-workout or affects your workouts the following day, then you need to supplement with carbohydrates post-workout. Remember, athletes have higher glycogen production in the 30 minutes to 1 hour follow exercise. This is a great time to supplement with carbohydrates and increase your glycogen levels for the next days workout.
My follow-up question for this paper is: how long does this increased fat oxidation last? The paper used a test 2 hours after the initial workout, but what if the test was 4, 8, 12 hours following the test? It would be interesting to see how long our bodies increased fat oxidation. I also wonder how much of the increase in fat oxidation is due to a lack of carbohydrates. Does the fat oxidation only occur because our bodies just don’t have any carbohydrates to use for fuel, or is there another mechanism. If it just increases fat oxidation because we have no carbohydrates, then the effects should wear off after our first carbohydrate rich meal. This is an interesting study, with ample opportunity to investigate more.