Mythbusters

The other day I found that one of my posts here was linked on another blog and tagged as "mythbusters". That kind of sums up what some of the blogs I write are, since sometimes I try to deconstruct training myths that are – often wrongly –ingrained in the triathlon community.

One myth that once in a while still surfaces is the fat-burning myth. Not the most particular representation of the myth that states that you should slow down to burn more fat, hopefully that one is dead and buried. I'm talking about the more general one that sees fat as some sort of untapped source of fuel that we need to access in order to become more "efficient". For the myth followers, "teaching our body" to ”optimize fat burning” by way of this or that type of training is the way to increased endurance.

This seems attractive! I can use this stuff around my waist to go further and faster! I can have a training plan taylored to ME and to the needs of MY body! I am a snowflake, and no other human is like ME!

Central to this approach is the O2 Metabolic Assessment test. From this test, we can determine the respiratory exchange ratio (RER) for a given exercise intensity. The RER is the ratio of carbon dioxide (CO2) production to the oxygen (O2) consumption and serves as an indicator of the nutrient mixture being utilized.

This is great, I can know what type of fuel MY body is burning at a given intensity and perhaps have my training designed so that I can burn more fat!

While googling around to write this post, I came across this interesting quote from none other than Dr. Andrew Coggan. Apart from being known to troll, er, frequent, triathlon internet fora and just having coached one person in his life, in real life Dr. Coggan is an expert in substrate utilization during exercise. On the relevance of performing an O2 Metabolic Assessment test, Dr. Coggan had this to say:

"Interesting, perhaps, but I don't really see much relevance to training prescription (…) Although the measurements are highly reproducible, I certainly wouldn't rely on such information to modify someone's training program."

I decided to dig in a little more and went to PubMed for a search on this subject. The evidence points to diet being the factor that has a major impact on the mix of substrates oxidized during exercise. And not just long-term diet trends, but also the composition of foods consumed in the days leading up to the test. So even athletes who generally consume high-CHO content diets will have the ability to oxidize mostly fat if they switch to a high-fat diet for a short period of time prior to such a test.

The scientific evidence also points to the fact that two athletes, with similar levels of fitness, that exercise at the same relative intensity (relative to lactate threshold) will oxidize a similar mix of substrates. This means that fitness and diet, not training, will determine how much fat you will burn.

The reason why the search for "efficient fat burning" is a red herring is that is not because an athlete is efficient at burning fat that he is fitter. Fitter athletes have undergone a set of training adaptations that are superior to other, less fit, athletes. Among them is an improved ability to oxidize lipids at a given absolute exercise intensity. Improved fat utilization is the consequence of becoming more fit and not the other way around.

For all this, the next time someone tells you that doing a O2 Metabolic Assessment test is the only way to understand your body, taylor your training to your needs and get to the next level, remind them that a review of the literature on the subject is in order. And go spend your money elsewhere.