Apparently, the fat utilization myth is alive and well, thinking from the reactions I had to my previous post on the subject. I had several comments to the original post, as well as questions on email, so I thought I would write a mini-FAQ about the subject. Here are some questions I was asked:
How do you explain the fact that some people burn a lot of fat and others don’t?
Because some of the determinants of the respiratory exchange ratio (RER) are dependent on individual characteristics. Research shows that there is a large interindividual variability in resting RER that persists during exercise of increasing intensity. Research also shows that the major determinants of RER included muscle glycogen content, training volume, proportion of type 1 fibers (only rest RER), free-fatty acids and lactate concentrations in the bloodstream, and % dietary fat intake. This last item is an important one, because it shows that changing your % of dietary fat intake will change your RER results.
I have recently repeated my baseline O2 Metabolic Assessment test and it showed that I had improved my fat utilization. How do you explain that?
Along with the initial O2 Metabolic Assessment test, the myth followers usually give nutritional advice that advises athletes to remove sugar and starch-rich foods from their diet. The consumption of “good fats” is also advised. What this does in practical terms is to reduce the CHO content of ones diet, and increase the fat content. So when you repeat the test, even if your fitness hasn’t improved, just because you increased your % dietary fat intake, your RER will show that you are now burning more fat.
How can my level of fitness be a factor on my fat utilization results?
Like it was mentioned on the original post, research 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 as you become more fit, for the same absolute intensity, you will burn more fat, even if diet is maintained.
Should the results from O2 Metabolic Assessment tests be used to guide my training?
The answer to that is no. The goal of training is to improve performance, not % of fat burning. And contrary to what the myth believers will tell you, improved fat burning does not translate to gains in performance. Improved fat utilization is the consequence of becoming more fit and not the other way around.
Do you think the use of O2 Metabolic Assessment tests is a scam and the people making money out of this quacks?
I wouldn’t go as far as to say that, as I'm sure that many people that use these tests have a strong (unfounded) belief in them. However, it is important to stress that using O2 Metabolic Assessment tests to guide your training is a self-fulfilling prophecy. After you do the first test, the usual advice is to train at your “maximum fat burning zone”. Often you also get nutritional advice that in reality increases your % of dietary fat intake. When you repeat the test, you will be likely more fit and have a diet with more fat. So the results of the test will show that you’re better at burning fat at absolute intensities. The “obvious” conclusion is that the advice that was given was spot-on and you’re better at burning fat. But in fact, you’re just more fit or/and are consuming a higher % of dietary body fat. So basically, repeating the test will always have the same result, because the repeat test is affected by increased fitness and changed diet. Even if an athlete never trained in the “maximum fat burning zone”, as long as her fitness improved or/and her % of dietary body fat increased, she would still have improved fat burning ability at absolute intensities.
Can I really burn 100% fat at rest?
The answer to that is NO!!! We burn approximately 55% to 65% fat and 45% to 35% CHO at rest.
Coyle EF, Jeukendrup AE, Oseto MC, Hodgkinson BJ, Zderic TW. Low-fat diet alters intramuscular substrates and reduces lipolysis and fat oxidation during exercise. American Journal of Physiology, Endocrinology and Metabolism. 2001;280:E391–398.
Coyle EF, Jeukendrup AE, Wagenmakers AJM, Saris WHM. Fatty acid oxidation is directly regulated by carbohydrate metabolism during exercise. American Journal of Physiology. 1997;36:E268–275.
Goedecke JH, Gibson AS, Grobler L, Collins M, Noakes TD, and Lambert EV. Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes. Am J Physiol Endocrinol Metab 279: E1325–E1334, 2000.
Helge JW, Richter EA, Kiens B. Interaction of training and diet on metabolism and endurance during exercise in man. Journal of Physiology. 1996;292:293–306.
Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983;32:769–776.
And for the last question
McArdle WD, Katch FI, Katch VL. Essentials of Exercise Physiology, 2nd Ed. Baltimore, MD: Lippincott Williams & Wilkins, 2000.