Monday, June 2, 2008

Chewing the fat

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?


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.

Some references:

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.


And for the last question

McArdle WD, Katch FI, Katch VL. Essentials of Exercise Physiology, 2nd Ed. Baltimore, MD: Lippincott Williams & Wilkins, 2000.


Ryan Denner said...

Hi Paulo-

Between this post and your previous one - there is a lot of information to digest here (pun intended!).

I saw on your last post, there was some discussion regarding one of your athletes' diet. Do you recommend a breakdown of fat% vs. CHO% vs. protein% for your athletes, or do you let them eat what works for them?

thanks in advance-

Thomas said...

Hi Paulo,
I am also interested to know your opinion about the question asked above. To add to the mix, I read in an interview in ST, that Joel recommends his athletes to loosely follow recommendations for day to day diet, which is definitely not in line with the conventional wisdom of endurance athletes. I also read that Jorda Rapp 's diet is also not that classic high carb diet, and he eats a lot of fat. So do you think, that the studies you quoted, that suggest dietary fat intake might increase fat metabolism, can have an implication on what the proper ratio of carb/fat/protein is more suitable for athletes.
Also about losing weight, I agree 100% that the only way is to eat less. My question is what is the easiest way to eat less ie. not feeling the hunger or get more satisfaction from the same number of calories.


Ryan Denner said...

Paulo, not to gang up on your or anything...

Thomas, was this the article/interview you were referring to?

Paulo Sousa said...

I will start out by saying that nutrition is not one of my favorite subjects. I certainly don't push any sort of special diet to my athletes, because I simply don't think that diet is a determinant in performance. I do stress the importance of the timing of food intake on recovery from workouts, and more relevant, the impact that correct body composition has on performance. In order to achieve the latter, a certain degree of discipline needs to be attained, and just sticking to some sort of balanced diet will help with that.

Lorenzo Coopman said...

Maybe you like this quote "Everyone always asks me, 'What did you eat?' I didn't know anything about nutrition; if there was a hot dog on my plate, I ate it. None of those little things make a bit of difference if you're not doing the real work. George Young 4times Olympian.

Paulo Sousa said...

I'll jump on the bandwagon for a minute, and quote this guy here (bad spelling and all, english is not his strong point):

"nutritian is over rated . sorry , i said it ." - Brett Sutton

Thomas said...

This is the interview with Joel I was referring to:

(I had to break the lines of
the URL to show up correctly)

particularly this question :

" ST: Do you have suggestions/thoughts for your athletes in terms of diet?

Joel: I like to keep nutrition simple. We eat for recovery - high quality, clean and organic are the goals. If you train enough you can replace all those burned calories with good clean nutrition, or processed junk, which will over time affect recovery, performance and optimal health. I agree with and recommend most of the nutritional advice found on We need enough carbs to replace what’s burned so we can back up quality training the next day, but otherwise limit carb based snacks, especially post dinner. Dairy, soy and wheat based products are limited for many of my athletes as well. "

From pure performance point of view it is hard to argue in favor of any particular diet, as it seems a variety of athletes use different approaches and as long as what they eat is not dramatically unhealthy, they seem to get results from their training. But this is also another example that fitness not always equates to health. FWIW, I am from Germany, and in my local community of cyclists/runners, I see less concern on some issues that are frequently discussed on mostly north american tri forums about weight and health. Things like winter weight gain, race weight, how to lose the last 5 pounds etc. might become a no brainer, or at least less of an issue for an AG athlete with a) portion control b) eating healthy.

Paulo Sousa said...


I agree with Joel in general, even if I am not a big Mark Sisson fan.

I also bet that in Germany, diet books are not as popular as in the US. It's really a cultural thing.

But like I said, not my favorite subject ;-)

Clint-Murphy said...


Reading the three posts you have on the fat-burning myth is raising a question in me.

I recently did Vo2 max testing with lactate threshold testing and determined my (a) aerobic threshold, (b) lactate threshold, and (c) Vo2 Max.

The training that I'm now doing is based on these zones - is what you're suggesting that this is wrong for me? Based on my understanding in talking to the coach, I'm not training in the lower zones because I burn more fat as fuel but to train my body to go faster at an intensity at which the body is able to remove lactic acid from the muscles faster than it builds up....Also, at least 1 x per week I would do interval workouts based on the lactate threshold to raise this up as well.

Is this something completely different from what you're saying is a "myth" or is it the myth?

Generally as a result of the testing, on my running and my riding, I have lowered my pace slighly to bring the heart rate down about 5-10 bpm as most of my training was in zone 2 where I was told it is "junk miles" and I want to focus just below the aerobic threshold for long events with, as noted, interval work to increase lactate threshold/speed.

Your thoughts?

Paulo Sousa said...

I described what is the myth in the previous post. What you are doing is using (hopefully accurate) metabolic testing to determine training zones. That is a valid method to determine training zones, if done correctly.
One extra note; there isn't such a thing as "junk miles." Given the continuum that exists between aerobic intensities, basically every mile ran at an intensity above say 65% of VO2Max is the same.