Thursday, May 22, 2008


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.


Leng said...

brgSo... if want to burn more fat, I do need to became more fit ... but to became more fit, is there any type of food mix that I should eat to accelerate the process of eliminate more fat ?

Paulo Sousa said...


My athletes have had great success in losing body fat by eating less.

Alex said...

Hi Paulo, I have been reading Alan Couzen's blog where he explains how metabolic testing works. I am a bit confused by your post because it reminds me of the chicken and egg problem, so let me ask a question:

Someone does metabolic testing on the bike and let's say that he burns more fat in the zone between 160-200Watts. He leaves the lab and then trains on that zone 100% of the time for the next 6 weeks and comes back for testing.Do you agree with the following:

1)He will have been burning mostly fat for his training and following the proper diet will have managed to lose weight that was mainly fat.

2)He will have increased the amount of calories coming from fat to produce this wattage and therefore it will be easier for him i.e produce less lactate, to get these Watts.

Hasn't he become more fit that way?

Thanks very much,

Paulo Sousa said...


First off, I think it's evident that when I say fit, I mean from a scientific point of view, which means cardiovascular fitness + metabolic fitness. The way you improve your fitness is if you improve either your cardiovascular fitness or your metabolic fitness or both.

Regarding 1), the fact that your example athlete changed his diet is not related to training.

Regarding 2), I don't know if you realize that increasing "the amount of calories coming from fat to produce this wattage" does not necessarily translate into being "easier" to produce the watts.

"Hasn't he become more fit that way?"

I don't know, since you didn't tell me anything about his cardiovascular fitness and his matabolic fitness.

Hope this helps clarify your questions.

leon said...


My athletes have had great success in losing body fat by eating less.


Ahhh, the bitter truth :(

MarkyV said...

I always laugh when I hear about the met cart testing and then think back to what the IMC 2nd place, IMAZ shoulda-been champ and soon to be IMLP champ eats. Sometimes nothing and the other times white rice, chicken and oatmeal.

Yeah... it's about fat burning and diet and nothing about getting fitter.

khai said...

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.

So... PubMed is telling me to fat load before my next race? I have to say, that brisket sandwich and fries consumed ~18hrs prior to last weekends Sprint seemed to do me pretty well... :p

Alex said...


You are right in that I should have been more precise. So here goes:

1)I was referring to becoming metabolically fitter (although I understand that by training consistently he will better his cardiovascular fitness as well but let's leave that out for now).

2)By saying "following the proper diet" I didn't mean that he changed it, just that he has been eating in a way that does not affect negatively his training before and after the test.

3)"...does not necessarily translate into being "easier" to produce the watts"

Maybe I haven't understood it correctly, so I'll ask again:

In order to produce this same amount of watts (let's say 200), doesn't he need to use the same fixed amount of energy before and after the test?

If yes, the fact that after his training he will use less glycogen and more fat to produce the same amount of power, doesn't it make it "easier" for him ,especially if he races long distance i.e. half or full ironman?

I hope I am not too much of a pain and appreciate the fact that you already answered my previous questions.

Thanks again,

Paulo Sousa said...


Here's some more answers for you:

1) By training consistently, the bigger changes in fitness will come from an increase in metabolic fitness (LT), as cardiovascular fitness (VO2Max) will increase far less.

2) Like Dr. Coggan says, results for the O2 test can be very reproducible. Having said that, having the same diet for a period up to 5 days before a test can be very hard. But that is really not the main issue here. The issue is that for example, athletes with a very high-CHO diet and that are very fit will have results in the test that "show" they are "very bad" at utilizing fat. That of course, doesn't prevent them from being fast...

3) Surely you realize that for a fixed intensity, "200 watts", as your metabolic fitness increases, relative intensity decreases. So if relative intensity is lower, relative fat utilization increases. But the bottom line is that, as your metabolic fitness increases, you are able to exercise for longer at a fixed intensity. But that is not what racing is about. Racing is about exercising at a relative (to LT) intensity in order to arrive to the finish line FASTER.

So to make up an example using your athlete, let's say he can push 200w for a IM. As his fitness improves, he will want to race at the same relative intensity that those 200 watts used to represent. So if fitness went up, maybe that relative intensity translates into an absolute intensity of say, 210w. So as fitness increased, he'll want to ride at 210w because he wants to ride FASTER. And like I mention in the blog, if relative intensity is the same and diet hasn't changed, then he will oxidize a similar mix of substrates.

Hope this helps.

Alex said...

Thanks very much again for the answers, certainly helped a lot.


Matt Calarco said...

Apparently, what you say here only applies to male athletes. ;-)

I jest, of course. Nice post.

Kyle Marcotte said...

I eat pretty much however much I want of whatever I want, whenever I want. Maybe that's one of the reasons why I haven't podiumed at IMC. Should I train more or just eat less?

Paulo Sousa said...


Obviously the answer to that is train more AND eat less ;-)

Also I know this guy that got second at IMC and has a blog that eats rice for breakfast all the time. Maybe you should try that!

Kyle Marcotte said...

Damn it. I knew you'd say that!

I think the guy who finished 2nd eats rice because that's all he can afford to eat. Is rice better than cold pizza and warm beer?

Maybe I should try to find of how much Vegemite the winner ate for breakfast instead?

Will said...

Speaking of that guy who came in 2nd at IMC he does eat a fair amount of oatmeal, rice, and way over processed chicken nuggets, but we are sneaking in some high quality protein as often as he lets us.

But I think the key to his upcoming race in Lake Placid is without a doubt Malt Vinegar and Sea Salt chips.

mat steinmetz said...
This comment has been removed by the author.
Paulo Sousa said...


First off, let me just say that the last time I read your blog was about 9 months ago and I don't remember you approaching this subject.

Second, I am not aware that there are two kinds of scientific evidence, one for elite athletes and other for the "far from elite" like you call them. My "target audience" is triathletes, which by definition are human beings. It's about humans that the literature is about and it's humans we're talking about here.

Third, what you "feel" or what you're "not sure of" has very little relevance when stacked up against the evidence on the subject. If you want to contradict that evidence, I suggest you continue your studies in exercise physiology and make what you "feel" your PhD thesis. Good luck in trying to find a supervisor and funding for a study that would contradict the existing evidence.

Lastly, your whole last paragraph does not make sense at all from a basic physiology point of view. Let me just say that I am just not impressed.