Symmetry Blog

Metabolic Flexibility - Part 2; The Testing!!

Symmetry Physio - Tuesday, July 16, 2019

Shay 'geeks out' as he steps up his training for the Hawaiian Ironman.


Metabolic Flexibility- Part 2; The Testing!!!

A few weeks ago, I put together a blog discussing the concept of metabolic flexibility or becoming fat adapted for ironman or endurance racing. In summary, this outlined why it is important for your body to be able to utilise not just carbohydrates, but fat as fuel for events of this length. If you haven’t already, I would suggest having a read of Part 1 which discusses the concept of Metabolic Flexibility and Fat Adaptation and the science supporting its application.

Qualifying and training for the World Championships in Kona I feel somewhat justified now in being able to “geek out” on some of these tests I’ve always wanted to do. And something I have always been interested in doing is an energy contribution test. This test uses a VO2 testing protocol whereby you ramp up the intensity of your work in 3-minute intervals until failure. When you have lost a lot of fitness and are just getting back into things, this isn’t a pleasant experience, and the results a little disappointing.

However, the real value is achieved when the scientists measure through your breath exhalation the percentage and amount of carbohydrate and fat you are oxidising or burning as fuel at any given effort.


The table and graph below outline the results!

In summary, you can see that at the lowest intensities (75 watts) 78% of my energy usage was burning fat. Then I had a steady mix of about 4:6 fat to carbs up to about 200 watts, before carbs start being used up almost exponentially.

So, it’s not hard to do the maths to work out how you will fare in terms of energy requirements for a given pacing strategy. For example, if we simplify things and just consider the Ironman Bike alone; At 195 watts (51% of VO2 max) I was oxidising 1.8 g/min of carbohydrates during the test which equals 108 g/hr. If on average the body can take on between 60-80 g/hr, that means, there is a deficit of let’s say about 40 gr/hr. Now at the start of the day our carbohydrate or glycogen stores should be topped up maxing out at about 500grams or 2000 cals. So, for a 5-hour ride at 195 watts we are looking at dipping into 200 grams of this 500 grams before we even start considering the run or swim and assuming we are consistently absorbing 70 grams of carbohydrates an hour or about 3 gels. This is valuable information!


Having an optimal race in terms of energy requirements means balancing this fine line between racing as fast as you can without running out of available glycogen and hitting the wall. The poorer your contribution of fat oxidation to the equation the quicker you will burn through glycogen stores. And then, when they do run out there is a much further fall or bigger ‘bonk’ where you simply won’t be able to work because your alternative and now only option of fat burning is very inefficient.


The other interesting feature of the test for me was how many carbs are required to work at higher intensities. To fuel 325 watts for a minute it required up to 8 grams of carbohydrates. That’s a gel gone in 4 minutes. This was an eye opener for me and really gives context to the term of ‘burning matches’ and avoiding any surging and instead working at keeping a consistent effort.

So, on the basis of this test there is work to be done for me. I think I got the equation about right at Ironman New Zealand pacing the bike at about 225 watts which I would no longer be able to do on the data from this test. But that’s what training is for! Time to start getting stuck into it! Stay tuned for the next instalment…..

Shay McLeod                                                                                                                                    Symmetry Physiotherapy




Post has no comments.
Post a Comment

Captcha Image

Trackback Link
Post has no trackbacks.

Our Clients