My metabolic efficiency status leading into IMCDA

In my work as a Sport Dietitian, I have guided many levels of triathletes with their daily and training nutrition leading up to Ironman.  But now it’s my turn to feel the challenges of Ironman nutrition. I have to tell you I feel a great deal of excitement to see how the carb-controlled nutrition patterns I have been following will work for me during such a long and challenging event.  Sure, I’m looking forward to the event itself.  Absolutely.

But as part of being a Sport Dietitian, it is good to challenge the nutrition norms, especially as we are in the midst of an evolving field of sports nutrition for endurance athletes.  Now, don’t get me wrong.  I’m not foolish to think my nutrition plan is 100% bulletproof, given that there are always so many things we cannot predict in a race setting.  Yet, I do have a good level of confidence in my fueling needs based on my metabolic efficiency status and training nutrition trials to date.

As mentioned in a previous post, I overhauled my daily nutrition in November last year to be more carb-controlled. I started Ironman training essentially at the beginning of this year, so about 5.5 months have passed as part of this experiment. An assessment of my metabolic efficiency status in December, about 6 weeks after implementing a carb-controlled nutrition pattern, showed that I was fairly efficient at using my fat stores as a fuel source:

Dec 2012 Metabolic Efficiency Assessment: Bike

Dec 2012 Metabolic Efficiency Assessment: Bike

The red line represents my percentage of expended calories coming from fat and the blue line is my percentage of calories coming from carbohydrate.  

I haven’t changed my daily nutrition patterns too much since this test, although amounts of foods have changed in bigger training blocks and I also have been experimenting more with nutrient timing to support my training sessions. By "nutrient timing", I am referring to the before, the during, and the after (training sessions), which does roll into quite a bit of my “daily nutrition” time on long training days.

I should clarify my use of the term “carb-controlled”. Please know this does not mean an absence of carbohydrates in my diet. Nor does it mean I am starving myself (I have a respectable appetite!). Rather, the amount of daily carbohydrates is about 50% less than what I would be eating (aside from training nutrition) if I had decided to train for an Ironman prior to last year. I can tell you it is much different than what the majority of sports nutritionists and sport nutrition companies recommend for athletes' to train, perform and recover well. Meals and snacks are focused on higher fat choices and a consistent and moderate amount of protein. I always have the goal of achieving stable blood sugar with my food combinations, but I do allow for some “misses” to occur.  

So, here are the results of my most recent Metabolic Efficiency Assessment:

May 2013: Metabolic Efficiency Assessment: Bike

May 2013: Metabolic Efficiency Assessment: Bike

Note that I opted to start the test at higher watts for my first stage and not go as long as I did during the December test since I will not be biking at 180 watts and a perceived effort greater than 8 for this Ironman race.  Even though I did this test after completing a 70.3 race a few days prior and was in a big training block, the results show I am still efficient at using fat as a fuel source at the intensities and heart rates I plan for IMCDA.

This is great news!