Burning more fat: some numbers

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Ironman racing is hard.  Developing the endurance and deep aerobic capacity to be successful at Ironman racing takes years.   Finishing an Ironman is a major accomplishment, but going as fast as you can over the distance requires the development of a strong endurance foundation through a methodical approach to progressive overload year after year.  The progressive overload needs to be supported through adequate recovery and rest to facilitate and allow the physical adaptation — more and larger mitochondria in your muscles.

Nutrition has often been called the 4th discipline in Ironman triathlon and due to the long duration of the event, most, if not all, athletes are fuel-limited at some point during the race.  Some basic accounting illustrates the point.  Most athletes store 2000-3000 Kcal worth of glucose in their liver and muscles, so at the beginning of the Ironman, this is the fuel store that we have to work with.  It has also been shown that supplementing this fuel supply by ingesting carbohydrate during exercise improves performance and has become common practice in endurance sports.  However, our digestive systems are limited and have been found to accommodate and process about 1.75 g/min of exogenous carbohydrate during exercise if multiple sources of carbohydrate are used – glucose and fructose as an example  (Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Asker E. Jeukendrup.  Current Opinion in Clinical Nutrition and Metabolic Care 2010, 13:452–457).   Carbohydrate is 4 Kcal/g, resulting in a maximum possible utilization of 420 Kcal/hr of exercise.   Most athletes will burn more than that on an hourly rate throughout the duration of the Ironman, even at low intensity exercise, resulting in a depletion of the fuel store.
To confound the problem, many athletes take in too many carbohydrates too quickly during exercise resulting in a concentration in the gut that is too strong and prevents digestion.  Or they do not pace correctly, going too hard and resulting in the body not being able to handle the amount or the concentration of what is ingested and burning through too much of the stored fuel supply too quickly.  There is evidence of this at every Ironman – athletes whose bodies have “shut down” and are not processing anything.  They end up dramatically slowing down, walking, or having the contents of their gut involuntarily ejected.

Those that cover the distance without nutritional issues or slowing down dramatically have paced the event correctly and likely also have a more “fuel efficient” metabolism.  What does this mean in terms of exercise nutrition?  We all, no matter how lean, have ample fat stores for several Ironman races in succession (80,000 Kcal+), an order of magnitude more than the aforementioned carbohydrate stores (2,000-3,000 Kcal).  To better preserve those precious glycogen reserves, we need to burn more fuel from our near endless fat reserves.

The question then becomes how do you improve this?  Can you train this ability?  While there is not yet published research on this topic (that I can find), there are many coaches and athletes that have experimented and developed evidence to suggest the ability to burn fat for fuel is trainable.  Some have suggested there are two ways to impact your ability to burn more fat (Seebohar, Bob. Metabolic Efficiency Training: Teaching the Body to Burn More Fat):

  1. “Periodizing” your nutrition to match your training demands — Generally, we get enough carbohydrates to support our daily training from fruits, vegetables, and dairy products, and do not need to supplement with large amounts of pasta, etc.  He also suggests that sports nutrition products are not necessary during most of the year and should only be used during times of the year when our training is intense.
  2. The second way we can impact our ability to burn fat is simply through consistent training and keeping the training at a moderate intensity most of the time.

How can you measure your ability to use fat as fuel?  This is done through analysis of the gases you breathe in and out during exercise, which gives indication of the mix of fuel you are burning at different intensity levels.  A mask is connected to the gas analyzer, which is connected to a computer for data collection.  The athlete subject then performs a graded test protocol where they increase the intensity level (biking on a stationary trainer or running on a treadmill) periodically and systematically.  The mix of fat versus carbohydrate used as fuel is then recorded as a function of intensity (heart rate and power on the bike or pace on the run).

I’ll use my own data as an example.  Here’s the recorded information from a test in April 2012:


This was a test on the bike and shows plots of power, heart rate, fat (in Kcal/min), and carbohydrate (in Kcal/min).   You can see from this plot that I start burning substantially more carbohydrate (CHO) pushing the power above 200 watts and my heart rate above approximately 135.  In addition, I burn about 7.5 Kcal/min of CHO and about 7.5 Kcal/min of fat at that intensity level.  This is very valuable information and can be used to plan a precise fueling strategy for Ironman racing.

Back to the question of whether or not you can train the ability to burn fat.  Although controlled studies have yet to be published (that I know of), it is intuitive that you might be able to impact the mixture of fuel through diet manipulations and there is ample anecdotal evidence that suggests this.  The data above is from one test at one point in time – April 2012.  But you can use this same protocol in a repeated fashion to monitor the athlete and any changes over time.

Here is the same data from 2010, but by using my heart rate to “normalize” the intensity level, we can compare the number of calories burned per minute from fat and CHO from two different points in time.  In this case, it was February 2010 to April 2012.

You can clearly see from this data that I have improved my ability to burn fat.  If you look through the range of heart rate from about 115-150 (between the blue lines), you can see a substantial change:

2010  ~ 14-15 Kcal/min ~ 5 Kcal/min from FAT ~ 9-10 Kcal/min from CHO

2012 ~ 14-15 Kcal/min ~ 7 Kcal/min from FAT ~ 7-8 Kcal/min from CHO

The difference of 2 Kcal may seem small, but that is PER MINUTE.   If you look at these numbers on an hourly basis, they start to tell you the whole story.

2010  ~ 900 Kcal/hr ~ 300 Kcal/hr from FAT ~ 600 Kcal/hr from CHO

2012 ~ 900 Kcal/hr ~ 420 Kcal/hr from FAT ~ 480 Kcal/hr from CHO

I am burning 15 total Kcal per minute, or roughly 900 per hour at that intensity level.  In 2010, I would’ve been using about 600 Kcal/hour of stored glycogen and only about 300/hour of fat.  In 2012, at this intensity level, I am now using about 480 Kcal/hour of stored glycogen and 420 Kcal/hour of stored fat, saving an additional 120 Kcal/hour of stored glycogen.  That can really pay off when racing the Ironman distance.   For example, in 5 hours on the bike, that would equate to 600 additional calories of glycogen that have been spared for the run!

Throughout this time frame, I have trained consistently around 12 hours per week on average and peaked out for a few weeks before Ironman races at 20-22 hours.  However, I have not maintained a strict “metabolically efficient” diet.  I have done my best and eat well most of the time, using lean sources of protein and fruits and vegetables, but I have two young children and I keep it realistic — eating everything in moderation and indulging occasionally (you might call it regularly).  While I have a long way to go to reach my goals, I have been able to see measurable, significant improvement in my ability to burn fat, which is a significant component and piece to the Ironman puzzle.

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