The impact of fueling for a race when Fat-Adapted...

I am fully geeking out on numbers as I start thinking about a fueling plan for the Mad City 100k coming up on April 12th.    This is some seriously obsessive number crunching, so proceed with caution if you aren’t a fan…

Armed with some of my preliminary results from the UConn FASTER study and a little extra research, I have a plan in place for how many calories I think I will need.   I thought I would share my thought process, as it illustrates a distinct advantage for competitors who are fat adapted.

The 100k distance on the road is still a speedy race and the more you push yourself past the lactate threshold, the more stress it puts on your system and the more challenging fueling becomes.  Part of the advantage of being fat-adapted is that I should (theoretically) be able to do the race and ingest fewer carbs and rely more heavily on stored body fat.  This is important in that needing to take in less calories will also leave more blood volume available for muscles instead of diverting some blood volume for digestion - keeping my stomach from rebelling.

During the FASTER study at UConn they were able to test the amount by grams per minute of fat and carbs that each participant was burning at various VO2 max levels.    65% of max VO2 is the level associated with lactate threshold – the tipping point level at which metabolism starts to become more anaerobic and lactate begins to build in the muscles. This would be a pace that is comfortable; conversation pace.  Elite marathoners (I am not one!) race near 85% of max VO2.  I doubt I will be running that close to my ceiling during the 100k, but I will assume 85% just to be safe.

To start the FASTER study, they did a VO2 max test to find the max of each participant and used the data to establish what 65% of max VO2 would be for each individual.  In this way they could ensure that each participant would be operating at similar efforts even though the actual pace might vary.

It is a fact that the level of carbs metabolized gets greater the more anaerobic the effort is and this was reinforced  during the treadmill test at the FASTER study.  This disparity was far less pronounced with fat adapted athletes who had trained their bodies to rely on primarily metabolizing fat at lower exercise intensities.  Conversely, an athlete who eats a traditional high-carb diet begins even low-level efforts by burning mostly carbs in the form of muscle glycogen.

My results were typical of the other high-fat/low carb trained participants.  For example, at 65% of max VO2 on the treadmill, I was running 7:50 mile pace and at this level I was burning 1.24 grams of fat per minute and zero grams of carbs.  As the intensity rose and VO2 levels got to 75%, I was burning 1.07grams/minute of fat and .95grams/minute of carbs.  (Keep in mind that a gram of fat yields 9 calories and a gram of carb is 4 calories, so even at this level fat is supplying me with 70% of my metabolized energy needs).   At 85%, I was burning 1.04g/fat per minute and 1.49g of carbs per minute.    This amounts to 1.04 x 9cal (9.36 calories from fat per minute) and 1.49 x 4cal (5.96 calories of carbs per minute).  At this level, 61% of my calories were coming from fat metabolism.

If we can assume 98 calories are burned per mile,
I will need 98 x 62 = 6,076 calories during the Mad City 100k race.

61% of the 6,076 calories (3,706 calories) should come from stored body fat.   This leaves 2,370 calories that will depend on carbs.  These carbs can come from glycogen stored in muscles and from carbs ingested during the race.  A typical male would be able to store 1,800-2000 calories in skeletal muscle in the form of glycogen.  I will assume that as a smaller, 130lb. male, I can store approximately 1300 calories.

If my body has around 1300 calories of muscle glycogen on board, then I technically only need to ingest:  2,370 – 1,300 =  1,070 calories of carbs during the race.

Since I normally run in a fasted state, I would like to wait until after 20k to start taking in calories just to let my body and stomach settle a bit.  If I take in 150 calories between each 10k loop starting after loop 2, I would get 8 x 150 = 1200 calories.  This should average out to taking in about 200 calories per hour.

Lets think about the case of a High-Carb trained athlete and how the numbers being reversed would effect fueling:

Again:  98calories x 62miles = 6,076 calories needed.

If 61% now came from carbs (instead of fat), 3,706 calories of carbs would be needed during the race. With 1300 calories of stored muscle glycogen, that leaves 2406 calories of carbs to ingest during the race.

If I took these carbs between each loop (9 times instead of the 8 times above) that would be 276 calories of carbs per loop or 356 calories need to ingest per hour.

This is nearly 4 gels an hour, the edge of what most people’s stomach can handle, especially when trying to run hard during a road 100k.  No way could my stomach take that.    If the stomach does rebel, and one can’t take in enough calories, you would be forced to rely on metabolizing fat only.  For an athlete who doesn’t train in fasted state and doesn’t primarily rely on metabolizing fat, their pace must slow to a lower VO2 max threshold – likely below 65%.  This means a big slow down late in the race as ingestion of carbs fails to keep up with need.  This is what I am hoping to avoid.

The further implication is that for a race of less intensity, say a 100 miler, a fat-adapted athlete ought to be able to rely almost entirely on stored fat for fuel as long as they stay under lactate threshold - 65% of max VO2 (In theory anyway!!).  On April 12th I will see what happens when theories and reality collide!  Hope it goes well…

(I’m sure I have messed up some of these numbers, so feel free to point out any mistakes!)

1 comment:

  1. Hi Kevin,
    good results :)

    Ultra-runners (like Zach Bitter) race at 85% VO2 max in 100 km race? That is more than marathon pace (70-80% VO2max) -

    Can you tell me what is their pace (for example Zach) in races like:
    Distance (km/miles):

    Thank you :)