Iron Math: Calories and Why The Back Half of an IM Marathon Is So Damn Hard

I was recently trying to explain to someone that has little-to-no familiarity with endurance sports why the back half of an Ironman marathon is so hard.  In an attempt to put things in a more common and very rational language, I decided to explain things using calories. I'd never thought through this on my own, and while the results certainly weren't surprising, they really illustrate why things tend to get so hard later on in the day. Here is a summary of the conversation and the math.  Please note I'm not a scientist and a lot of this is based on assumptions which are based on various articles I've read; so while my analysis is far from precise, it is directionally correct.

Race Morning: Let's say I nail my carb-load and pre-race nutrition pretty well and start the race in a fully loaded state. I am going to call this having 2200 calories on board. 

Swim: For the hour and change (and a bit of change in my case) it takes me to swim 2.4 miles, I am going to assume I burn 700 calories. I'll finish the swim with 1500 calories on board....probably explains why I feel so good at the start of my bike!

Bike: Based on an accurate power meter reading, I'm fairly certain I burn about 700 calories an hour on the bike.  I am consuming 450 calories an hour of straight carbohydrate.  I am going to be generous and assume I'm actually absorbing 400 of those calories.  So, 700-400 = 300 calories net burn in every hour.  My IM bike took a bit under 5 hours, so I had a net loss of 1500 calories (300x5) on the bike, which means I'll finish the bike even.  Not ideal for starting a marathon (think about starting an open marathon after eating a low-carb diet for 3 days), but not terrible either...explaining why most (myself included) feel pretty good in those first few miles.

Run: Now this won't be as accurate as the power meter data from the bike, but running at between 7:50-8:30 pace (my range this last IM) and in fairly hot conditions, I'll estimate a burn of about 800 calories per hour. Between sips of sports-drink at aid stations and energy gels, I probably consume around 250-300 calories an hour on the run...so I'll call it 275.  Let's say I absorb 250 of these calories (again, super generous, but a nice round number).  With this burn and replacement, I'm running a net deficit of 550 calories an hour.  Here is how it shakes out over the course of the run:

• 1 hour (7.5 miles) in a 550 calorie hole
• 2 hours (15 miles) 1100 calorie hole
• 3 hours (21.5 miles in, I've slowed down a bit) 1650 calorie hole
• 3-3.5 hours (to race finish) the hole grows to 2200 calories as it is becoming harder to take down anything, and the time it takes to absorb anything becomes greater than the remaining time left in the race. 

Like I said in opening, this is nothing that should be surprising; the back half of an IM marathon is hard! But it is a nice and fairly logical way to understand why most start to feel a big drop-off in overall energy around mile 13-15.

A few notes: This model assumes a lot (haha) including that 100% of energy during the race is coming from carbohydrate stores.  I'm sure (because I've seen Alan Couzens' charts) that some of the burn is coming from fat, but probably a pretty minimal amount at the watts I'm riding (200-215) and pace I'm running (7:50-8:30).  Also, every individual is different in their metabolism...and also their pacing and the strain it places on their energy systems...but from a thematic standpoint, for any athlete that is "racing hard" this story holds true.

What this means to me: I think Mirinda Carfrae said it best on the recent NBC Ironman special.  When asked what is going through her mind during the run, she simply said "Eat. Drink. Run."   The IM marathon is a beast with fangs, and all energy should be used on running, and if not running, making sure you eat and drink. Carfrae nailed it, and she is a total killer, and I have a mini-crush on her because of it (sorry Caitlin).

Final interesting thing: I also think this type of approach to thinking about the IM marathon explains why when top-end professional's blow up, they often end up walking or DNFing, whereas for many age-groupers, at "blow-up point" the run turns into a jog which turns into a shuffle.  For the former, with body fat % that is extremely low, there isn't a lot of fat on board so literally the tank becomes empty and the car stops.  For age-groupers, even the pretty lean (e.g,. 7% body fat and up) there is enough usable body fat on board to fuel the end of the run, just at a significantly reduced pace.