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 quantifiable language, I decided to frame my explanation in calories. I'd never thought through this on my own, and while the results certainly are not surprising, they illustrate why things tend to get so hard into the afternoon of an Ironman. 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 in turn are based on various articles I've read; so while my analysis is far from precise, I am confident 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. I've seen a whole bunch of research that seems to center on a 'maxed out' carb load state between 2000-2800 calories, and given that I don't live in a lab, I think the lower end of the range most likely reflects my reality.

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

Bike: Based on an accurate power meter reading, I'm fairly certain I burn 700 calories an hour while riding. 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 (research here says the upper limit of absorption per hour is 100 grams of CHO, or 400 calories, so this is a very aggressive estimate, but it's also a nice round number).  So, 700-400 = 300 calories net burn in every hour.  My IM bike took a bit under 5 hours, which means I had a net loss of 1500 calories (300x5) on the bike, so I'll finish the bike even.  Not ideal for starting a marathon (just think about starting an open marathon after eating a low-carb diet for 3 days), but not terrible either, which explains why most (myself included) feel pretty good in those first few miles of the run.

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, and this is all carbohydrate.  Let's say I absorb 250 of these calories (again, super generous, but keeping numbers round).  With this burn and replacement, I'm running a net deficit of 550 calories an hour.  Here is how that shakes out over the course of the run:

• 1 hour (7.5 miles) 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 carbohydrate becomes greater than the remaining time left in the race.

Like I said in opening, this should not be surprising; the back half of an IM marathon is hard! But I do find this back-of-the-envelope analysis a nice and straight forward 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, 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 the strain that respective pacing places on their energy systems; so while this cannot be applied with any precision, from a thematic standpoint, this story holds true for any athlete that is "racing hard," whatever his or her hard might mean in watts, pace, etc.

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: 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.