I can honestly say in my 24 marathon finishes (these include races, pacing, and a few trail marathons), that I have never been a victim of the infamous marathon bonk. The closest I have ever come was at the Cincinnati Flying Pig in 2007. For whatever reason that day I did some really stupid things with my nutrition and suffered major stomach issues after the race. I remember hitting some tiny incline around mile 24 and after having been on pace for my goal all day I watched it slip away.
Determined not to let that happen again, I researched like crazy and devised a nutrition plan based on the information I found. Because I’m a big geek, I came up with a mathematical formula to calculate exactly how many calories one needs to take in during the marathon to prevent bonking and minimize GI distress.
Are you ready to see how many calories my magic math says you need to take in per hour of your next marathon? Well get your calculators and come along and see!
Marathon nutrition comes down to preventing two things: 1) bonking and 2) GI distress. In other words, when coming up with your race day nutrition strategy you need enough calories to provide the energy you need to cover 26.2 miles without upsetting your stomach.
I found it hard to believe that everyone’s consumption would be the same, and while the general recommendation of a gel every 30 – 45 minutes seems a good baseline, I think it generally causes people to overconsume gels and thus get into GI distress territory. In my experience, custom designing your nutrition plan using my magical math formula has resulted in no stomach issues and sustained energy throughout my best marathons.
Alright, let’s get to the good stuff. Here’s the formula to determine how many calories you need to take in during your next marathon!
Weight x .63 x GMP x .3 = Amount of Calories to Consume per Hour of the Marathon
Ok, let’s explain!
Step 1! What is your weight in lbs.? This is the “Weight” in the formula.
Step 2! The .63 is used to determine how many calories you burn per mile of running. You multiply your weight x .63 and this tells you approximately how many calories you’ll burn each mile of your marathon.
Step 3! Now we need your “GMP” or goal marathon pace, but in miles per hour (like on a treadmill). Here’s a calculator for converting your pace to miles per hour.
Step 4! When you multiply the first three steps together, you get the amount of calories you burn per hour, but when you’re running you can only replace/digest at most 30% of those calories. So now we need to take 30% of our calories burned per hour and this is our target calorie intake per hour of the marathon! We’re done!
Let’s do a quick example. Say Jenny is 130 lbs and plans to run a sub-3:30. A sub-3:30 is about 8:00 pace or 7.5 mph. Here’s how the math works for Jenny: 130 x .63 x 7.5 x .3 = 184 calories per hour.
Remember, this formula tells us how many calories we can digest and use in race, at most! So if in doubt, err on the side of taking in a little less, especially if you are prone to GI distress. Jenny would probably be best off rounding down to 150 calories per hour, so taking a gel every 40 minutes, or 5 miles in her case, would be about right.
Conveniently most commercial gels have 100 calories per packet, which helps make the math easy when it comes to deciding what to take to get in those calories.
- When to take your last gel. Remember gels don’t “kick in” for 30 minutes, so consider that when deciding when to take your last gel in the race.
- To sports drink or not to sports drink? I do not take any sports drinks on the course. I am getting all the sugar I need from my gels. If you do take Gatorade rather than water, you would need to adjust the timing of gels if any to figure out how many calories you want to take in at each aid station. This obviously makes things trickier, but if you can’t stomach a whole gel at once and prefer your calories liquid style, it is easy to adapt the equation to your needs.
What marathon day nutrition plan do you use Salty Readers? Do you have a stomach of steel and can consume more than 30% of what you expend? Are you lucky enough to never have bonked in a marathon? Share your tips for nailing race day nutrition!