Science Friday: The Wall of Carbs

We’ve all heard of carbo-loading. Big heaping bowls of pasta the night before a race. Bags of pretzels and morning bagels. We’ve known about the benefits of filling up on carbs before endurance events for a long time. See below, from a 1971 experiment. In that study, subjects ran in two 30 km (18.6 mile) races. Before one race, they filled up on carbs and before the other, they ate their normal diets. The groups were mixed (some ran the carb-filled race first, some ran the normal diet race first) and the results are shown below. Across the top is the distance, broken up into eight increments. Each line represents a different subject (their initials are at right) and the time difference between the carbo-loaded race and the normal diet race. All the subjects slowed down when on a normal diet, and in particular at the end of the race.

Ok, you knew all this already. (But isn’t it nice to see it in a graph?) What have we learned since then? Exactly how many carbs do we need to consume? Are midrace GUs necessary? Or should we carry those Sport Beans that sound like maracas the whole race? Does pacing matter? What about our individual body compositions?

Based on a 2010 paper, one thing we’ve learned is how to make more complicated and colorful graphs. See below.

Don’t freak out. I’ll try and simplify it.

In this study, which got a lot of press when it came out, Benjamin Rapoport looked at the physiology behind hitting the wall. Interestingly, he did this because he was so annoyed when he himself hit the wall in a marathon.

It’s an interesting, dense examination of the factors that contribute to marathon pace. He spells out (in equation form!) how carbs are more efficient than fat. And how higher intensities means burning more carbs. Remember acetyl coA from high school biology? Yea, that has a role. He also discusses the importance of keeping an even pace. If you slow down, you also slow your carb consumption, but if you want to hit your goal pace, you’ll eventually have to pick back up. When you do pick the pace up, your carb consumption sky rockets and you end up using more carbs then you saved when you slowed down.

In the graph shown above, he models how VO2 max and glycogen loading (carbo-loading) can influence your marathon time. Each colored line represents a different VO2 max. For example, someone with a VO2 max of 50 is modeled by a light green line. If this person doesn’t carbo-load, his projected marathon time would be a 3:42 (the light green line hits the bottom of the gray glycogen loaded box at the vertical line corresponding to 3:42). If he does carbo-load, he could break 2:45 (the light green line hits the top of the gray glycogen loading box a little past the vertical line corresponding to 2:45). Rapoport also goes into how muscle mass plays a factor (on the right) but I’ll ignore that for now. I think the main take away is how much we can gain in that gray box. You want to be at the top of that box, and therefore further to the right of the graph, in faster territory. If you do like equations and physiology, it’s an interesting paper you should give a read. Find it here. 

Although he focuses on midrace fueling in most of the paper, in the discussion and in interviews he notes that prerace fueling is best. Better to carb up than GU up. One of the things that got the article so much press was a calculator he created and made available online. You can enter your age, sex, weight, resting heart rate, and goal time and it will tell you how many carbs you need to consume before the race. The problem is the models explained in the paper are much more rigorous than this calculator. People’s weight and heart rate are not the same as their leg muscle mass and VO2 max. When I plugged my data in, I got a range of 3:59-2:44 for my predicted marathon time, which I found slightly unhelpful.

The most useful information I got was from an article in Runner’s World that interviewed Rapoport and asked for a simple answer of how many carbs we should eat in the days before the race. He suggested we aim to get 4 g per pound of body weight per day. For a 150 pound runner, that’s 600 grams. In a review of Rapoport’s paper and calculator, Alex Hutchinson, the Sweat Science guy, mentioned that you can carbo load effectively with 10 g per kilogram of body weight for just one day. For a 150 pound runner, that’s 680 g. Which is essentially the same, although for one day instead of three. Most conventional wisdom is a longer, three day carbo load is better, possibly because muscles can’t absorb that much glycogen all at once and so spreading it out helps. Either way it's a lot of carbs.

I suggest you take the time to count carbs, the same way some people count calories. Look up how many carbs are in the foods you normally eat before a race and count them up. Be honest with yourself. I was shocked at how many more carbs I was supposed to be getting and how far off I was. As runners who are hyper aware of what goes into our bodies we are concerned with balanced meals: protein, veggies, some healthy carbs. But in the days before a marathon you really need to be emphasizing those carbs, even if it comes at the price of less dairy, less protein, etc. Don’t overeat, just focus on bread, pretzels, rice, etc. Count what you eat and see the range you fall in. Since I started doing that I’ve drastically increased the carbs I eat before a race and have broken old PRs and avoided the wall. If you need more convincing, look again at the graph above and consider which side of the gray box you could be on.

Tune in over the next few days for a Boston infused blog. And good luck to all those running Monday! Carb up!

Dream big,

Teal