What's the deal with Fat Loading and Carb Loading?

Apologies to my non-marathon-running readers and friends, because this space is going to be monopolized for the next couple of weeks with posts on strategies for the couple of weeks before the marathon.

And they’re really big weeks!

We marathon runners (and half marathon runners, too) have been training for months, but now, we’re cutting waaaaaaay back on our mileage to rest and restore our bodies after all those pounding, runs. Sadly, that means we lose the benefit of the powerful endorphins we’ve been releasing during tough workouts, and we need those feel-good fuzzies more than ever because the impending race is weighing heavy on our minds and our nerves. Speaking of heavy, one of the biggest concerns for runners during this time is weight gain. For months we’ve been feeding ourselves to fuel intense runs, and now we’re not burning calories at the same level. Try and tell that to our appetites! And then we’re supposed to carb load carefully so as not to put on weight? It’s enough to push me—I mean, someone—over the edge!

But fear not.

In the next couple of posts, I’ll break down the carb load into easy to understand concepts.

Simply put, carb loading involves eating lots of carbohydrate-rich foods before an endurance event to load up your muscles with glycogen (aka stored carbohydrates). We can only store a small amount of carbs in our muscles—90 minutes to two hours at a hard effort of exercise. We have hours of fat to burn, but using fat for fuel is far less efficient than glycogen, so we need to maximize the carbs stored before we hit the start line, or we risk a very painful finish (click for a story on a particularly painful run I had last year).

For my half marathoners: Carb loading is only effective for events lasting more than two hours. Most half-marathoners don’t need to undertake a carb-loading regiment of several days, though they definitely benefit from enjoying carb-rich foods the day before their race and in their evening meal the night before the night before the event (that wasn’t a typo: that all important carb-filled dinner happens two days before the half marathon!).

The two-phased carb load for the full marathon is a widely accepted practice (Writer/Athlete/Coach/Nutritionist extraordinaire Matt Fitzgerald has written extensively on this topic and informs my process considerably):

Step 1 is adapting a low-carb, high-fat diet of about 65% fat for several days—up to 10 if you can manage it. Your body adapts to burning fat for fuel during this phase, especially during the short workouts you’ll be undertaking without the benefit of carbohydrates.

Step 2 is the carb load, a diet of 70% or greater carbohydrates for about three days to fill up then top off your bodies carbohydrate stores.

Marathoners can opt to skip fat loading, but they tremendously benefit from a multi-day carb load. If a marathon runner waits until the day before the race to carb load, it’s already too late! Most of us need more than a day to fill those stores. 

I’ve heard quite a few questions about how much more we should be eating to carb load. That’s actually a bit of a misconception. The carb load doesn’t necessarily require us to eat lots more food—don’t forget that our bodies can only store a relatively small amount of carbohydrate. If you over do it by forcing down significantly more calories than you need, you could be carrying extra poundage to your 26.2 miler.

Carb loading is more about shifting the proportions on your plate.

Allow me to illustrate what a healthy everyday dinner plate looks like:


Should a runner opt to try the fat load (again not a requirement for every marathoner), the plate shifts to more fat, a regular sized portion of protein, and a small amount of starch vegetable or whole grain.



And for the carb load (my personal favorite illustration!), the shift is toward a larger serving of carbohydrate rich food with smaller amounts of protein and fat.

The easiest way to jump into the carb and fat load is to think of the meals and snacks you already like and to make simple adjustments to proportions and ingredients. In my next post, I’ll share practical easy swaps you can make to meet the nutrition requirements of race week.

Need help developing a custom dietary plan for your race week? Let's talk: lauren@live-full.com