How to Burn Fat Instead of Sugar and Never Bonk Again

By: Matt Frazier

You know what it feels like to run a 5K, a half marathon, or even a marathon.  I want you to imagine that feeling now.

Now think about running three marathons, back to back to back.  We’re in the realm of pretty serious ultrarunners now—this is something few people will ever do.

Now imagine doing that the next day.  And the next day.  And every day for the next nine days after that, totaling 1000 miles in just less than 12 days.  We’re talking stupid mileage now.

That’s exactly what Stu Mittleman did.  He did it by burning a better fuel than anyone else.

Why Sugar Will Only Take You So Far

Most of us run on sugar.  We consume tons of it throughout the day.  And since we now live a go-go-go society, we’re in a constant state of stress that tells the body it needs to burn sugar to help keep us going.

But think about this.  According to Stu Mittleman, you have about 160,000 calories’ worth of energy in your body at any given time.  Of that, only 4500 to 5500 calories are in the form of sugar, and a lot of that is reserved for your brain and nervous system.

That doesn’t leave much for distance running.  The way most of us run, those sugar reserves are quickly depleted, at which point the options are (a) stop running; or (b) refuel with more sugar.  If you don’t do one of the two, your body physically shuts down, as a way of hanging onto what little sugar it has left for brain function.  And that’s what we call a bonk.

Since (a) isn’t an option for crazy NMA readers like you, you’re left with refueling as your only choice.  The problem with that, though, is that when you eat more sugar, you encourage your body to burn even more of it.  Soon, you’ve got to fill up again, and eventually you’ve sucked down so many gels that your stomach and GI system plot a coup to overthrow whoever is in charge, which happens to be you.

There’s an alternative to this vicious sugar cycle.  It’s called burning fat, and—surprise—you’ve got plenty of it to burn (sorry, you do).

Why Burning Fat Is Phat

Remember those 160,000 calories you’re holding onto?  Well, something like 85% of that is fat.

I suppose this could be bad news if your goal is zero percent body fat and the resulting death.  But it’s great news if you want to run far: If you can find a way to tap into fat as your primary fuel source, then the distance you can run will be limited by muscle failure or injury long before your fuel source runs dry.

That’s how Stu Mittleman ran 1000 miles in 12 days, and it’s how he ran across the country in about 50 days.  And lucky for us, Stu and others like him aren’t shy about sharing how they do it.

How You Can Train Yourself to Burn Fat for Fuel

It’s possible to change the way you run and eat so that your body learns to run on fat from the very start of your run, rather than waiting until sugar supplies are depleted, shifting to fat only as a last resort.  Pretty exciting stuff, huh?

Hold it right there.  Before you swear off sugar and start packing your old energy gels in your kids’ lunches, remember: This is a gradual process.  If you currently take in a lot of sugar before and during your runs and you suddenly stop supplying it to your body, you’ll bonk, and it’ll be dangerous.  Introduce these concepts slowly and gradually, and always carry a few gels with you for emergencies.

With that out of the way, let’s get into the nuts and bolts of what you can do to start burning fat.

First, note that we’re only talking about the long, slow run. Your body starts sugar-burning as your exercise intensity crosses the lactate threshold.  (A good indicator of when this happens is when it becomes difficult to carry on a conversation, or when your mouth drops open to start taking in air while you run.)  You can gradually increase the level of intensity at which you cross the threshold, so that you can eventually run faster while staying in a fat-burning state.  For speedwork and hill workouts, your body will still rely on sugar, and that’s fine, since they’re short, and sugar is great for hard, short runs.

Extend your warmup period. If you’re standing still and you suddenly bolt off running, your physiology changes.  Your body senses something is up (perhaps you’re being chased by bears and zombies?) and starts burning the sugar fires, since sugar is great for short bursts of energy.  But that’s exactly what you don’t want to happen on your long run.

So warm up extremely slowly.  Walk for the first few minutes.  Then start running so slow that you have to hide your face when you pass people you know.  Relax everything and enjoy it.  Over the course of 10 or 15 minutes, build up to your long-run speed.  Speaking of which…

Run slow! You want to stay below your lactate threshold for as long as possible, so your body can get used to burning fat for fuel.  So go really slow.  If you use a heart rate monitor, stay at 60 to 70 percent of your max.  Make sure you easily carry on a conversation while you run.  Your goal is to do this enough that your threshold increases, i.e., you can run faster yet still stay in this aerobic, fat-burning zone.

Practice running in a carbohydrate-depleted state. Running coach Greg McMillan has a great article about depriving your body of carbs so that it learns to burn fat.  This means restricting sugar intake both before and during your long runs.

Stu Mittleman personally told me that he would never even eat a banana while he was running, and instead carries raw almonds and vegetable purees with him.  He recommends fatty, alkalizing foods and a little protein.  So besides nuts, you might also try nut butters on vegetables, or perhaps avocados and even oils if you can stand taking them straight.

Since you’ll be drinking water instead of sugary sports drink, you’ll need to replace electrolytes.  For that, you can get salt pills or use something like Nuun in your water.

Again, be careful with this.  I’ve found that it’s a slow process to transition to completely carb-free runs.  I’m at the stage now where I’ve eliminated a lot of the sugar from my long-run routine, but I still eat some non-sugary carbohydrates.  This is why I’m a big fan of pinole, and also things like whole-wheat pitas with hummus, or a wheat bagel with almond butter or peanut butter, though I’ve tried to limit gluten recently.  Keep in mind that this is still very much a transition phase, as complex carbs are ultimately converted into sugar before they’re used for energy.

A Better Way to Run

You’ll probably find, as you start consuming less sugar, that running becomes more enjoyable.  The sugar-burning state is a stressful one, one in which other processes in your body slow down as part of the fight-or-flight response that sugar is so well suited for.

You’ll find that your mind calms.  You become more relaxed and more creative while you run, and the whole experience is more spiritual and more enjoyable.

What I didn’t tell you was where Stu ran his 1000 miles.  He did it on a track.  Yes, a track.

Even if you were physically able to run 75 miles a day 12 days, would your mind be able to handle that?  You can tell me you don’t care about running 1000 miles, and that even 50 sounds like the worst use of a Saturday known to man.  But don’t even try to tell me you couldn’t use the mental strength that Stu’s feat.  And that’s what kicking the sugar habit, even in your everyday life, can do for you.

This post is part of a series of posts designed to teach you how to run long and strong.  Go check out the rest!