This article came out my own curiosity rather than some quest for a perfect diet or protocol. I have often wondered myself, and others have asked, just what and how much are we burning for fuel during exercise? Ever run five or ten miles in the summer? Ever squat a heavy weight for 20 reps? Moved furniture for four hours? It certainly feels like you've done something strenuous; you're gasping for air, possible dizzy, muscles begging you to just.stop.moving. Personally I avoid most junk and processed food 95% of the time but admit to eating pizza and drinking Gatorade after moving furniture all day because I feel like I burned through a whole pizza's worth of calories. Maybe I did.

Before delving further, it must be noted that the exercise discussed is usually strenuous in nature, whether it be aerobic exercise or weight lifting. And to add to that, doing 3 sets of curls and 100 crunches is not the kind of lifting I am discussing, nor is briskly walking for 20 minutes. Neither exercise session such as those really require excess calories or special attention to how you are refueling. This is not to bash those of us who exercise in that way, but having a giant protein/carb shake after an “ab” day just isn't necessary. There, I've said my peace.

Digging back a bit from college physiology, there is a system of measuring how much fat or carbohydrate you are burning at rest or exercise called a respiratory quotient. This measures the level of oxygen and carbon dioxide being exchanged through your mouth and nose to determine what fuel you are burning. This is measured on a scale of 0.7 to 1.0. What about 0 to 0.69? Don't ask, it doesn't matter! What matters is 0.7 means you are burning almost entirely fat for fuel and 1.0 would be almost entirely glycogen and glucose (carbohydrates in essence, and how I will refer to them in the remainder of the article). I am sure many low-carb dieters would be closer to 0.7 and those of us consuming carbohydrates all day every day would be close to1.0. There are always freaks who eat lots of carbohydrates and STILL burn fat but let's not hold grudges.

In general, though, your diet will reflect at least somewhat in this respiratory quotient. What does this mean for the exerciser though? Just consider this, exercise that favors carbohydrate consumption, such as weight training, will benefit from eating carbohydrates and thus being in a purely fat burning state can hamper performance in that exercise most of the time. Likewise, if you are always eat and burning carbohydrates and you do some low-intensity aerobic exercise like brisk walking, which burns mostly fat, you could be holding back your progress. Here's a few examples:


At Rest = Almost entirely aerobic and burning fat for fuel

Walking/Gardening/Light Activity = Mostly aerobic and burning fat for fuel

Moving furniture/Heavy Yard Work = A mix of fat and carbohydrates leaning more towards carbs

Running/Jogging = A mix of fat and carbohydrates for fuel

Weightlifting = A mix of fat and carbohydrates for fuel


These are VERY general but I outline them to make it easy to see that depending on the level of intensity of your exercise, you need to eat different types of food. One thing to keep in mind is that the longer you sustain an exercise, the more likely it is to be fueled by fat, whereas shorter intense bursts rely more on carbohydrates. This is because fat cannot be accessed readily enough during intense exercise like a squat or moving a huge rock to fuel that activity, so your body uses a substance called ATP for the first few seconds, followed by carbohydrates for the next 30 seconds to few minutes and largely fat beyond that mark.

If you grasp this concept and related it to your lifestyle and exercise, you can pretty easily figure out what types of foods to eat. Have a desk job where you sit 40 hours a week, followed by some light walking a few nights plus housework and light yard work on weekends? Not very many carbohydrates needed. However, the carpenter who works hard all day in a demanding environment then heads to the gym 4 times a week to lift weights will definitely need some carbohydrates to fuel his activity(yes, I know this guy). As with most things, most of us fall somewhere in the middle.


You Put Some Pop-Tart Behind That Squat

This leads me to the fun stuff. And the explanation for the title of this article. During my internship, while lifting at the gym with Greg, he finished his squat workout with the most reps he could get with 365 pounds. I believe he got 17. I recall him telling me he was eating Pop-Tarts the night before to get in calories, so when he finished the set, panting and red-faced, I remarked “You put some Pop-Tart behind that squat!”. It was a joke (and I thought it was quite funny) but in essence, some of that Pop-Tart probably made it into the energy used to fuel his exercise. Which is pretty cool.


How Much Are We Burning?

I'll keep the research brief here, but know that the muscle tested were the ones used during the exercise, so legs were tested in squats etc. This is important because the more strenuous the exercise and more total muscle used, the more carbohydrates and/or fat burned. So even that super long and hard arm workout isn't really requiring that much fuel. People also differ so much in size and fitness too, that one size does not fit all here. A 200 pound man may be storing up to 400 grams of glycogen(carbohydrates) in his body, while a smaller woman may only be storing 250. So whatever reductions take place in percent of fuel used, it is in relation to how much you already have stored. Remember that.


Aerobic Exercise

In general, carbohydrates are considered to be burned at 1 gram per minute for sustained aerobic activity, such as a 5 mile run. So a 7 1/2 minute mile, for 5 miles is 37 1/2 minutes, requiring about 38 grams of carbohydrate. There will be some fat burned as well but overall a shorter run like this only requires a modest carbohydrate replenishment. The longer you sustain aerobic exercise, the more you will rely on fat for fuel as well as protein, as the body simply won't be able to meet the fuel demand with just fats and carbohydrates. Studies have shown that sustained aerobic exercise can increase fat oxidation(burning) over baseline by 600%. At very low levels of aerobic exercise, fat makes up nearly all the fuel used. At more moderate intensities, like jogging at 65% of VO2 max, glycogen makes up about 40-50% of fuel used with fat taking up the rest, even at just 30 mins of duration. However, training for a marathon and running for 2+ hours requires more aggressive nutrition, so don't short-change yourself on recovery if you are training for athletic performance.


Anaerobic Exercise

Exercise like weightlifting, strongman and bodybuilding require different fuels at different times, because unlike running, which is a sustained activity, lifting is intermittent. This means that you will be using more carbohydrate for energy during your exercise and then more fat during the rest between sets. A few studies have shown that lifters doing five sets each of front squats, back squats, leg presses, and leg extensions to fatigue, comprising 30 minutes of exercise used about 26% of their muscle glycogen content, as well as 30% of their muscle triglyceride(fat) content. The rest is most likely made up of protein being turned over for fuel.

A quick generalization just for reference. Let's say the lifters had 350 grams of stored glycogen in their muscles. If they trained their lower body as in this study, they probably taxed about 60-70% of the muscles in their body. This is about 210-245 grams of potential glycogen. If they decreased it by 26%, that is about 59 grams of carbohydrates used during exercise, with the rest being fat and protein. Other studies show a decrease in glycogen by up to 40%, which would have been about 90 grams of glycogen used.

Doesn't sound like much considering that a large sweet potato is about 75 grams of carbohydrates. So why bother eating more than the minimal amount? There are a whole host of things to consider like the energy used during recovery which burns more fuel, to water retention to signaling through our nervous system, which all are impacted by carbohydrate consumption. A study that aimed to investigate this question had 5 people perform weightlifting on a diet with carbohydrates and 5 people on a very low carbohydrate diet. During a squat workout, the low-carb group performed 6 reps less on average on their first set than the normal group and 3 less on their second set, which is significant.

On the opposite spectrum, they studied a group of lifters who trained twice a day. Half consumed a whopping 250 grams of carbohydrates in a drink between sessions and the other half had water. The group with the carb drink were able to train 30 minutes longer than the water group and performed significantly more reps in their second workout.



I outlined the research to make a point. Eating carbohydrates fuels exercise and enhances performance. The question is, how much do YOU need.!? I would be surprised if the lifters I just mentioned actually burned through all 250 grams of carbs in their first session, so did they really need that many? Physiology is complex and I would say that that many carbohydrates not just replenished glycogen but blunted cortisol, spared protein and retained water, all aiding in performance.

So, the answer is to consider your goals. If fat loss is your only concern, I've shown some research that both weight training and aerobic exercise burn fat for fuel. If you restrict carbohydrates, your body will be MORE reliant on fat during exercise and this will aid in your fat loss. But restricting carbohydrates can also severely inhibit performance, so if your looking for a faster 5k time or a bigger squat, consuming enough to not only fuel that exercise but enhance recovery becomes more important than aesthetics. So if a protein/carb shake and 2 cups of rice after training is not sufficiently recovering you for your next sessions, you know you need to up the amounts.

And then comes the question of diminishing returns for fat loss. There are plenty of lean, fit people who run but there are also plenty of runners who don't look so lean or fit either. But I thought aerobic exercise burns fat, so the more I do the leaner I'll get? Some people seem to truly thrive on running and look great doing nothing else. But if you're burned out, tired, lethargic and all that running still hasn't gotten you leaner, than ditch it and lift some weights. Research shows it burns not just glycogen(carbs) but fat too. Plus, a break from the massive cortisol spikes that excess running brings will improve your sleep, recovery and overall energy.

Find your goal, find your exercise and THEN find your fuel.

But hey, sometimes you just gotta put some Pop-Tart behind it.


Haff, G. G., et al. 1999. The effect of carbohydrate supplementation on multiple sessions and bouts of resistance exercise. Journal of Strength and Conditioning Research, 13, (2), 111-7.

Leveritt, M. & Abernethy, P. J. 1999. Effects of carbohydrate restriction on strength performance. Journal of Strength and Conditioning Research, 13, (1), 52-7

Jensen, Jorgen et al.  The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise.

Frontiers in Physiology.  30th December,  2011.  Online.