Back to Fitness: Eating Before Exercise?


By John Andersen

One of the most common questions I get about running is, “should you eat before you run?” My Dad used to tell me, “its best to start a run with an empty stomach and an empty colon.” Interestingly, I find that most people have never really given this topic much thought, even though it does impact your exercise and your body’s response to exercise quite significantly.

Sometimes we do things a certain way because that’s how we were taught or told long ago. Sometimes we do things a certain way because we tried it once and nobody died. But one key concept in fitness is that we want to get the most gain out of each workout or exercise we do, so it’s best to give some thought to how we train and to reconcile that with our overall current fitness goals.

Here are some common goals I think most people would agree with regarding their exercise routines:

• Better health

• Weight loss

• Better performance/endurance/stamina


So in taking those goals, let’s consider the topic of eating a meal before exercise, or not, and you can decide what’s best. Please note, however, that the science of fat burning and insulin usage during exercise is growing, and people are very different! Also, hard work pays off, always! We’re just talking about trying to maximize the effects of your hard work so you can get the most benefit. So don’t take this as gospel, but take it as more information as you figure out what works best for your individual body in your fitness path.

In speaking with people on this topic, many people say they take in some food right before exercise, i.e. eating a light breakfast before a morning run. The quick thinking on this would be “I’m gonna go burn some calories, I may want to take some in so I don’t get tired.” Although this may make sense on the surface, you may actually be making it a little harder to achieve the health goals you are striving for.

First, a quick review of our body’s energy use during exercise:

Our muscles burn a hybrid engine, using a mixture of glucose (sugar) and fat for energy. Our body gets about 36 molecules of ATP from one molecule of glucose (ATP is the “dollar” of energy currency in our body), while we get a whopping 460 ATP from one molecule of fat. Fat is clearly more efficient. It also “burns cleaner” with less byproducts such as lactic acid compared to glucose. Our body has about 60-90 minutes of glucose stores before we run out and “bonk,” whereas even the thinnest athlete has essentially endless supplies of fat stores for exercise.

Now let’s examine what happens when we eat a meal right before exercise.

When we eat a typical pre-exercise meal (cereal, banana, energy bar, etc.), we are usually taking in a meal that has a decent amount of carbohydrates, and many times simple sugars. Again, on the surface, this seems to make sense.

However, when we take in those carbs, our body releases insulin into the bloodstream. Insulin is the hormone that takes sugar from the blood and brings it into the tissues where it actually gets put to use.

Unfortunately, insulin also stops lipolysis, our body’s ability to break down fat.

So, it seems that the meal we ate just before going on that bike ride or run is now priming our body to burn mainly glucose/carbs as an energy source, and inhibiting our body’s ability to use fat.

There are two problems with this:

First, taking in carbs right before exercise is simply unnecessary (for most of us).  Our muscles and liver have large glycogen (sugar) stores that will usually last us for 1-1.5 hours of exercise on their own. So for most of us who are waking up early and doing less than 1.5 hours of exercise, you simply do not need calories.

Second, we are not teaching our body how to become an efficient fat burner. Yes, fat burning! We all should strive to be better fat burners! Why?

We will have less fat stores!

We will have better endurance because burning fat is so much more efficient.

We will start to wean ourselves off the “carb/insulin” cycle, where we get hungry, eat carbs, spike our insulin, have a blood sugar drop, get tired/weak/hungry again, eat more carbs, and repeat…

It makes sense if we start our exercise on an empty stomach, with no insulin spike, our body will be better able to start burning more fat in our hybrid engine, and the more we burn fat, the more efficient our body becomes at breaking down fat, which will then carry over into our non-exercising life!

Now let’s revisit the “empty colon” part of this story. Let’s face it, nobody wants to have to take care of business in the middle of a run or workout. Typically getting up a little earlier and having some coffee is very helpful, or even a glass of water and just walking around a bit. But remember that you can give yourself a big insulin spike from that sugary coffee drink, so try weaning yourself off the sugar and even off the cream if you can stand it!

Last, I will say this conversation always leads to the question of “what about my long runs/rides/workouts, where I’m exercising for 2-3 hours?” Well, now you need food because even though your hybrid engine may be getting better at burning fat, it’s still going to use carbs, and you will eventually run out and bonk. So if you’re planning on longer endurance exercise, consider starting to take in calories about 45 minutes into your run/workout. This should be simple carbs and most people tolerate things like sports drinks, gels, blocks, etc. pretty well, although bars and PBJs work pretty well for many folks too! It’s all about what you can digest, and this is where you just have to get out there and experiment. For your runners and cyclists, aim for 200 calories per hour during your long workouts. However, if you keep your effort low, your body will use fewer carbs and you can ultimately “train” your body over time to go for hours without any calories, even fasting overnight.

Remember, if you’re only exercising for an hour, you don’t need any calories!

Think about your routine and just get out there and experiment. There’s rarely a “right” answer, but there’s a lot of good exercise science out there these days. It’s always good to “train smarter, not harder”!


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