Running on an Empty Stomach
What do you eat before you go on a run? Toast or a bagel? Oatmeal? Fruit? Pancakes? (Ok, probably not the last one.)
What about going for a run with nothing in your stomach but some coffee? Now how about running for over an hour just carrying water? The idea of running without eating any food is certainly not a new phenomenon, but as far as I can tell in the last couple years it has started to catch on as an important training tool.
Really, no food? I, like many runners, always tried to make sure that I had a decent meal before my runs—enough to fill me up, but not so much as to lead to cramping or bloating. It took a while to get my body used to eating before a run, and the amount of digestion time my body needed before i could run gradually grew smaller and smaller until I became able to eat a bagel & peanut butter and then head out the door 10-15 minutes later without cramping.
Last year I started cutting down on the food I had before runs even more, heading out with only a cup of coffee in my belly. I won’t lie: it took some time. I was sluggish; had headaches; got fatigued quickly. But after a bit I could go farther and farther, eventually being able to go for a hour and a half on technical trails with just water (and electrolytes if needed).
The Point Why should you run on an empty stomach? When most of us run we are relying on carbohydrates, which are stored as glycogen in the muscles, liver, and blood—it is “the primary, quickest, and most accessible fuel source for medium to high-intensity activities.” However, relying on our supply of carbs can only get us so far, as we can burn through them quickly and can only replenish so much per hour. When you are low or completely depleted of carbs you will enjoy the experience of “hitting the wall,” aka “the bonk.”
So if you can train yourself to take in less carbs and instead tap into your bountiful fat reserves you can help to stave off the effects of bonking as well as learn the signs of bonking and how to deal with it.
However, running on an empty stomach can be challenging. And it is not recommended for everyone…at least not right away. Take my advice: start slowly.
- Decrease pre-race food slowly. E.g., If you are eating oatmeal maybe try just a piece of toast or some juice. Eventually ween yourself down to just a little food, and then nothing.
- Keep your runs shorter to begin with. Eventually go longer and farther.
- Always bring water and some form of electrolytes (e.g., liquid or in capsule form).
- Important: always bring some carbs with you just in case (e.g., a Gu or two).
Also, there are a number of problems that can arise from not eating before a run (these can be heighten depending on the fitness of the runner or conditions such as hotter temperatures, lack of hydration, and the type of terrain). For example, you might experience:
These are many of the symptoms that occur when carb depletion is reached, but:
when you teach your body to rely on fat for fuel, your combustion of carbohydrates goes down, thus “sparing” carbohydrates…You become very fatigued when you run too low on carbohydrates. We store only a very limited amount of carbohydrate (glycogen) in our bodies. Compare this with a relatively unlimited supply of fat. Even an athlete with only 6 percent body fat will have enough fat to fuel exercise lasting for many hours. When you use more fat, you generate more energy and your carbohydrate supply lasts longer. 
So next time you head out for a run try and do it on little to no food. There are lots of benefits that can come from such an approach so long as it is done smartly and responsibly.
One last thing: I usually do not run on an empty stomach for more than an hour and a half. Beyond that I find that I need some kind of carbs. There are some endurance athletes who run their long runs (read: 2-4 hours) on nothing but water and electrolytes. While it might be beneficial to extend my empty-stomach runs to 2 hours I haven’t done so yet. Thus, I really wouldn’t consider myself an “endurance” athlete, and therefore I am not speaking to that kind of endurance training.
*I am not a doctor (well, I am not a medical doctor). I am not a sports nutritionist. These are my thoughts and some references to back them up. Be careful doing any new kind of exercise, especially if you have any pre-existing conditions.*