Have you ever tried running after having a large breakfast? Chances are you quickly realized that the decision to eat may not have been the wisest idea. The combination of demanding exercise plus food often results in not so great effects like cramping or worse, a strong feeling of nausea. However, it’s also not a good idea to hit the pavement on an empty stomach, either. So, what should you do?

Provided you didn’t overdo it on a meal the night before, the body’s level of glucose (the energy needed to nourish the muscles) is likely low. If that’s the case, you may be able to squeeze in an easy to moderate intensity run under an hour. If you’re thinking about extending it beyond an hour or upping the intensity while on an empty stomach, you may begin to experience effects such as lethargy and perhaps even lightheadedness.

Having a meal before an intense run ensures glucose levels are up so that the muscles have the “fuel” reserves to endure the heartier run. As mentioned above, however, this could also lead to not so pleasant side effects.

What you may wish to do is have a snack before heading out on a run as it will help with fueling your workout and it will also assist with greater calorie burn versus running on an empty stomach. Besides the aforementioned side effects, insufficient fuel for your run may also increase cortisol, a stress hormone that increases your odds of becoming ill by briefly suppressing the immune system.