This was an interesting question posed to me, here's my answer;
The quick answer is, it all depends. If one has not eaten in 5 to 6 hours then it would probably not be a good idea to go running. But then this would all depend upon many things. What was the person’s last meal, are they a seasoned/conditioned runner, do they suffer from any medical problems most notably diabetes, heart disease, high blood pressure.
If the person is already a conditioned runner and does not have any medical problems for which they take any meds for, they could most likely run in the morning before having anything to eat. I have patients in my practice who run first thing in the morning without any adverse effects. But it needs to be stressed they are in shape and are not amateurs to the running world.
Even if the person does take meds for certain medical problems, as long as they gradually built themselves up to the level they are at, they’ll be fine too. It’s all about consistency and continuity.
One very, very important side note. I have had many a patient over my nearly 20 years in the practice of medicine who I have ordered stress tests on who responded back, “Well, I don’t need a stress test, I run 20 or so miles a week; surely if something was wrong with my heart I would know it.” My response is always the same, “When you are running, you are not being monitored like you are on a treadmill in the cardiologist’s office.” And yes, if I had a nickel for every runner that I found out had blockages in their coronaries, due to my gentle but firm persuasion to have a stress test…well, we all know the rest
Now, if one is just starting out on a running routine, they would want to have had a meal at least 2 hours prior to the run. This meal should, interestingly enough, consist of more protein/fat/cholesterol with minimal carbs (carbohydrates). The carbs, contrary to popular belief, will actually make one more tired prior to a run. The carbs in the diet will be stored first as glycogen (muscle and liver) and then as fat, and will also cause a shift in the ratio of sleep inducing amino acids causing the patient to experience the classic ‘food coma.’ Another biochemical fact is that after the glycogen stores are used up, the body has to rely on free fatty acids for fuel (fuel in the form of something known as ATP, our life molecule, we really don’t ‘burn’ anything) and this is what the skeletal muscles and heart will use to function.
Of course the converse of this question would be “Is it OK to run on a full stomach” and the answer would be a resounding no. This is because as we exercise we will shunt blood from the gastrointestinal track to the muscles and this will slow down the digestive process and could cause abdominal cramping, pain and diarrhea.
Dr Jim and Bones Fighting the Fight against the Medical Establishment's Dietary Ignorance!