Running on an empty stomach is also not advisable, as the low levels of glucose in the bloodstream mean you can get light-headed very quickly.
Both cases show that as recreational athlete, the run is not all fun. You should be doing some hard thinking about your diet.
But when should a runner eat, and how much - and most importantly, what?
BEFORE DOING SPORT:
You should never head off on a run or go to the gym on an empty stomach, otherwise your blood-sugar levels could fall dangerously low, says Herbert Loellgen a sports physician and professor.
However, you should leave some time between eating and doing sport, so the body can digest the food. It's different for everyone - some people only need half an hour between having a meal and heading to the gym, others need two hours.
Even if you do have enough time to fit in a meal or a snack, you should avoid fatty food on gym days. These take longer to digest and even eating something greasy during the day can affect you if you are doing sport in the evening, says sports college instructor Hans Braun.
If you feel like you need an extra energy boost, try having a small snack. Fruit, such as a banana or some raisins, is always a good option, as it will raise your blood sugar levels quickly.
If you're taking part in a longer leisure-exercise session such as an extended cycling tour, the glucose molecules (glycogen) in muscles and the liver need to be properly tanked up, explains Antje Gahl from the German Nutrition Society.
Therefore, you should always make sure to get enough carbohydrates beforehand.
If you haven't eaten enough, you might have problems with coordination and are more likely to twist your ankle, for example, warns Loellgen.
Drinking is just as important, even before sport. You can drink a bit more than you think you need and as a result the body cells will already be supplied with liquid.
When doing sport, people sweat - some more than others. The loss of fluid is best balanced by water. There are no fixed rules on how much, but if you sweat a lot, you should drink more.
The fluid requirement can be estimated with a quick glance at the scales. You should weigh yourself before and after doing sport. According to the amount and intensity of the session, you will usually weigh less than normal after you are done. However, the weight loss should not be extreme.
"Two-per-cent weight loss is fine," says Braun. For example, if you weigh 80 kilograms, a weight loss of 1.6 kilos is OK. If you lose more weight than this during a sports session, next time you might want to think about drinking more to compensate.
AFTER DOING SPORT:
If you've been doing light sport, you can wait a bit before having a bite to eat. However, if you've been training intensively for an hour or two, you should not wait too long. After around half an hour you should eat carbohydrates to replenish muscle glycogen, says Loellgen. You will also need to top up on protein.
"The functioning of the muscles depends on protein." They not only help to build muscle but keep existing muscles flexible and powerful.
Protein shakes can help you get enough protein, but you should always look carefully at the list of ingredients beforehand. Ideally they should contain amino acids such as leucine, isoleucine and valine.