: Why am I advised not to eat immediately before exercise? It is commonly said that, after eating, you should wait 2-3 hours before exercising. It isn't recommended to eat and right afterwards to exercise (or swim). But I

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You have two main nervous systems when it comes to energy usage / conservation.

Sympathetic

Increased Heart Rate
Dilation of bronchioles [increased intake /
exhalation of air]
Dilation of Pupils
Vaso constriction in skin [tightening of blood vessels near skin]
Vaso dilation in skeletal muscles [widening of blood vessels near
muscles]
Slowing of peristalsis conversion of glycogen to glucose in liver
secretion of epinephrine and norephinephrine [ hormones to
increase heart rate]

Parasympathetic

Constriction of pupil
Constriction of bronchioles
Slowing of heart rate
Increase secretion of digestive glands

Tabers Cyclopedic Medical Dictionary (21)

When you decide to eat, the parasympatheic nervous system begins conserving energy so that more of it goes to digesting food and other bodily functions.

When you decide to exercise, the sympathic nervous system begins expending energy so that you can do exercise x.

In both of these, the other nervous system is reduced, permitting more energy to be given as needed.

When you both consume food and exercise the body is placed in a tricky position. It can't say no to digesting food, it will begin to rot (gas anyone?) and it needs that food to generate more energy later on. But it can't say no to exercise, that could very realistically kill you in a survival situation.

So it does a compromise, both will function at decreased levels.

This doesn't mean you can't eat food while exercising. As otherwise mentioned, marathoners do consume food. But it's unlikely they will eat complex foods like fat or protein and go straight to simple foods like carbohydrates since it is easier for the body to break it down. Despite simple foods, energy must be used to digest so it makes exercising harder. But the benefit of the energy is more quickly greater then not consuming it. Consuming too much can override this though. 1 There aren't any long term marathoners I know of that eat a thanksgiving dinner prior and expect to make it very far.

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Your body needs time to digest (which depends on how much you've eaten) as it takes a lot of energy. Too much fat or protein can only slow down the movement of foods from the stomach making you feel uncomfortableNHS. It's also important to be well hydrated when exercise, and when digesting food the water is important as part of the digestion process to break down the fat molecules, so you can be more easily dehydrated.

During the digestion process, your body produces more digestive enzymes (stomach acid) and if you'd change your body position during the exercises, you put more pressure on your LES (lower esophageal sphincter). In normal condition it holds the top of the stomach closed, however if you've a weak sphincter or you're pre-disposed to develop GERD (acid reflux disease), the changes in the barrier can cause stomach acids coming up from the stomach into the esophagus (see reflux disease).

Heartburn is usually associated with regurgitation of gastric acid (gastric reflux) which is the major symptom of gastroesophageal reflux disease (GERD). Especially after eating spices, high fat or high acid food.

If you're young and healthy, it is fine, but it all depends on your health condition (GERD predispositions and risk of disease).

Therefore if you're planning an exercise, consider limit your meal or aim for a snack.

See also:

Is eating one's dinner right before going to sleep bad for health?
What are the health tradeoffs between eating while standing vs. sitting vs. in reclining position?

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It takes energy (and blood) to digest the food. So when you eat shortly before exercising, you need the energy at two places (digestion and muscles) and the muscles win.

This does not mean it is bad to eat before (or during) exercise. Some exercises (like running a marathon) requires you to replenish the energy (and the food) but you can't eat anything. You need to time your consumption. (Heavy foods take longer to digest so the time gap needs to be longer).

I have most of this information from a book on sport injuries. But I have found a reasonable reference.