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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 haven't really understood why that is. Does this bad behavior affect me immediately (if I'm in robust health) or does it affect me in the long term? Αnd how?

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    Close Voters: Surely there are good answers to this question? Surely someone knows what happens if you exercise too soon - it can't all be opinionated can it? – Tim Apr 2 '15 at 20:10
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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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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.

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    There are a lot of marathoners that eat regular food items during events, as do half and full ironman athletes (Events of 70.3 and 140.6 miles respectively). Additionally, there is an entire industry around quickly digested items for endurance athletes (Gu, Stinger Waffles, etc). What to eat as well as when to eat it is extremely variable from person to person. – JohnP Apr 2 '15 at 18:21
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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:

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    This is an example of what I talk about above. It's a basic description of the digestive process, with "you could", and "can cause", finalized with a summation of "if you're healthy, probably it won't affect you much", which is a complete guess. – JohnP Apr 7 '15 at 0:29
  • If the best answer is simple information about possibilities, then it should be vote to close as either too broad or opinion based. And the other question that you linked has nothing at all to do with eating before exercise. – JohnP Apr 7 '15 at 14:53
  • @AkshayVasu This is not another question. Please read the answers more carefully while reviewing them. – kenorb Apr 20 '15 at 9:31
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    @kenorb pardon me it was by mistake, and it won't happen again. – Akshay Vasu Apr 20 '15 at 9:57
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    I like this answer, +1. I don't think that long-term health risks are the only concern here (although you addressed those as well). I've tried both swimming and yoga less than 3 hours after a (lunch-sized) meal (being young and in good health) and I did both things once and never again, because I felt all the physiology you described above. I simply take the sentence "it all depends on your health condition" as a disclaimer in line with the rules that we don't give personal health advice. If you are in good health you may not have serious consequences, but the exercise still won't feel good. – Lucky Jul 23 '15 at 22:30

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