How to prevent cramps when exercising?

It's hard to tell what the cause of the cramps really is....are you doing strength training type workouts, or cardio? If you're doing cardio, it's probably due to a lack of oxygen to your muscles and not so much that you ate food earlier. You don't have to stop, just lower the intensity of whatever you're doing and take longer deeper breaths until the pain subsides. The stronger you get and the higher endurance you build, the more you can push yourself. Like I said before, without knowing what you're doing it's hard to guess what's causing them, but that is one possibility.

Do you stretch properly and do some light warm up work? Also, when you drink water during exercise take regular, small sips rather than gulping down loads of water in one go.

Also great points...stretching after a warmup is an awesome tip, and try to be well hydrated before your workout as well as sipping some water throughout your exercises.

When I had that problem, my trainer told me to eat a banana a day - I do that right after working out with some protien and no more cramps. It can also be caused by gas so as someone previously mentioned - small sips of water.

The bananas are a huge help with the side cramping. It's the potassium in them that does the trick. If I drink to much water before I run I have that problem so the sips is a great idea.
Proper streching BEFORE and AFTER your workout is also a must to avoid pulling muscles and to helping them grow.
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