I'm not sure why this is happening, but it is common. Dairy is certainly a possible culprit but you say you've cut it out before you run.
Here are some additional tips.
1. Avoid caffeine and warm fluids. They can increase intestinal motility.
2. Limit high-fiber foods. You may want to track the food you eat the day before too.
3. Avoid any other foods that you know upset your stomach or give you gas.
4. Stay well hydrated (dehydration slows digestion of food and cause stomach problems). I recommend 16-20 ounces of water 60-90 minutes before you run, and 8-10 ounces every 20 minutes during the run. Importantly, avoid replacement beverages like Gatorade since the sugar can increase motility.
6. Avoid aspirin, ibuprofen, products with sorbitol, or large doses of vitamin C. All of these can cause diarrhea or otherwise upset your stomach.
7. Try a bland food like half a bagel and nothing else 30-45 minutes before you train.
8. Pepto Bismol can help too.
9. If possible, empty your bowel before you run.
If it continues you should consult your doctor to rule out any medical problem.
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