Berberine appears to be effective even against giardia, a tenacious parasite responsible for many serious gastrointestinal infections. Berberine hydrochloride, a purified drug compound sold in tablets, is used specifically to treat diarrhea in the Orient and Mexico. If you find the tablets in your travels, they are worth having, as they have shown activity against a variety of bacteria and even amoebae. Though I have found it quite difficult to find, one of the most historical antidiarrheals is guava leaves, which slow the intestines and are astringent and antibacterial. It's a good idea to also carry something for absorbing bacterial toxins, like pectin or kaolin-pectin compounds.
Some people find the opposite to be a problem, constipation.
If increasing fiber intake doesn't do the trick, I would try senna or cascara sagrada. These are stimulant laxatives and should ONLY be used for temporary relief when needed. If you are crossing more than two time zones, the effects on your biological clock can range from minor restlessness to total disruption. They can include drowsy days and sleepless nights and digestive distress, and can take days to readjust. Recent research has shown that light-therapy can help too, by resetting your body's production of the natural sedative, melatonin.