Nylon ukulele strings are rather durable and are less susceptible to damage from oils and sweat than metal wound strings which are often found on guitars. How often you need to change your strings depends on how much you play it. This could be every other month to about every six months. Restring your strings, if they need it, well before performances as the string will stretch considerably when your uke is first strung.
A piece of toast with peanut butter. Peanut butter should not cause diarrhea. In fact, the Cleveland Clinic recommends adding peanut butter to your diet if you develop diarrhea. If you are allergic to peanuts, however, diarrhea may occur after you eat peanut butter.
If you discovered that peanuts and peanut butter cause constipation while on your elimination diet, it may be best not to reintroduce them. If you really enjoy peanut butter, you could try to have very small amounts a few days apart to test your tolerance to moderate quantities.