Low-impedance headphones are in the range 16 to 32 ohms and high-impedance headphones are about 100-600 ohms. As the impedance of a pair of headphones increases, more voltage (at a given current) is required to drive it, and the loudness of the headphones for a given voltage decreases.
Yes, taking to much Vitamin D can cause some people to get diarrhea. Some others may experience constipation from to much Vitamin D.
Supplements can be a good way to make sure you get enough nutrients, but many vitamins and minerals can be toxic if you take too much. A lot of vitamin A can cause nausea, vomiting, and liver damage, for example. Too much vitamin D can cause weakness, heart rhythm problems, and confusion.