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.
Vitamin D has numerous effects, including regulating the body's calcium levels by increasing the absorption of calcium from the gut. Taking large doses of vitamin D supplements can cause toxicity, called hypervitaminosis D. The main effect of this is hypercalcemia -- high calcium levels. Symptoms of hypercalcemia include nausea, loss of appetite, excessive urination, itchiness and weakness.