Most people are vitamin D deficient, and so should be taking supplements. Vitamin D is synthesized by sunlight hitting our skin, but in our modern lifestyles and places we live, we hardly get any direct sunlight. Vitamin D deficiency is the reason that there was such strong evolutionary selection pressure to lower melanin production in people's skins as early populations of people migrated away from areas with consistent sunlight - the danger of sun damage to skin was far outweighed by the need to produce enough vitamin D. And while you can get some of it form food, it's not nearly enough (vitamin D infused milk isn't enough!) You should be taking the large 5000IU (125 mcg) doses every day, and even that is on the low side of what your body would normally produce if you worked outside for a living.