The OP is right that Vitamin D deficiency is common, and D3 supplementation is a good idea for almost everyone. It's super-easy, too; if you don't like taking i

The OP is right that Vitamin D deficiency is common, and D3 supplementation is a good idea for almost everyone. It's super-easy, too; if you don't like taking it every day, you can actually take a big dose once a week because it's fat-soluble. (Vitamin D's fat-solubility means that there's a theoretical possibility of overdose, but in practice this almost never happens. You'd really have to go nuts with the dosage to be at risk.) Last year, prior to beginning supplementation, I had my blood levels checked for Vitamin D (per the recommendation in Tim Ferriss's 4-Hour Body). I was amazed to discover that my levels were at the low end of normal, despite a diet rich in Vitamin D and regular direct sunlight (including frequently wearing a "suntan shirt" to let in the necessary UV). (I live in Pasadena, which is sunny year-round.) After supplementing for a couple of months, a second blood test confirmed that my blood Vitamin D levels were in the ideal range.