"I count vitamin D as secondary only to wheat elimination as among the most powerful strategies I have ever witnessed to regain ideal health. While a wheat-free diet is richer in vitamin D from eggs, mushrooms, fish and meats, most people nonetheless remain deficient.
And because we lose the ability to activate vitamin D as we age, supplementation is necessary for most people to achieve desirable blood levels of vitamin D. Having restored vitamin D in thousands of people over the past six to seven years, I have found most people require 4000-8000 units per day to achieve a desirable 25-hydroxy vitamin D level; I aim for 60-70 ng/ml (150-180 nmol/L). (Tablets are poorly or erratically absorbed.)
The list of potential benefits of restoring vitamin D include relief from winter “blues,” heightened mental clarity, elevation of mood/relief from depression, improvement of memory; increased bone density/protection from osteoporosis and fracture; increased HDL cholesterol, reduced blood sugar/enhanced insulin sensitivity, reduced blood pressure; enhanced athletic performance; protection from colon, breast, prostate and other cancers; among many, many others." - Dr. William Davis, M.D. and author of the book, "Wheat Belly"
P.S. Vitamin D can cut your risk for colds and the flu by up to fifty percent.