Recent research confirms that low blood levels of Vitamin D are clearly linked to higher rates of influenza, many types of cancer as well as diseases such as coronary disease, hypertension, adult onset diabetes, osteoporosis, MS, migraine headaches, and depression to name some.  Optimum blood levels of Vitamin D are linked with significantly lower rates of all of these diseases.  The benefits from optimum levels of this vitamin are nothing short of astonishing.  A well done study indicates that if everyone in the world had optimal Vitamin D blood levels, there would be 1,000,000 fewer deaths from cancer every year.  We used to think that 400 IU per day for an adult was excellent, but the optimal dose turns out to be around 5,000 IU per day.  The dose for children is 1,000 IU under one year of age, 1,500 IU for children one to four years, 2,000 IU for children four to ten years, and 5,000 IU for children over ten years old.  It was found that Vitamin D can also be used for an acute problem, which brings us back to cold and flu season.  Someone getting a cold or the flu could take 20,000 to 25,000 IU per day for five days to reduce the severity of the illness.  Yes, Vitamin D can be toxic, but a person would have to take 40,000 IU per day for weeks to develop a toxic level.  The only form you should supplement is Vitamin D3, and it is best taken with a meal.  Blood levels can easily determine if you are in the optimum range.  Make sure you have been on Vitamin D for at least three months before getting a blood test.