Patients commonly ask, "Why can't I get vitamin D from food? I drink milk and eat fish."
They're absolutely right: both vitamin D and some oily fish contain vitamin D. However, it's a matter of quantity. An 8 oz. glass of milk contains 100 units of vitamin D (at least it's supposed to; this is not always true). A serving of oily fish like salmon or herring may contain up to 400 units. Thus, if you ate fish three times a day like the Eskimos or the Inuit, you might obtain sufficient vitamin D to prevent the broad and alarming spectrum of phenomena associated with deficiency.
I suspect that most people don't want to eat fish three times a day, nor drink the 20 to 50 glasses of milk per day that would be required to obtain a truly healthy quantity of vitamin D.
The vocal and outspoken Dr. John Cannell of the Vitamin D Council (www.vitamindcouncil.com) has written eloquently on the potential relationship between influenza and vitamin D deficiency. He and his co-authors on a recently published paper point out that the peculiar and unexplained seasonality of influenza corresponds to vitamin D levels. Read his eloquent discussion in Medical News Today at http://www.medicalnewstoday.com/medicalnews.php?newsid=51913.
In the article, Dr. Cannell explains:
"The vitamin D steroid hormone system has always had its origins in the skin, not in the mouth. Until quite recently, when dermatologists and governments began warning us about the dangers of sunlight, humans made enormous quantities of vitamin D where humans have always made it, where naked skin meets the ultraviolet B radiation of sunlight. 
We just cannot get adequate amounts of vitamin D from our diet. If we don't expose ourselves to ultraviolet light, we must get vitamin D from dietary supplements...Today, most humans only make about a thousand units of vitamin D a day from sun exposure; many people, such as the elderly or African Americans, make much less than that. How much did humans normally make? A single, twenty-minute, full body exposure to summer sun will trigger the delivery of 20,000 units of vitamin D into the circulation of most people within 48 hours. Twenty thousand units, that's the single most important fact about vitamin D. Compare that to the 100 units you get from a glass of milk, or the several hundred daily units the U.S. government recommend as “Adequate Intake.” It's what we call an “order of magnitude” difference.
"Humans evolved naked in sub-equatorial Africa, where the sun shines directly overhead much of the year and where our species must have obtained tens of thousands of units of vitamin D every day, in spite of our skin developing heavy melanin concentrations (racial pigmentation) for protecting the deeper layers of the skin. Even after humans migrated to temperate latitudes, where our skin rapidly lightened to allow for more rapid vitamin D production, humans worked outdoors. However, in the last three hundred years, we began to work indoors; in the last one hundred years, we began to travel inside cars; in the last several decades, we began to lather on sunblock and consciously avoid sunlight. All of these things lower vitamin D blood levels. The inescapable conclusion is that vitamin D levels in modern humans are not just low - they are aberrantly low."
Like Dr. Cannell, I am absolutely convinced that vitamin D deficiency plays an important role in a number of illnesses, including coronary disease. The more we mind our patients/participants vitamin D status (blood levels of 25-OH-vitamin D3), the more easily we gain control over LDL cholesterol, pre-diabetic patterns, blood pressure, blood sugar, and coronary plaque. In fact, I am becoming rapidly convinced that vitamin D deficiency is an extremely important coronary risk factor.
Because I live in Wisconsin (bbrrrrr!) where seeing the sun is a cause for celebration and sun exposure is possible three months a year, I take 6000 units per day vitamin D. This is the amount necessary to raise my blood levels into the true, physiologic range of 50-70 ng/ml. My wife takes 2000 units per day, and each of my kids takes 1000 units per day, though I believe that my 14-year old son (my size now) should take more. We'll judge by blood levels.
If there is a little-known secret to reducing heart scan scores, vitamin D is that "secret".
To read more from Dr. Cannell or to subscribe to his free and very informative newsletter, go to Vitamin D Council