Vitamin D has been a media darling for the past decade.  It has been attached to bone density, myriad cognitive changes, endless lab requests for 'levels', and autoimmune disorders. And much of this stems from we northerners inability to acquire enough through our skin because of the simple concept known as geography. Aside from intake of fortified foods or a supplement, the skins ability to convert it into an active form that the body can use largely comes down to where you live as latitude and longitude, along with time of day and time spent in the sun, determine the intensity and how much vitamin D is created for use.  Here's what we think we know about this 'sunshine vitamin'.

Vitamin D is referred to as a vitamin, but it is actually a hormone.  Among the many roles it plays it is essential for the absorption of calcium. That is why you often see vitamin D and calcium sold together as one supplement.  We also know that those who have very low vitamin D levels are more likely to have quicker bone loss as they age, as well as fractures.  And the reason vitamin D lowers fracture risk is because it helps build muscle. We know this because muscle biopsies studies have shown vitamin D receptors suggesting that it at the very least plays an important role. With stronger muscles - or an increase in lean muscle mass - people are less likely to fall.  And falls are what most often cause fractures. 

Deficiencies have also been associated with chronic inflammatory diseases, e.g. lupus, Type 2 diabetes (T2D), rheumatoid arthritis (RA), multiple sclerosis (MS), and more recently some mental illnesses including depression and bipolar disease.

But too much of a good thing is often just that:  too much, and that means risk is associated with taking in vitamin D through supplementation.  Vitamin D is one of four fat soluble vitamins, and these vitamins are stored in and around the vital organs and in fat. That is why people who are obese have, on average, lower vitamin D levels as it stays absorbed within their fat and is not available in their blood.  If you take more than that is recommended we see an increase in blood calcium. This elevated level, referred to as hypercalcemia, increases your risk of kidney stones.  So how much is too much?

The International Osteoporosis Foundation recommends 800-1000 IU (International Units) per day and that includes your intake through food.  So, if you live northwards of Atlanta, GA then you are likely at risk simply because six months of the year the earth is tipped on its axis away from the sun, and that means the 'sunshine vitamin' is barely accessible because of low intensity.  You see, the skin has remarkable receptors on it that chemically alter the suns ultraviolet-B radiation by stimulating the production of what is commonly known as vitamin D3.  From there it enters the circulation and is transported to the liver and on to the kidneys.  If you wear sunscreen, stay covered up, are elderly and are largely housebound, or live in a northern latitude, then you may likely have a deficiency that could easily be addressed through changing your intake by increasing your consumption of fortified foods e.g. milk and yogurt, or integrating more swordfish and salmon into your diet. A single supplement of 600 IUs with your meal - when it would be best absorbed - may make sense for many who have limited intake of those foods.  

Here is a list of those foods with notable amounts of vitamin D

Cod liver oil, 1 tablespoon 1,360 IU

Swordfish, cooked, 3 ounces 566 IU

Salmon (sockeye), cooked, 3 ounces 447 IU

Tuna fish, canned in water, drained, 3 ounces 154 IU

Orange juice fortified with vitamin D, 1 cup (check product labels, as amount of added vitamin D varies) 137 IU

Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup 115-124 IU

Milk, nonfat, reduced fat, and whole vitamin D-fortified almond or flax milk, 1 cup 115-124 IU

Yogurt, fortified with 20% of the DV for vitamin D, 6 ounces 80 IU

And what about those other six months of the year in New England? Well, it is likely that 20 minutes twice a week in the sun sans sunscreen will allow that 'sunshine vitamin' to do what it does so naturally.  (Just remember to apply sunscreen afterward.) And for those who do learn of very, very low Vitamin D levels from their lab work?  You will benefit from large doses taken weekly for a defined period of time - like six to eight weeks - and then have it reassessed.

U.S. Department of Agriculture, Agricultural Research Service. 2011. USDA National Nutrient Database for Standard Reference, Release 24. Nutrient Data Laboratory Home Page, http://www.ars.usda.gov/ba/bhnrc/ndl.

For @PAC