"The Core 4" - Vitamin D

Vitamin D
  Also known as the "sunshine vitamin," vitamin D is important for building strong bones and teeth because it is necessary for the absorption of calcium. Not only does it help build and keep your bones strong, recent studies also show that it plays a major role in healthy cell development and helps keep your immune system strong. While you may think that being outside in the sun and drinking milk fortified with vitamin D is enough, it actually isn't. The problem is that most of us wear sun protection when we're outside and it blocks our vitamin D production. That's fine, because skin cancer is far scarier than having low levels of vitamin D, but if you want to get it from the sun your skin needs to be unprotected. Foods fortified with vitamin D even don't have the therapeutic levels we need. Living in a northern climate, I know the winters are brutal and there is no way to get the sun exposure needed during those chilly months. These reasons are why studies are showing that around 50% of the general population and estimates of 95% of the US senior citizens are deficient. 
  Getting your levels checked is relatively inexpensive and I recommend everyone at least consider it. If you find you are deficient, the best way to raise your levels naturally is from the sun. Fifteen minutes of exposed skin a day can go a long way to boosting your levels. Many of us work when the sun is at its peak, making this difficult to do on a daily basis. That's when supplementation comes in. Around 8,000 IUs of vitamin D3 daily can make up for those days or months you lack the proper sunlight exposure. Vitamin D is very important to our overall health and well-being. Making sure we maintain proper levels is key to not only strong, healthy bones, but also the prevention of chronic diseases. 

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