Vitamin D

Did you know how important Vitamin D is?  
Low Vitamin D levels have been associated with increased incidence of Cancer, Heart Disease, Multiple Sclerosis, High Blood Pressure, Depression, Schizophrenia and even Autism.  A lot of the research is based on higher incidence of these health issues in areas with less sunlight in the winter months...a combo of the angle of the sun and the cold weather.
Your body can make Vitamin D, but only in the presence of sunshine. This is affected by: where you live on the planet...if you live in northern USA or anywhere in Canada the angle of the sun is too low for 5 months of the year to allow for Vitamin D synethesis, plus it is probably too cold to have short sleeves on anyway; we spend a lot of time indoors working, learning, playing; we wear sunscreen which inhibits the UVB rays from reaching our skin; the older we are the less we are able make; we are overweight and as Vit D is fat soluble it tends to sit in the fat rather than being available for use by our body...we tend to avoid too much sun for so many reasons that we limit this natural production.
Solutions: Spend 20 minutes in the sun every day or supplement.  Vitamin D is in milk, orange juice (which states added D), fatty salmon, mackerel and other fatty fish.  Vitamin D3 is recommended, also seen as cholecalciferol on the back of the label.  A maintenance level is 2,000U a day for adults, 7,000U for nursing mamas, 5,000U for elderly and heavier adults.  You can even get a blood test to see where your blood levels are to start as the goal is to keep your blood level at 50-60 ng all the times.
Vitamin D deficiency has also been related to muscle power and force and can affect the various ways muscles work!

Reference: Whitcomb, John E.,Vitamin D: An Introductory Guide. AFAA CEU Express. July 2009