Studies have shown that 80-90% of people north of Atlanta are deficient. Why is this important? Sure, it helps with bone and calcium metabolism, but there is a lot more to the story. Vitamin D is involved in nearly every metabolic process in the body. Cardiovascular health, heart failure, hypertension, dyslipidemia, fatigue, inflammation, blood vessel lining function, platelet function, and many more functions are being discovered and reported every day.
If you want your level checked, you should check a 25-Hydroxy Vitamin D (25OH Vitamin D). Under 20 is considered severely deficient. A level 20-30 is deficient. They have found that most problems begin to occur at a level of 40 or less. So, while 30 or more is considered normal, we should aim for a target of over 40.
Vitamin D comes from the sun. Humans try to avoid the sun at all costs now with clothing, sunscreen and working indoors. So we need to be supplemented.
It’s easy to replace Vitamin D with very small pills that you can take daily or weekly. Normally we start supplementing people with doses like 50,000 units one a week for 12 weeks, then put them on a maintenance dose of about 15,000 per week. You can purchase over the counter pills of 5000 units. Take three per week. It's very difficult to reach toxic doses of Vitamin D, so don't worry about this.
Fifteen minutes of bright sun exposure is the equivalent of taking 20,000 units. Getting Vitamin D from the sun converting it in our skin is natural and you can never get toxic this way. But sun exposure makes you look older, harms your skin, and results in skin cancer.
Unfortunately, the USRDA thinks that we only need 400-800 per day. These numbers are being revised and looked at more closely. Most Vitamin D experts would agree that we need at least 2000 per day. As a reference, an 8oz glass of "Vitamin D" milk usually contains 200 units of Vitamin D. Future, USRDA requirements for children will likely be 800 per day, and adults probably 2000 per day.