With the arrival of Fall, there are going to be changes in our daily routine. School is back in session and we may find ourselves spending more time inside working and catching up on what didn’t get done during the energetic summer season. Winter will be a few months away, the days will get shorter, and we will naturally be indoors more to avoid the cold that will (eventually) come. More time inside means less time in the sun. Less time in the sun can mean a shift in our vitamin D status…and vitamin D status has implications for our health.

Vitamin D is a fat-soluble vitamin that is important for a number of cellular processes in our bodies. It plays a significant role in calcium balance and bone metabolism. We need vitamin D in order to absorb other nutrients from our gut. Vitamin D helps muscles work, the immune system function, cells differentiate, and it is involved in the balance of hormones such as insulin. Very few foods naturally contain vitamin D. We can get some from fatty fish, for example, or foods specifically fortified with the vitamin. Most vitamin D is obtained from synthesis in the skin. When our skin is exposed to UVB from the sun, precursors of vitamin D are formed that enter a cascade of enzymatic reactions in the liver and kidney to become the active form of the vitamin used by the body.

Because of food fortification, profound vitamin D deficiency is rare in developed countries. Subclinical vitamin D deficiency, however, is very common. Research has shown that fifty percent of the adult population in America is deficient in vitamin D. Some experts have even estimated that up to ninety percent of adults may have some degree of deficiency.

People can become deficient in vitamin D for a number of reasons. Because our primary source of vitamin D comes from sun exposure, levels tend to decline in the winter months when we cover ourselves up and spend more time indoors. People with darker skin are at greater risk of vitamin D deficiency because the pigment in their skin does not absorb as many UV rays. Those who have difficulty absorbing fat (for example, in Celiac Disease, Crohn’s Disease, or pancreatic insufficiency) are more likely to have low levels of vitamin D. Being of older age, obesity, taking certain medications, or being hospitalized are all risk factors for vitamin D deficiency.

Some well-known complications of vitamin D deficiency are osteoporosis and fractures. Associations have also been made between vitamin D deficiency and cancer, infections, autoimmunity, and cardiovascular disease. Those with lower vitamin D levels may have muscle weakness and be more likely to fall. People who are deficient in vitamin D may complain of fatigue or musculoskeletal pain. Those with low vitamin D levels score lower on mini-mental status exams, a test used for evaluation of dementia. Vitamin D status of the mother appears to affect pregnancy outcomes. It has even been hypothesized that the drop in vitamin D that occurs in the winter months may be somewhat responsible for the increase in infections, like the cold and flu, during that time of year.

Vitamin D status can be easily determined by a blood test called 25-hydroxy Vitamin D. The optimal vitamin D level, however, is not known. Deficiency is typically defined as a serum level of less than 20 ng/mL and excess as greater than 100 ng/mL. A reasonable target serum level in most healthy adults is 50 ng/mL. The Recommended Daily Allowance (RDA) of vitamin D for adults is 600-800 IU. Fifteen minutes of daily sun exposure may be adequate for maintaining a normal vitamin D level. If your vitamin D is not optimal, supplementation is easily available over-the-counter. Testing for vitamin D is widely available and usually affordable, but if you are without insurance, have a high deductible, or find that your insurance policy does not cover this test, many accessible commercial laboratories have reasonable cash pricing (approximately $20).

As always, the answer to optimal health does not lie in one vitamin, mineral, food, or practice. Vitamin D is, however, important for the balance of health. Ask your doctor about testing vitamin D as part of your health maintenance evaluation and have the discussion about the optimal level for you and how you can reach it.