Most of us have a vague notion that, when we expose ourselves to sunlight, our bodies make vitamin D. Most of us assume that if we spend any amount of time outside, we must be good with our D levels. In reality however, vitamin D deficiency is rampant in America.

The National Health and Nutrition Examination Survey found that 50 percent of children aged one to five years, and 70 percent of children between the ages of six and 11, are deficient or insufficient in vitamin D. It is estimated that 50% of the population of the US is deficient. Researchers have also noted that vitamin D deficiency is prevalent in adults of all ages who always wear sun protection (which blocks vitamin D production) or limit their outdoor activities. People with increased skin pigmentation (such as those whose ancestors are from Africa, the Middle East, or India) are also at risk, as are the elderly. It’s estimated that over 95 percent of US senior citizens may be deficient in vitamin D, not only because they tend to spend a lot of time indoors but also because they produce less in response to sun exposure (a person over the age of 70 produces about 30 percent less vitamin D than a younger person with the same sun exposure).

Vitamin D is really a steroid hormone more than a vitamin. D is a very powerful epigenetic regulator, which simply means that it triggers our genes to express and do what they are meant to. It has been estimated that D plays a hand in regulating as many as 2000 of our 3000 genes. We know that when D is present in appropriate levels in the body, it is antibiotic, anti-microbial, and anti-inflammatory. We also know that D works to inhibit cancer cell growth, and helps prevent autoimmune disease and infections. In autoimmune disease and with cancer, large doses of vitamin D can be beneficial in fighting inflammation, disease progression, and cell division. Vitamin D is also key to the metabolism of calcium in the body. Without adequate levels, bone mineralization cannot happen, and calcium levels cannot be maintained. Severe D deficiency results in a bone-weakening disease called rickets.

When we are exposed to the UVB rays in sunlight, our bodies manufacture vitamin D in the form known as D3, which the liver then converts into an active form for use throughout the body. Many foods and dairy products are fortified with vitamin D, in the D2 ergocalciferol form. Unfortunately, only D3, also known as cholecalciferol, is the active useful form in the body. D2 cannot be converted to D3, and is therefore toxic to the body: it halts the D3 conversion and desensitizes the D3 receptors throughout the body. It’s very important to ensure that any supplemental vitamin D is taken in the D3 or cholecalciferol form.

It’s also important to realize that only bare skin–without sunscreen–can absorb the UVB rays from the sun that stimulate D production. Sunscreen use is clearly very important to prevent aging and skin cancers, but its widespread use is contributing to plummeting vitamin D levels. SPF 8 sunscreen decreases the body’s vitamin D production by 97%; SPF 15 reduces it by 99%. Additionally, sunlight filtered through a window will not trigger D production either. The best recommendation for ensuring adequate vitamin D levels is 20 minutes of sunlight a day, directly on bare uncovered skin. Before 10 am or after 3 pm are the best times to avoid the most harmful rays of the sun.

Most people won’t ever know they are deficient, as there are few overt signs and symptoms of deficiency. However, if you have darker skin, are over the age of 50, live in a northern climate, or you have compromised digestion (meaning you aren’t breaking down the fats necessary to absorb D), you can be almost certain your levels are low. If you are overweight or suffer from depression, it’s also quite likely that you are deficient.

Determining your vitamin D levels can be a bit tricky, even though there is a simple blood test. It has been shown that the standards used in laboratory D testing vary wildly, and it is hard to standardize results across companies. To complicate matters further, some people convert their D differently, such that the standard D blood test (known as OH-25) will show low levels, even though their active levels might be adequate or high. Even so, blood tests remain the best assessment tool.

If your blood tests show you need to increase your D levels, the best supplements consist of D3 drops suspended in an oil (usually olive or sesame), that you drip directly into your mouth daily. Because D is a fat-soluble vitamin, it needs to be ingested in the presence of fat in order to be absorbed and used. Oil-based drops are an easy solution. As with any supplement, consult your physician (or nutritional therapist!!) to determine what’s best for you.