Over the July 4th weekend, my non-physician husband with a history of skin cancer tried to justify not wearing sunscreen in order to get some vitamin D. My husband, of course, has no idea how much vitamin D he needs or why, and I suspect he is not alone.
Why do we need vitamin D?
The easy answer is for bones. Vitamin D facilitates absorption of calcium and phosphate, which are needed for bone growth. Without sufficient vitamin D, bones become brittle (in children this is called rickets and in adults it is called osteomalacia) and break more easily. Vitamin D is likely beneficial for other parts of the body as well; studies suggest an overall decrease in death in addition to reductions in blood pressure, respiratory illnesses, cancer, heart disease, and depression. Adequate vitamin D during pregnancy also appears to reduce the chances of having a low-birthweight baby. However, most of the effects of vitamin D have not been studied in controlled settings. Studies looking at the benefits of vitamin D on various conditions are ongoing. A small study published earlier this month suggested that high-dose vitamin D could reduce redness and inflammation following sunburns, but the dose tested far exceeded the recommended daily dose of vitamin D.
How do I get the vitamin D I need?
Don't run out to the drug store to buy vitamin D pills just yet. Your body produces vitamin D when the skin is exposed to sun, and it is estimated that most people need 1,000 to 1,500 hours of sun exposure throughout the spring, summer, and fall to obtain the necessary amount of vitamin D. Vitamin D deficiency is very common and on the rise. This is mostly due to vigilant sun protection, since sunscreen with SPF 30 reduces vitamin D production by 95%. Of course, as a dermatologist I am not advocating for prolonged sun exposure, but small amounts can go a long way, as the skin produces vitamin D that can last at least twice as long the vitamin D you take in through foods or supplements. Vitamin D can also be obtained through other sources, including fatty fish (such as tuna, mackerel, and salmon), foods fortified with vitamin D (such as dairy products, soy milk, and cereals), beef liver, cheese, and egg yolks. Of course, vitamin D supplements are needed for people most at risk for deficiency, including breastfed infants, older adults, people with limited sun exposure, darker skinned individuals, and overweight individuals.
Why shouldn't everyone just take vitamin D supplements?
Too much vitamin D can be harmful, including (ironically) increasing fractures, falls, and kidney stones, and can be toxic by causing excessive levels of calcium. Although not proven, high vitamin D levels have been associated with prostate cancer, pancreatic cancer, and mortality. A recent study found that use of high-dose vitamin D supplements increased over a 15-year period.
So how much is too much? For adults, toxic effects increase above 4,000 IU per day. The recommended dietary dose of vitamin D is 600 IU each day for adults 70 and younger and 800 IU each day for adults over 70. To put this into perspective, 4 ounces of cooked salmon contains approximately 600 IU of vitamin D.