Around the 1700s, a previously rare disorder known as Rickets — which causes soft, weak bones in childhood — became very common in European cities. Armed with the knowledge that certain foods could prevent diseases like beriberi and scurvy, scientists considered a dietary link for Rickets. They discovered that Cod liver oil, and surprisingly, sunlight could prevent and cure the disease. It was not until the 1930s that vitamin D was identified as the curative compound.
Vitamin D is found naturally in very few foods (Cod liver being one of them); the primary natural source for vitamin D is synthesis by the skin in response to ultraviolet (UV) light exposure. It is estimated that sun exposure to the face and arms for even a short period is equivalent to consuming approximately 200 IU of vitamin D per day. Despite the efficiency of this processes, many of us do not get enough regular sun exposure to meet the recommended daily allowance (RDA) for vitamin D. The RDA is the daily amount of a particular substance that would meet nutritional requirements for most (97.5 percent) healthy people.
Because it can be difficult to get enough vitamin D through natural sources, many foods are fortified with the vitamin to prevent deficiency. These fortified foods (like cereals and dairy products) are an important source of vitamin D. If you are not able to get adequate vitamin D through dietary sources (including fortified foods) and sun exposure, you may benefit from supplements to help meet the RDA, and prevent deficiency.
So what is the RDA for vitamin D?
600 IU/day for adults under age 70 years.
800 IU/day for adults over 70 years.
Is it possible to get too much vitamin D?
The toxic level of vitamin D has not been clearly defined.
According to the institute of medicine (IOM), the safe upper limit for vitamin D is 4000 IU per day for healthy adults, including pregnant and lactating women.
Vitamin D deficiency that is severe enough to cause Rickets or osteomalacia (softening of the bones) is now rare in most populations. More mild vitamin D deficiency (vitamin D level below 20 or 30 ng/mL), however, remains common. Having low vitamin D results in an increased risk for osteoporosis, and for fractures and falls in older adults. Prevention and treatment of vitamin D deficiency can help reduce the risks associated with low vitamin D. It has been suggested that low vitamin D may also contribute to problems like muscle weakness, cancer, autoimmune disease, and cardiovascular disease. Further research is needed to better understand the role of vitamin D in these non-skeletal disorders.
Factors that may increase your risk for vitamin D deficiency include:
Older age: as we age, our skin is not able to synthesize vitamin D as efficiently; dietary intake and sun exposure is more likely to be inadequate.
Limited sun exposure: many people do not spend much time outdoors, or wear protective clothing or sunscreen when they are outdoors.
Dark skin: higher amounts of melanin reduce the ability of the skin to synthesize vitamin D from sunlight.
Medical conditions that could impair fat absorption (like inflammatory bowel disease, history of gastric bypass surgery).
Certain medications (like phenytoin, or long term, high dose glucocorticoids)
Strict vegan diet
If you are concerned that you may have low vitamin D or may not be getting enough vitamin D, talk to your healthcare provider. Screening for vitamin D deficiency is not generally felt to be necessary for average risk adults. There is, however, a blood test that can be used to diagnose vitamin D deficiency, and testing is recommended for those at high risk. If you are diagnosed with vitamin D deficiency, your provider will advise you about treatment with over-the-counter or prescription supplements.
Information provided in this article sourced from: UpToDate, WebMD, National Institutes of Health, Mayo Clinic, U.S. Preventive Services Task Force, Nature Structural & Molecular Biology.
Elizabeth Gudmand is a nurse practitioner working with Dr. Sid Pani, at Emerson Practice Associates, at 490 Boston Post Road, Sudbury, MA.