Found in virtually every tissue and cell in the body and enabling many biological functions, vitamin D comes from three sources: food, supplements and the sun. It's also one of four fat-soluble vitamins, the other three being vitamins A, E and K.
Vitamin D has two forms that are relevant to human health: vitamin D2 (produced by some plants) and vitamin D3 (synthesized when skin is exposed to ultraviolet rays). Its primary role is in the regulation and absorption of calcium and phosphorus. Rickets in children and osteomalacia, osteopenia and osteoporosis in adults are the most well-established consequences of vitamin D deficiency.
In addition, hundreds of studies over the past 15 years underscore interest in vitamin D's potential 
role in risk reduction of other diseases, including some cancers, heart disease, stroke, diabetes, autoimmune diseases, infectious diseases, neurocognitive disorders and mortality. While much of this research is ongoing and may provide more definitive answers in the future, the current general consensus is that — with the exception of bone health — the relationship between vitamin D and other health conditions is still unclear.
However, there is little consensus as to what constitutes vitamin D deficiency and optimal intake.
What Is the Recommended Intake of Vitamin D?
Vitamin D status is assessed through a simple blood test and measured as nanograms of 25-hydroxy vitamin D per milliliter of blood, or ng/ml. Depending on who you ask, sufficient levels range between 20-50 ng/ml and 40-80 ng/ml, while recommended intakes for adults are as low as 600 IU per day and as high as 5,000 IU per day.
Where experts do agree is that most American adults fall short in the vitamin D department. At particular risk are overweight or obese individuals or those who have undergone gastric bypass surgery, older adults, people with limited sun exposure and people with inflammatory bowel disease or other conditions causing fat malabsorption. Identified as “an underconsumed nutrient” in the 2015-2020 Dietary Guidelines for Americans, achieving adequate vitamin D intake from food alone may be challenging. There is a relatively limited number of foods rich in vitamin D — the best sources being fatty fish such as salmon, sardines, trout, mackerel and tuna, in addition to fish liver oils. In fact, most dietary vitamin D comes from fortified foods such as milks, juices and breakfast cereals.
While some sources advocate sensible sun exposure for vitamin D production, exposure without sunscreen to block UV rays poses an increased risk of skin cancer — although this potentially could change pending the release of vitamin D-friendly sunscreen products.
In the meantime, supplemental vitamin D may be necessary to fill nutrient gaps. Vitamin D3, which is made in part from sheep's wool, is the preferred form because it is more efficiently converted in the liver to the form the body uses than vitamin D2 (which is commonly found in medical prescriptions but is harder to measure in blood). When counseling vegan clients, however, RDNs should recommend vitamin D2 supplements. Supplements can be taken daily, weekly or monthly, and many health-care providers recommend starting with a higher dose, then reducing after a month or two and continuing with a maintenance dose. Multivitamins also may provide some vitamin D — typically 400 IU, but with a range between 200 IUs and 10,000 IUs.
Vitamin D Toxicity
Whereas safety concerns often abound with high levels of fat-soluble vitamins, vitamin D appears to be the exception. Vitamin D toxicity, one of the rarest medical conditions, is typically due to extremely high doses of 50,000 IU daily but can occur at as low as 10,000 IU per day. The tolerable upper intake level is 4,000 IU for adults. “Too much vitamin D can increase calcium levels, which can lead to calcification of kidneys and blood vessels, high blood levels of calcium and phosphorus usually occurring when serum levels reach 200-250 ng/ml,” says Michael Holick, PhD, MD. “When toxic levels are reached, consequences disappear once the supplement is discontinued unless the toxicity has persisted for long periods of time.”
Kathleen Zelman, MPH, RDN, is the nutrition director of WebMD.
