by Katelyn Castro
There is always one nutrient that seems to linger in the media for a while. Lately, vitamin D has been the lucky winner! Considering that over 40% of Americans are vitamin D deficient, according to the National Health and Nutrition Examination Survey (NHANES), it’s worth taking a closer look at vitamin D.
Depression, cancer, heart disease, and type 1 diabetes are some of the many health conditions that have been linked to vitamin D deficiency. While it is too soon to point to vitamin D as a cure-all, this vitamin may be more important for our health than previously thought—especially during the winter months in New England!
Why is Vitamin D Important?
Vitamin D is most often known for its role in bone health, increasing calcium absorption and helping with bone mineralization alongside calcium and phosphorus. Historically, rickets in children and osteoporosis and bone fractures in adults have been the most common signs of vitamin D deficiency.
As a fat-soluble vitamin and a hormone, vitamin D is also involved in many other important metabolic processes. Did you know vitamin D activates over one thousand genes in the human genome? For example, vitamin D is needed for protein transcription within skeletal muscle, which may explain why vitamin D deficiency is associated with poor athletic performance. Vitamin D also regulates blood pressure by suppressing renin gene expression, supporting the possible relationship between vitamin D deficiency and risk of heart disease. Additionally, vitamin D status may alter immunity due to its role in cytokine production. Studies have found that vitamin D deficiency is associated with upper respiratory tract infections. While more research is needed to explore these connections, these findings continue to suggest that vitamin D plays an integral role in bone, muscle, cardiac, and immune health.
Where Do You Get Vitamin D?
Only a few foods are natural sources of vitamin D, including eggs and fatty fish like salmon, mackerel, tuna, and sardines. Instead, vitamin D-fortified foods like dairy products, juices, and breakfast cereals make up the majority of Americans’ vitamin D intake.
Sun exposure, on the other hand, can be the greatest source of vitamin D for some people–hence vitamin D’s nickname, the “sunshine vitamin.” Unlike any other vitamin, vitamin D can be synthesized in the body when the sun’s ultraviolet B rays reach the skin and convert cholesterol into a Vitamin D3, the precursor for vitamin D. Then, Vitamin D3 diffuses through the skin into the blood, where it is transported to the liver and kidneys and converted into vitamin D’s active form, 25(OH)D.
Research has found that exposing arms, legs, and face to the sun for 15 to 30 minutes twice a week provides about 1000 international units of vitamin D (equal to about 10 cups of milk!). Despite this robust source of vitamin D, deficiency is surprisingly common in the U.S.
Who is at Risk of Vitamin D Deficiency?
Many circumstances can alter vitamin D synthesis and absorption, increasing risk of vitamin D deficiency. Some of the factors that have been found to impact vitamin D status include the following:
- Season: According to research, during the months of November to February, people living more than 37 degrees latitude north or south produce little or no vitamin D from the sun due of the angle of ultraviolet B sunrays. While vitamin D is stored in fat tissue and can be released into the blood when needed, our stores typically only last one to two months.
- Limited Sun Exposure: Vitamin D synthesis can also be blocked when sunscreen is applied correctly or when long robes or head coverings are worn for religious reasons. For example, sunscreen with a sun protection factor (SPF) of 8 decreased vitamin D synthesis in skin by about 95% in one study.
- Skin Color: People with darker skin pigmentation have also been found in research to have lower levels of vitamin D due to decreased synthesis. This is supported by the high prevalence of vitamin D deficiency among certain ethnicities, with 82% African Americans and 69% Hispanics found to be vitamin D deficient according to NHANES.
- Weight: Studies also suggest that overweight and obese people may have higher Vitamin D requirements. Since they have more body fat and since vitamin D is a fat-soluble vitamin, vitamin D is more widely distributed in fat tissue, making it less bioavailable. As a result, more vitamin D may be needed for it to reach the blood stream for distribution in the body.
- Age: Older adults have been found to have lower levels of the vitamin D, likely due to both decreased sun exposure and inefficient synthesis. One study found that 70 year-olds had about 25% of the vitamin D precursor compared to young adults, which decreased vitamin D synthesis in the skin by 75%.
- Fat Malabsorption: When any gastrointestinal disorder or other health condition impairs fat absorption (i.e. liver disease, cystic fibrosis, celiac disease, or Crohn’s disease), vitamin D is also poorly absorbed and utilized since Vitamin D is a fat-soluble vitamin.
Vitamin D deficiency can be especially concerning because symptoms like bone pain and muscle weakness may go undetected in the early stages of deficiency. Although physicians do not routinely check vitamin D levels, those at risk of deficiency may benefit from a serum 25(OH)D test. This is a simple test used to measure the level of vitamin D circulating in blood, with levels less 20 nanograms per milliliter commonly used to diagnose deficiency. However, some organizations like the Endocrine Society argue that levels greater than 30 nanograms per milliliter should be recommended for optimal bone and muscle metabolism.
How Much Vitamin D Do You Need?
Similar to vitamin D serum levels, no ideal vitamin D intake has been well established since many factors contribute to vitamin D status. The U.S. Institute of Medicine recommends 600 to 800 international units (IU) of vitamin D daily for adults, assuming minimal sun exposure. On the other hand, the National Osteoporosis Foundation recommends larger doses of 1000 to 1200 IU daily for adults to support adequate bone health. Although vitamin D toxicity is rare, an upper level of 4000 IU has been set by the Institute of Medicine since extremely high levels can lead to calcium buildup, and could cause poor appetite, nausea, vomiting, weakness, and kidney problems.
With limited amounts of vitamin D provided from food, even fortified foods, diet alone is usually inadequate to meet vitamin D needs. For example, you would need to drink about 8 cups of milk every day to reach 800 IU of vitamin D from diet alone! While sun exposure can supplement food intake to meet vitamin D needs, many Americans still fall short of their needs due the factors outlined above.
For the 40% of Americans who have been found to be vitamin D deficient, vitamin D supplementation can be an effective and safe way to meet needs. Whether you’re an avid sunscreen-user or living here in New England during these fall and winter months, a daily vitamin D supplement can ensure that vitamin D stores are adequate. Multivitamins typically provide 400 IU of vitamin D, but a separate vitamin D supplement (D2 or D3) with 800 or 1000 IU may be needed to meet daily intake recommendations.
Katelyn Castro is a second-year student in the Dietetic Internship/MS Nutrition Program at the Friedman School. During the summer, she enjoys soaking up the sun if only for an excuse to get her daily dose of Vitamin D. During the winter, you can find her trekking through the snow, bundled up like the boy in A Christmas Story, and contemplating whether she needs a D supplement.