7 vitamin D deficiency causes

Experts estimate that one billion people are vitamin D deficient worldwide. Many don’t realize it because symptoms can be mild. But they can also be severe, causing bone deformities called rickets in children and a bone softening condition called osteomalacia in adults. Due to the potential for severe symptoms, knowing what the various vitamin D deficiency causes are and what you can do about them is important (Sizar, 2021; Pilz, 2019). 

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Having a vitamin D deficiency means that you don’t have enough circulating vitamin D in your blood. The effects of a vitamin D deficiency can lead to symptoms of bone pain, muscle weakness, forgetfulness, or fuzzy thinking. These can worsen depending on the severity of the deficiency. Long-term vitamin D deficiency may be a factor in cardiovascular disease, cancer, and autoimmune diseases (Holick, 2017). 

A healthcare provider may order a simple vitamin D blood test if they suspect you are at risk. But not everyone needs to get tested for a deficiency. Usually, experts recommend that only people in high-risk groups or people with symptoms get tested (Kennel, 2010).

This blood test measures the amount of a form of vitamin D called calcidiol (25-hydroxycholecalciferol) that circulates in the blood. An extreme deficiency is anything less than 30 nmol/L (12 ng/mL). Levels ranging from 30 to 50 nmol/L (12mg/mL-20 ng/mL) are not high enough for good bone health, and your healthcare provider will likely recommend treatment. Levels of 50 nmol/L (20 ng/mL) or higher are considered normal. If your levels are over 125 nmol/L (50 ng/mL), it’s too high (NIH, 2021). 

Vitamin D is produced in the body when sunlight’s ultraviolet B (UVB) rays reach the skin. It is also in some foods. To use vitamin D, the body needs to convert it into calcitriol—the active form of vitamin D that circulates in the blood. Converting it is a complex process that involves cholesterol in the skin, absorption in the intestines, and vitamin D conversion in the liver and kidneys (Bikle, 2017). 

If the body doesn’t get enough sun or dietary vitamin D, it lacks the building blocks to make enough active vitamin D. Another reason for a deficiency is that the body isn’t properly absorbing, converting, or retrieving the vitamin D or sunlight it does have access to (Wacker, 2013).

Ultimately, there are a variety of vitamin D deficiency causes that involve lifestyle, genetics, and medical conditions: 

Sunlight exposure is the most efficient way to get vitamin D. About 50–90% of vitamin D is from the absorption of sunlight through the skin. Experts consider about 20-minutes a day of sun hitting over 40% of the skin enough to boost the average adult into a healthy vitamin D range. Because sunscreen does limit UVB rays, healthcare providers say it's best to balance and make sure you limit sun exposure and that you always apply enough sunscreen to prevent sunburn (Sizar, 2021; Kim 2020).

Living in northern latitudes in the winter months reduces UVB rays, and these periods can make it difficult for many people to get enough vitamin D from the sun. Limited sunlight, working inside year-round, and clothing that covers a majority of the skin surface can all lead to a vitamin D deficiency (Wacker, 2013).

More than 40% of adults in the U.S. are vitamin D deficient. The numbers go up to about 63% in Hispanic adults and 82% in African American adults. Researchers say the differences are due to the amount of melanin in the skin (Parva, 2018). Melanin protects the skin from harmful ultraviolet B (UVB) rays from the sun. By absorbing and scattering UVB rays, melanin also limits the amount of sun the skin absorbs. The result is that people with dark skin need more sunlight to get the same level of vitamin D as those with lighter skin. If you have dark skin, it makes sense to talk to your healthcare provider about supplementation to counter a lack of sunlight (Felton, 2016; Kim, 2020).

Aging may slow down the body’s ability to convert sunlight into usable vitamin D. A small study had healthy young adults and older adults expose themselves to the same amount of UVB radiation in a tanning bed. The increase in the blood level of vitamin D in the young adults was three times greater than that of the older adults. A similar study’s results show older people could achieve the same vitamin D level through UVB rays, but it took three times the exposure. A lack of physical activity, poor health, and being bedridden can keep people from getting outside in the sun (Wacker, 2013).

Aging also increases the risk for vitamin D deficiencies linked to medications, kidney disease, and disorders that make it hard to absorb nutrients. (Maurya, 2017; Meehan, 2015).  

Many people don’t get enough vitamin D in their diets. That’s because there are only a few food sources naturally high in vitamin D. 

Oily fish such as salmon, sardines, and mackerel are the best sources of natural vitamin D3 (cholecalciferol). Egg yolks and mushrooms also are natural sources. Some foods, including milk, some other dairy products, breakfast cereals, infant formula, and some brands of orange juice, are fortified with vitamin D2 (ergocalciferol), which is plant-based. Both types effectively boost circulating vitamin D in the body, though vitamin D3 seems to boost circulating vitamin D to a greater extent (Chauhan, 2021; Shieh, 2016).

Because breast milk contains only small amounts of vitamin D, the American Academy of Pediatrics recommends that exclusively or partially breastfed infants receive vitamin D supplementation beginning in the first few days of life. Healthcare providers typically suggest Vitamin D supplements for all ages due to a lack of sun and vitamin D in the diet (CDC, 2021; Fink, 2019).

Vitamin D is often added to calcium supplements because it helps with calcium absorption. Studies show that this combination is better at preventing hip fractures linked to osteoporosis and low bone density. Calcium supplements can affect the heart, so it’s best to talk with your healthcare provider about all supplements you take, sources of vitamin D, and which approach offers the most health benefits for you (Yao, 2019; Yang, 2020).

Dozens of medications can lower vitamin D levels. It’s best to ask your healthcare provider about the risk of vitamin D deficiency when taking drugs that fall into the following categories (Sizar 2021; Nordqvist, 2019; Grober, 2012):

• Corticosteroids (such as prednisone) 

• Laxatives

• Cholesterol-lowering drugs

• Certain anti-epileptic drugs (Teagarden, 2014)

• Certain tuberculosis drugs (Chesdachai, 2016)

Studies link obesity, specifically a body mass index (BMI) of 30 or more, to vitamin D deficiency. Researchers think that since Vitamin D is a fat-soluble vitamin stored in body fat, more fat tissue makes it less available to circulate. 

To counter trapped Vitamin D stores, healthcare providers usually advise those managing obesity to take extra vitamin D supplements to reach normal levels. If the excess weight keeps people from going outdoors, a lack of sunlight can add to a deficiency (Wacker, 2013; Maurya, 2017).

Several medical conditions can affect vitamin D synthesis or absorption. Malabsorption syndromes such as Crohn's disease, celiac disease, inflammatory bowel disease, chronic pancreatic insufficiency, and cystic fibrosis can lead to vitamin D deficiency. These conditions affect digestion and the absorption of nutrients. Some make it harder to process fat, which is needed to absorb vitamin D.  Gastric bypass surgery for weight loss can also make it harder to absorb vitamin D (Margulies, 2015; Sizar, 2021; U.S. NLM, 2021). 

Since the liver and kidneys help convert vitamin D to its active form, conditions that affect these organs, such as cirrhosis, chronic liver disease, and chronic kidney disease, can reduce vitamin D levels as well (Sizar, 2021).

The U.S. The Preventive Services Task Force (USPSTF), an independent panel of national experts, does not recommend population-based screening for vitamin D deficiency. Unless you have symptoms or risk factors, your healthcare provider will likely tell you it’s not necessary. There are still questions about how much vitamin D supplementation should take place, citing concerns about vitamin D toxicity (JAMA, 2021).

If you think you may have a vitamin D deficiency, talk to your healthcare provider to see if you need to test your levels, especially if you have signs of weakened bones or other risk factors (JAMA, 2021).

Fortunately, treatment is usually straightforward and effective with the right amount of vitamin D supplements. Warning: taking too much vitamin D can lead to toxicity and a condition called hypercalcemia. For this reason, it’s important to work with your healthcare provider to correct a vitamin D deficiency. 

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Current version

October 20, 2021

Written by

Kristin DeJohn

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.