Wondering what causes low vitamin D levels—and how you can avoid becoming deficient in this key nutrient?
Here, we’ll cover common causes of low vitamin D levels, symptoms to look for in the case of deficiency, and ways to get more vitamin D—so keep reading. (Note that you can take a vitamin D blood test at home to see if your levels are high enough.)
From age and skin color to chronic conditions like celiac disease, here are 8 common factors that can contribute to low vitamin D levels.
When considering what causes low vitamin D levels, specific chronic conditions are often the culprit. Cystic fibrosis, Crohn's disease, and celiac disease, for example, can reduce the ability of the intestines to absorb enough vitamin D into the bloodstream—leading to low circulating levels of vitamin D, which means there’s less of it for different parts of your body to use.
Lower vitamin D levels are often associated with a body mass index of more than 30. Several explanations have been described in the scientific literature for why this is the case. One possible reason is that vitamin D accumulates in fat cells—so there’s less of it in circulation throughout the body if one has more fat cells. Research continues in this area to more fully tease out the mechanism responsible for the link between obesity and low vitamin D.
Kidney disease can reduce the amount of an enzyme needed to change vitamin D into the form that’s usable by the body. If your body can’t convert vitamin D into its usable or “active form,” this can lead to a deficiency.
As we age, our skin becomes less efficient at making vitamin D, which can contribute to low levels.
Those who have darker skin are more likely to have low vitamin D levels. This is because darker skin has a greater amount of melanin—a pigment that absorbs much of the sun’s rays before it can trigger vitamin D production.
Certain drugs can also affect your body’s ability to convert vitamin D into its circulating form and can lead to a deficiency. These include:
Bariatric surgery can help with weight loss in some people by making surgical changes to the digestive system. If you’ve undergone bariatric surgery, your intestine may have a reduced ability to absorb vitamin D.
People who are homebound—such as those in nursing facilities—or who otherwise don’t get much sun exposure are at risk for having a low vitamin D level. According to the National Institutes of Health (NIH), some researchers suggest that regular exposure to the sun for 5-30 minutes, twice per week, is typically enough to provide adequate vitamin D levels.
Of course, sunlight exposure also needs to be balanced with the risk of skin cancer, so it’s best to avoid getting sunburned. Also keep in mind that wearing sunscreen, clothing, or other UV-blocking measures may help prevent skin cancer, but it will also reduce your body’s production of vitamin D.
Now that we’ve covered what causes low vitamin D levels, let’s talk about the symptoms of vitamin D deficiency.
In adults, symptoms of vitamin D deficiency may include:
Among children, long-term deficiency can cause rickets, a rare disorder that leads to stunted growth, weakness in muscles, pain in bones, and deformities in the joints. Children who are deficient in vitamin D may also experience sore, weak, or painful muscles.
Low vitamin D levels have been associated with an increased risk for certain health conditions, including the following.
In the United States, over 53 million adults have (or are at risk of developing) osteoporosis. This bone disease occurs when the body experiences bone loss or makes too little bone, which increases the risk of bone fractures. A lack of vitamin D reduces calcium absorption, which can lead to osteoporosis. That’s why adequate levels of vitamin D are key for optimal bone health.
Osteomalacia is most often caused by severe vitamin D deficiency and is characterized by the softening of the bones. In children and young adults, this can lead to bowing of the bones, particularly in the legs. In older adults, osteomalacia can lead to fractures, bone loss, and deteriorating bone health.
Exposure to sunshine, specific foods in one’s diet, and vitamin D supplementation provide three effective ways to get vitamin D.
Sunlight that lands on your skin triggers vitamin D production. Most people can get substantial levels of vitamin D from the sun during the often sun-filled months of April through September.
The amount of vitamin D your skin produces through sunlight exposure is influenced by other factors, as well. Time of day, amount of cloud cover, and air pollution, your altitude, and the amount of melanin you have in your skin all affect how much vitamin D your skin makes.
It’s rare for vitamin D to occur naturally in the food you eat. Thankfully, though, it can be found in fatty fish like salmon, swordfish, and tuna—and (in small amounts) in beef liver, cheese, and egg yolks. Many food products, like milk and breakfast cereals, are fortified with vitamin D (meaning vitamin D is added to the product) to help people increase their vitamin D intake.
If you find that you aren’t getting enough vitamin D through food and sunshine, your healthcare provider may suggest vitamin D supplements. Vitamin D supplements can come in two forms—vitamin D2 and vitamin D3. Vitamin D2 is also found in fortified foods and naturally occurs in some plants. Vitamin D3, on the other hand, is naturally produced in our bodies and can be found in various animal-based foods. Some experts believe vitamin D3 is a better form of vitamin D because it’s naturally produced by the body.
The amount of vitamin D someone needs depends on their age, as highlighted below. (Note that the amount of vitamin D in foods and supplements is usually expressed in terms of International Units, or IU, so “IU/day” refers to “International Units per day.”)
Infants 0-6 months:
Infants 6-12 months
Children 1-3 years old
Children 4-8 years old
People 9-70 years old
People over 70 years old
Women 14-50 years old who are pregnant or lactating
That being said, other factors can affect how much vitamin D you may need—so it’s best to ask your healthcare provider what they advise for you.