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11 Sneaky Causes of Vitamin D Deficiency

Vitamin D keeps bones strong, boosts the immune system, and may even help prevent some types of cancer. But many people don't get enough. Here are the surprising reasons your levels may be low.

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Not enough sun

When your skin is exposed to sunlight, vitamin D synthesis occurs. But some people don’t get the sunlight they need in order for this to happen. This includes people who live in northern climates, office workers who spend hours a day behind a desk, and even children who play indoors rather than outside. “Not getting enough vitamin D affects bone health as well as many other metabolic processes in the body,” says Niket Sonpal, MD, assistant professor of medicine at Touro College of Osteopathic Medicine. It’s a myth, experts say that wearing sunscreen year round is one of the causes of vitamin D deficiency—because most people don’t apply it well enough to prevent the skin from producing D. Check out 10 more sunscreen myths that make dermatologists cringe.

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Getting older

“Everyone is at increased risk of developing low vitamin D levels as they age because the skin cannot synthesize the vitamin as efficiently,” says Ginger Hultin, a Seattle-based registered dietitian and spokesperson for the Academy of Nutrition and Dietetics. Older adults may also spend more time indoors. Check out the 23 vitamin D benefits that can save your life.

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Dark skin

Having a darker skin pigment can be one of the causes of vitamin D deficiency. “Higher amounts of the pigment melanin in the epidermal layer of the skin causes both a darker skin color and a reduced ability of the skin to produce vitamin D from sunlight,” says Hultin. “This can mean lower blood levels of vitamin D.” Do you know the 9 signs that you’re not getting enough D?

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Kidney disease

People who have kidney disease are at higher risk of a vitamin D deficiency. Why? A family of proteins known as fibroblast growth factor increase as the disease worsens, and they interfere with the body’s metabolism of vitamin D, explains Dr. Sonpal. One major problem: Only 10 percent of people with kidney disease know they have it. Here are 7 silent signs that your kidneys could be in big trouble.

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Celiac disease

While sunlight is a key to getting vitamin D, other important sources include fortified foods like milk and breakfast cereals. People with digestive issues like celiac disease may not absorb enough D from their food (and their illness may keep them indoors more), which could explain why 64 percent of women and 71 percent of men with the condition are deficient in vitamin D. “Some patients end up developing long-term consequences of vitamin deficiency before we even know they have celiac,” says Dr. Sonpal.

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Cystic fibrosis

This disease damages the lungs, but it can also target the gut, blocking the absorption of fat molecules. Because vitamin D is a fat-soluble vitamin, people with cystic fibrosis won’t get much D from food. “If you can’t absorb fat, you can’t absorb fat-soluble vitamins like D,” says Dr. Sonpal. The damage to lungs also means cystic fibrosis patients are less likely to spend time outdoors in the sun, he points out.

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Crohn’s disease

Another example of a digestive condition interfering with vitamin D absorption: This inflammatory bowel disease that causes intestinal inflammation can mean the sufferers glean very little vitamin D from their diet. Don’t miss these symptoms of celiac disease and Crohn’s disease you need to pay attention to.

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Obesity

Studies have shown that being obese—you have a body mass index (a measure that combines height and weight) of 30—can cause vitamin D deficiency. “Obesity does not affect the body’s ability to make vitamin D,” Hultin says. “Rather, a greater amount of subcutaneous fat in the body sequesters more vitamin D and lowers the amount that is able to circulate in the body.” Read about the dangerous connection between obesity and cancer.

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Gastric-bypass surgery

Unfortunately, one of the solutions for obesity—gastric bypass—can also cause a D deficiency. That’s because the surgery removes part of the intestine, which reduces the amount of the vitamin that the body can absorb from food. Here are the pros and cons of the top weight-loss procedures.

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An intestinal infection

Whipple’s disease is a rare bacterial infection caused by Tropheryma whipplei. It interferes with normal digestion by hindering the breakdown of foods, such as fats and carbohydrates, as well as the body’s ability to absorb nutrients. “Bugs block the intestinal pores in which we absorb nutrients, so vitamin D can’t be absorbed,” says Dr. Sonpal. Check out these 12 other diseases you didn’t know you could get on vacation.

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Where you live

Living in a cold, dark climate can increase the risk for vitamin D deficiency. So can living in a place where the fortification of bread and cereal isn’t common. “India is one such place,” says Dr. Sonpal. “Most Western nations typically have fortification.” Read about the diets that put you at risk for vitamin D deficiency.

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How to tell and what to do

A blood test can help determine whether you’re low in vitamin D: Normal levels of are approximately 30 ng/mL. Between 20 and 30 ng/mL is considered insufficient, and a level below 20 ng/mL is considered deficient. If you are low, Dr. Sonpal recommends taking a daily supplement of 1,000 IU. Keep in mind that too much D can be toxic, leading to bone pain, gut pain, constipation, nausea, and vomiting. Don’t try to diagnose yourself, he says. “See a doctor who can tell you how much to take for how long.”