Vitamin D deficiency occurs when your body doesn’t have enough of this essential nutrient needed for healthy bones and muscles. This widespread condition affects roughly one billion people worldwide, yet many don’t realize they have it because symptoms can be subtle or absent altogether.

How Common Is Vitamin D Deficiency Worldwide

Vitamin D deficiency has become a significant global health concern that touches communities across every continent. About one billion people worldwide have vitamin D deficiency, while an additional fifty percent of the global population experiences vitamin D insufficiency, which means their levels are lower than ideal but not yet critically low.^[1] In the United States alone, approximately thirty-five percent of adults don’t have enough vitamin D in their bodies.^[1]

This condition doesn’t discriminate by geography or economic status. It affects people in both wealthy and developing nations, making it truly a universal health challenge. The problem has persisted despite efforts dating back to the 1930s, when North America began adding vitamin D to milk to eliminate a childhood bone disease called rickets.^[3] Even with these preventive measures in place for nearly a century, subclinical vitamin D deficiency, where people have low levels without obvious symptoms, remains remarkably common throughout the world.^[3]

Who Is Most Likely to Develop Vitamin D Deficiency

Anyone can develop vitamin D deficiency, from newborn babies to elderly adults, but certain groups face higher risks than others. People over age sixty-five are particularly vulnerable because as we age, our skin becomes less efficient at producing vitamin D when exposed to sunlight, and our kidneys become less capable of converting vitamin D into its active form that the body can use.^[1]

People with darker skin face a greater challenge in producing adequate vitamin D. Higher levels of melanin, the pigment that gives skin its color, act as a natural sunscreen that reduces the skin’s ability to make vitamin D from sunlight. This means that individuals with naturally dark skin, including people of African, African-Caribbean, or South Asian backgrounds, need to spend more time in the sun than those with lighter skin to produce the same amount of vitamin D.^[1][7] Non-Hispanic Black people generally show higher rates of vitamin D deficiency compared to other populations.^[5]

Breastfed infants represent another high-risk group. Human breast milk, while nutritionally complete in most ways, contains very little vitamin D—only about twenty international units per liter.^[11] This is why health authorities recommend that breastfed babies receive vitamin D supplements starting from birth. Babies getting less than five hundred milliliters of infant formula daily also need supplementation, since formula contains added vitamin D.^[7]

People who spend most of their time indoors face obvious risks because they’re not exposed to sunlight, which is the body’s primary source of vitamin D production. This includes individuals who are frail or housebound, those living in institutional settings like nursing homes, and people who wear clothing that covers most of their skin when outdoors for cultural or personal reasons.^[7] Additionally, people living in northern regions where sunlight is limited during winter months are at increased risk, particularly between October and early March when the sun isn’t strong enough to trigger vitamin D production in the skin.^[7]

Several medical conditions can interfere with the body’s ability to absorb or process vitamin D. People with gastrointestinal disorders like Crohn’s disease, ulcerative colitis, or celiac disease struggle to absorb vitamin D from food properly.^[2] Those with obesity face unique challenges because vitamin D is fat-soluble, meaning body fat binds to some of the vitamin and prevents it from entering the bloodstream where it’s needed.^[2] People who have undergone gastric bypass surgery for weight loss have difficulty absorbing sufficient vitamin D because the surgery creates a bypass of part of the small intestine where vitamin D absorption normally occurs.^[2]

Chronic kidney disease and liver disease both affect the body’s ability to convert vitamin D into its active, usable form. Certain medications can also interfere with vitamin D metabolism, including some cholesterol-lowering drugs, anti-seizure medications, steroids, and weight-loss medicines.^[2][4]

What Causes This Deficiency

The root causes of vitamin D deficiency generally fall into two main categories: not getting enough vitamin D in the first place, or having a condition that prevents your body from properly absorbing or using the vitamin D you do get.^[1]

The most common cause is simply insufficient exposure to sunlight. When ultraviolet B radiation from the sun hits your skin, it triggers a chemical reaction that allows your skin cells to manufacture vitamin D. Your body is designed to obtain about eighty to ninety percent of its vitamin D this way.^[8] However, modern lifestyles often keep people indoors for most of the day, whether at school, work, or home. Your geographical location matters too—living far from the equator, particularly in northern latitudes, means less sun exposure during much of the year, making it harder to maintain adequate vitamin D levels.^[1]

Diet alone rarely provides sufficient vitamin D for most people. Very few foods naturally contain significant amounts of vitamin D. Fatty fish like salmon, tuna, and mackerel are among the best natural sources, along with fish liver oils, egg yolks, and beef liver.^[2] Many countries have responded to this scarcity by fortifying common foods with vitamin D, including milk, breakfast cereals, orange juice, and certain brands of margarine and yogurt.^[2][5] Even with these fortified foods available, it’s difficult to obtain adequate vitamin D through diet alone.^[8]

Malabsorption problems represent another significant cause. When your digestive system can’t properly absorb nutrients from food, vitamin D intake becomes ineffective regardless of how much you consume. This occurs in conditions affecting the gastrointestinal tract, preventing the vitamin from entering your bloodstream.^[2]

Sometimes the cause lies in how your body processes vitamin D after you’ve obtained it. Your liver and kidneys play crucial roles in converting vitamin D into its biologically active form. When these organs aren’t functioning properly due to chronic disease, they can’t complete this conversion process efficiently, leading to functional deficiency even when initial vitamin D levels seem adequate.^[2]

What Symptoms Might You Experience

One of the most challenging aspects of vitamin D deficiency is that many people have no obvious symptoms at all. The condition can be clinically silent, meaning you might be deficient without feeling any different from usual.^[5][10] When symptoms do appear, they’re often subtle and can easily be mistaken for other common health problems or simply dismissed as effects of aging or stress.

Fatigue and exhaustion are among the most frequently reported symptoms when vitamin D levels drop too low. This isn’t just ordinary tiredness from a busy day—it’s a persistent lack of energy that doesn’t improve with rest.^[1][5] Many people also experience difficulty sleeping well, which can create a vicious cycle where poor sleep leads to more fatigue.^[5]

Bone and muscle problems often develop when deficiency becomes more pronounced. You might notice bone pain or a general achiness throughout your skeletal system. This pain can be particularly noticeable when pressure is applied to certain bones, such as the sternum in your chest or the tibia in your lower leg.^[11] Your muscles may feel weak, achy, or prone to cramping, making everyday activities more difficult than they should be.^[1][5]

Changes in mood represent another common manifestation of vitamin D deficiency. People may experience depression or persistent feelings of sadness that seem to come without clear cause.^[1][5] While researchers haven’t definitively proven that low vitamin D causes depression, studies consistently show that people with depression are more likely to have insufficient vitamin D levels.^[4]

Other symptoms can include getting sick more frequently, as vitamin D plays an important role in immune system function. Some people notice hair loss, loss of appetite, or that their skin appears paler than usual.^[5] In severe cases, particularly in older adults, vitamin D deficiency may contribute to an increased risk of falls due to muscle weakness and impaired balance.^[4]

In children, vitamin D deficiency causes a condition called rickets, which presents with very specific symptoms. Children with rickets may have delayed growth patterns, with bones that appear bowed or bent rather than straight. They might experience muscle weakness and bone pain, and develop visible deformities in their joints.^[1] Children may start walking later than expected or prefer to sit for prolonged periods because standing and moving is uncomfortable.^[12] Fortunately, rickets has become quite rare in countries where milk and other foods are fortified with vitamin D.^[3]

In adults, severe vitamin D deficiency leads to a condition called osteomalacia, which means “soft bones.” This condition shares some symptoms with osteoporosis, another bone disease, but the underlying problem is different. While osteoporosis involves bones becoming porous and brittle, osteomalacia affects the bone matrix itself, preventing proper mineralization.^[1][3]

How to Prevent Vitamin D Deficiency

Prevention strategies for vitamin D deficiency involve a balanced approach combining sensible sun exposure, dietary choices, and supplements when necessary. The key is finding the right balance for your individual circumstances, taking into account your age, skin tone, where you live, and your overall health status.

Sunlight exposure remains the most effective natural way to maintain healthy vitamin D levels. During the warmer months, specifically from late March or early April through the end of September in many regions, most people can produce all the vitamin D they need through modest sun exposure on their skin when outdoors.^[7] You don’t need to sunbathe for hours—often just a few minutes outside on most days of the week is sufficient for people with light to medium skin tones. Being physically active outdoors helps your body produce vitamin D more effectively.^[23]

During autumn and winter months, particularly between October and early March in northern regions, the sun isn’t strong enough for your skin to make adequate vitamin D regardless of how much time you spend outdoors.^[7] This seasonal variation means that people in these areas need to rely more heavily on dietary sources and supplements during the colder months.

It’s important to balance sun exposure with skin cancer prevention. While you need some sun to produce vitamin D, you should still use sunscreen to protect sensitive areas of your skin. Remember that even on cloudy days, UV radiation can reach your skin.^[5] The UV index, which measures the strength of ultraviolet radiation, can help you determine when sun protection is most necessary. Sun protection including sunscreen, hats, clothing, sunglasses, and shade is recommended whenever UV levels reach three or above.^[23]

Incorporating vitamin D-rich foods into your regular diet provides another layer of protection against deficiency. Focus on including fatty fish such as salmon, trout, tuna, mackerel, herring, or sardines several times a week. Canned fish options like herring and sardines offer convenient, affordable choices.^[5] Other natural sources include egg yolks, beef liver, and fish liver oils.^[2]

Take advantage of fortified foods, which have had vitamin D added during processing. Check nutrition labels on breakfast cereals, milk (dairy, almond, or soy), orange juice, yogurt, and margarine to confirm they contain added vitamin D.^[2][5] These fortified products can significantly contribute to your daily vitamin D intake, especially during winter months.

Supplementation plays a crucial role in prevention for many people. Health authorities generally recommend that everyone consider taking a daily vitamin D supplement during autumn and winter months when sunlight is insufficient.^[7] The recommended amount for most adults is ten micrograms (which equals four hundred international units) per day.^[7] Children from age one year through age four should take a daily supplement containing ten micrograms throughout the entire year.^[7]

Infants have specific supplement needs that differ from older children and adults. Babies up to age one need between 8.5 and 10 micrograms of vitamin D daily. Breastfed babies should receive supplements from birth. Formula-fed babies need supplementation only if they’re consuming less than five hundred milliliters of formula per day, since infant formula already contains added vitamin D.^[7]

People at high risk for deficiency should take daily supplements year-round rather than just during winter months. This includes individuals who aren’t often outdoors, those who are frail or housebound, people in institutional care, those who usually wear clothing that covers most of their skin outdoors, and anyone with naturally dark skin.^[7]

How Vitamin D Deficiency Changes Your Body

Understanding what happens inside your body when vitamin D levels drop helps explain why this deficiency causes the symptoms and complications it does. Vitamin D functions more like a hormone than a typical vitamin, orchestrating complex processes throughout multiple body systems.^[3]

The most critical role of vitamin D involves calcium regulation and bone health. Your body needs vitamin D to absorb calcium and phosphorus from the food you eat in your intestines. These two minerals are the primary building blocks of bone tissue. Without sufficient vitamin D, even if you consume plenty of calcium through dairy products or other sources, your intestines can’t absorb it efficiently into your bloodstream.^[1][4]

When vitamin D deficiency becomes chronic or severe, your calcium and phosphorus absorption declines significantly. This leads to hypocalcemia, which means you have abnormally low calcium levels in your blood. Your body has mechanisms to detect this drop and responds by increasing activity of the parathyroid glands, small glands in your neck that regulate calcium balance. This compensatory response is called secondary hyperparathyroidism, meaning your parathyroid glands become overactive trying to normalize blood calcium levels.^[1]

Both hypocalcemia and hyperparathyroidism can produce symptoms when severe enough. You might experience muscle weakness and cramping, profound fatigue, and mood changes including depression. These symptoms occur because calcium is essential for proper muscle and nerve function, not just for bones.^[1]

To compensate for low blood calcium, your body begins stealing calcium from your skeleton, where ninety-nine percent of the body’s calcium is stored. This process is called bone demineralization—your bones break down faster than they can rebuild and reform. Over time, this leads to bones becoming progressively weaker and more fragile.^[1]

In adults, this bone demineralization results in osteomalacia, where the bone matrix itself becomes soft because new bone tissue can’t mineralize properly. People with osteomalacia face increased risk of bone fractures, especially in the hip, spine, and wrist. The condition also increases the risk of developing osteoporosis, where bones become porous, brittle, and prone to breaking even from minor impacts or falls.^[1][3]

In children whose bones are still growing, demineralization causes rickets, a more visible condition than adult osteomalacia. Because children’s bones are actively lengthening and developing, the lack of proper mineralization causes bones to bend and bow under the weight of the child’s body. The long bones in the legs are particularly affected, creating the characteristic bowed appearance associated with rickets. The growth plates at the ends of bones also become affected, leading to widening and deformity at joints.^[1]

Vitamin D also influences your muscular system directly. The vitamin D receptor is present in muscle tissue, and adequate vitamin D is necessary for optimal muscle function, strength, and coordination. When levels are low, muscles become weaker and less efficient, which helps explain why people with vitamin D deficiency often experience muscle aches, weakness, and increased fall risk.^[3]

Beyond bones and muscles, vitamin D plays roles in your nervous system, immune system, and cellular processes throughout your body. The vitamin helps regulate cell growth and proliferation, supports immune function to help fight infections, and may influence inflammation levels.^[1][17] Recent research suggests potential connections between vitamin D deficiency and various health conditions including cardiovascular disease, diabetes, autoimmune disorders, certain cancers, and depression, though more research is needed to fully understand these relationships.^[3][9]

The body has a sophisticated system for managing vitamin D once you obtain it. Whether from sunlight, food, or supplements, vitamin D starts in an inactive form. Your liver performs the first conversion step, creating a compound called 25-hydroxyvitamin D, abbreviated as 25(OH)D. This form circulates in your blood and has a half-life of about three weeks, making it the best indicator of your vitamin D status when doctors order blood tests.^[8]

Your kidneys then perform a second conversion, transforming 25(OH)D into the biologically active form called calcitriol or 1,25-dihydroxyvitamin D. This active form has a much shorter half-life of only four to six hours, so it’s not useful for measuring your vitamin D status.^[8] Interestingly, many tissues throughout your body can also perform this final conversion step locally, generating active vitamin D for use within specific cells and tissues, which may explain some of vitamin D’s wide-ranging effects beyond bone health.^[8]