What Is Caused By Vitamin D Deficiency? Explained

Vitamin D deficiency causes weakened bones, muscle pain, and fatigue — and in severe cases, it leads to rickets in children and osteomalacia in adults. The vitamin controls how your body absorbs calcium and phosphate, the two minerals that harden bone. When levels drop, bones soften and muscles weaken. A growing body of research also links low vitamin D to immune and metabolic problems, though those connections are less settled than the bone effects.

What Is Caused By Vitamin D Deficiency?

The best-established consequence of vitamin D deficiency is bone disease. Vitamin D’s main job is to help your gut absorb calcium and phosphate. Without enough of it, you absorb less calcium from food, and your body pulls calcium out of your skeleton to keep blood levels normal. Over time, that withdrawal weakens bone.

In children whose bones are still growing, this produces rickets. The growth plates at the ends of long bones fail to mineralize properly, leading to bowed legs, a softened skull, and delayed walking. In adults, whose growth plates have closed, the same process produces osteomalacia — soft, painful bones that fracture easily and cause aching in the hips, spine, and legs.

Beyond bone, deficiency is associated with muscle weakness and bone pain. Severe deficiency can also cause a specific pattern of muscle aching that improves when vitamin D is corrected, though this is not universal.

What deficiency does not reliably cause is more contested. Low vitamin D has been linked in observational studies to depression, heart disease, certain cancers, and autoimmune conditions. But observational studies cannot tell whether low vitamin D causes these problems or is simply a marker of poor health. Large randomized trials — which can test cause and effect — have generally not confirmed that raising vitamin D levels prevents these diseases in people who start out replete.

How Does Vitamin D Deficiency Affect Bones and Muscles?

The bone effects come down to calcium handling. Vitamin D increases calcium absorption in the small intestine and helps regulate parathyroid hormone, which controls calcium balance. When vitamin D is low, parathyroid hormone rises to compensate, and that hormone signals the skeleton to release calcium.

The result is a slow drain on bone mineral. In adults this shows up as osteomalacia, and it also contributes to osteoporosis when bone loss outpaces bone formation. The two conditions are different — osteomalacia is a mineralization defect, osteoporosis is a loss of bone mass — but low vitamin D can be a factor in both.

Muscle weakness is well documented in severe deficiency. Vitamin D receptors are present in muscle tissue, and deficiency can cause a waddling gait and difficulty climbing stairs in marked cases. The evidence for milder muscle effects, like vague aches in people with only modestly low levels, is weaker and inconsistent.

What Are the Symptoms of Vitamin D Deficiency?

Most people with low vitamin D have no obvious symptoms. That is why it is often called a silent problem. When symptoms do appear, they tend to be nonspecific:

• Bone pain, especially in the lower back, hips, and legs
• Muscle weakness or aching
• Fatigue
• Frequent fractures or slow healing
• In children, delayed growth, bowed legs, or irritability

Because these symptoms overlap with many other conditions, they cannot confirm a deficiency on their own. A blood test is what actually establishes the diagnosis. Symptoms alone are not a reliable guide.

Who Is Most at Risk for Vitamin D Deficiency?

Certain groups are far more likely to be deficient. The main reason is limited sun exposure, since most vitamin D is made in the skin in response to ultraviolet B light.

• People with darker skin, because melanin reduces vitamin D synthesis
• Older adults, whose skin makes vitamin D less efficiently
• Anyone who spends most time indoors, including those who are housebound or institutionalized
• People living at high latitudes, where winter sunlight is too weak for vitamin D production
• Those who cover their skin for cultural or religious reasons
• Exclusively breastfed infants, since breast milk contains little vitamin D
• People with conditions that impair fat absorption, such as celiac disease or Crohn’s disease, because vitamin D is fat-soluble
• People with obesity, since vitamin D is sequestered in fat tissue
• Those taking certain medications that speed up vitamin D breakdown

These risk factors are well established. The precise thresholds for “deficiency” versus “insufficiency” have been debated, and different organizations have set slightly different cutoffs over the years. What is not debated is that severe deficiency causes bone disease.

How Is Vitamin D Deficiency Diagnosed?

Diagnosis is made with a blood test for 25-hydroxyvitamin D, often written as 25(OH)D. This is the main circulating form of the vitamin and the standard marker of status. The active form, 1,25-dihydroxyvitamin D, is not used to assess deficiency because it can appear normal even when stores are low.

There is genuine disagreement about where to draw the line. Many clinical guidelines consider levels below 12 ng/mL (30 nmol/L) as deficient, and some use 20 ng/mL (50 nmol/L) as the threshold for insufficiency. Others recommend higher targets. Because these cutoffs differ, a result near a borderline may be interpreted differently by different clinicians.

What most experts agree on is that the lower the level, the clearer the case for treatment — especially if there are symptoms, abnormal calcium or phosphate levels, or a condition like kidney disease or malabsorption.

How Is Vitamin D Deficiency Treated?

Treatment depends on how low the level is and why it is low. For mild cases, a daily supplement is often enough. For severe deficiency, clinicians may prescribe higher-dose regimens for a period, then step down to a maintenance dose.

Two forms are commonly used:

• Vitamin D3 (cholecalciferol) — the form your skin makes; widely used and generally preferred
• Vitamin D2 (ergocalciferol) — plant-derived; also effective but may raise blood levels slightly less efficiently

Specific dosing should come from a clinician, because the right amount depends on your level, your health conditions, and any medications you take. Taking very high doses for long periods can cause harm, including high blood calcium, which can damage the kidneys. More is not better.

Addressing the underlying cause matters too. If a medication or a malabsorption condition is driving the deficiency, correcting that is part of the fix. And sunlight exposure helps, though how much is needed varies by skin tone, latitude, season, and time of day — there is no single universal recommendation.

Can Vitamin D Deficiency Be Prevented?

Prevention focuses on the same factors that cause deficiency in the first place. For many people, some sun exposure, dietary sources, and a supplement cover the gap.

Food sources include fatty fish like salmon and mackerel, egg yolks, and fortified milk, cereal, and some juices. It is difficult to get enough from diet alone, since few foods naturally contain much vitamin D.

For infants, the American Academy of Pediatrics has long recommended vitamin D supplementation starting soon after birth, because breast milk does not supply enough. This is a case where supplementation is a well-established standard rather than a matter of debate.

Whether routine supplementation helps people who are not deficient is a different question. Large randomized trials testing whether vitamin D supplements prevent fractures, cancer, or heart disease in generally healthy adults have mostly shown little or no benefit. That does not mean deficiency should go untreated — it means supplements are not a blanket solution for people who already have enough.

What Vitamin D Deficiency Does Not Cause

It helps to be clear about what the evidence does not support. Many headlines claim vitamin D prevents or treats a long list of conditions. The strongest human trials do not back most of these claims.

Observational studies often find that people with low vitamin D have higher rates of various diseases. But people who are sick, older, or less active tend to have lower vitamin D for many reasons. When researchers ran randomized trials to test whether supplements changed outcomes, the results were largely negative for cancer, cardiovascular disease, and depression in people who were not deficient to begin with.

This does not mean vitamin D is unimportant. It means the link between low levels and these diseases may reflect poor health rather than cause it. The bone and muscle effects remain the clearest and best-supported consequences.