What Will A Vitamin D Deficiency Cause?

Vitamin D does far more than build strong bones. It helps your body absorb calcium and phosphorus, supports muscle function, and plays a role in immune and nervous system activity. When levels drop too low, the most well-established consequences are bone-related: rickets in children and osteomalacia (softening of the bones) in adults. Severe deficiency can also cause muscle weakness and bone pain. A growing body of research links low vitamin D to other conditions, but for most of those, the evidence is not yet strong enough to say deficiency causes them.

What Does Vitamin D Actually Do in Your Body?

Vitamin D works more like a hormone than a vitamin. Your skin makes it when exposed to sunlight, and you also get some from food and supplements. The liver and kidneys then convert it into its active form.

In that active form, vitamin D’s main job is to regulate calcium and phosphate levels in your blood. It signals your intestines to absorb more calcium from food. It tells your kidneys to hold onto calcium instead of losing it in urine. And when blood calcium runs low, it can trigger the release of calcium from your bones.

That last point matters. Your body treats blood calcium as a priority because calcium is needed for nerve signaling, muscle contraction, and heart function. If you are not absorbing enough from your diet, your body will pull calcium out of your skeleton to keep blood levels normal. Over time, that weakens bone.

Vitamin D receptors exist in many tissues beyond bone and intestine. That includes immune cells, muscle tissue, and parts of the brain. This is why researchers study vitamin D in relation to so many different health conditions. But the presence of a receptor does not prove that low vitamin D causes disease in that tissue. It only means the tissue responds to vitamin D in some way.

What Will A Vitamin D Deficiency Cause in Your Bones?

The best-documented effects of vitamin D deficiency are on the skeleton. Without enough vitamin D, your body cannot properly mineralize bone tissue.

In children, this causes rickets. The growing ends of long bones do not harden properly. This leads to bowed legs, a soft skull, delayed growth, and dental problems. Rickets was once common in industrialized cities where smog blocked sunlight. It is still seen in some parts of the world today.

In adults, the equivalent condition is osteomalacia. The bone matrix is laid down but not fully mineralized, so bones become soft and weak. Symptoms include:

• Dull, aching bone pain, often in the lower back, hips, and legs
• Muscle weakness, particularly in the thighs and shoulders
• Difficulty walking or climbing stairs
• Bone tenderness when pressure is applied

In older adults, deficiency contributes to osteoporosis and raises fracture risk. This is especially true for hip fractures, which carry serious consequences for older people. The relationship between vitamin D, calcium absorption, and bone density is one of the most firmly established findings in nutrition science.

Can Vitamin D Deficiency Cause Muscle Weakness and Pain?

Yes. Muscle weakness is a recognized symptom of moderate to severe vitamin D deficiency, and it is separate from bone pain. Vitamin D receptors are present in skeletal muscle tissue, and the active form of vitamin D appears to influence muscle cell function.

People with deficiency often describe difficulty rising from a chair, climbing stairs, or lifting objects. In older adults, this muscle weakness can increase the risk of falls, which in turn raises fracture risk. Some research suggests that correcting deficiency may improve muscle function, though results across studies vary.

The muscle symptoms of deficiency can be mistaken for other conditions, including fibromyalgia, chronic fatigue, or general aging. This is one reason deficiency sometimes goes unrecognized for a long time.

What Symptoms Are Linked to Vitamin D Deficiency?

Many people with mild deficiency have no obvious symptoms at all. When symptoms do appear, they tend to be vague and easy to attribute to other causes.

Commonly reported symptoms include:

• Bone pain and tenderness
• Muscle weakness or aching
• Fatigue
• Mood changes, including low mood
• Frequent illness or slow recovery from infections

Fatigue and mood changes are frequently mentioned in relation to vitamin D, but the evidence here is weaker than for bone and muscle effects. Some studies have found associations between low vitamin D and depression, but it remains unclear whether deficiency causes depression or whether people with depression are simply more likely to have low vitamin D for other reasons. The same caution applies to the link between vitamin D and frequent infections.

Severe deficiency can cause more serious neurological symptoms, including seizures and tingling or numbness around the mouth and in the hands and feet. These are related to very low blood calcium levels, which severe vitamin D deficiency can trigger. This is a medical emergency and requires immediate care.

What Health Conditions Are Linked to Low Vitamin D?

Researchers have studied low vitamin D levels in relation to a wide range of conditions. It is important to separate conditions where the evidence is strong from those where it is still uncertain.

Strong evidence: Vitamin D deficiency causes rickets in children and osteomalacia in adults. It contributes to bone loss and fractures in older adults. These are established findings supported by decades of clinical research.

Moderate or mixed evidence: Low vitamin D is consistently associated with cardiovascular disease, type 2 diabetes, certain cancers, autoimmune conditions, and respiratory infections in observational studies. However, observational studies cannot prove cause and effect. People with these conditions may have lower vitamin D for many reasons. When researchers have tested whether vitamin D supplements prevent these conditions in large randomized trials, the results have generally been disappointing. For example, large trials found that vitamin D supplementation did not reduce the risk of cardiovascular events or most cancers in generally healthy adults.

Weak or uncertain evidence: Links between vitamin D and conditions such as multiple sclerosis, autism, and certain mental health disorders remain under investigation. No clinical evidence currently confirms that vitamin D deficiency causes these conditions.

The pattern here is worth understanding. A nutrient can be associated with a disease without preventing or causing it. Low vitamin D may simply be a marker of poor overall health rather than a direct cause of many conditions attributed to it.

Who Is Most at Risk for Vitamin D Deficiency?

Several factors make deficiency more likely. People who get little sun exposure are at the top of the list. That includes those who live in northern latitudes, spend most of their time indoors, wear covering clothing for cultural or religious reasons, or use sunscreen consistently.

Other risk factors include:

• Having darker skin, because more melanin reduces vitamin D production in the skin
• Being older, since skin becomes less efficient at making vitamin D with age
• Being breastfed as an infant without vitamin D supplementation
• Having a body mass index of 30 or higher, because vitamin D can be sequestered in fat tissue
• Having conditions that affect fat absorption, such as celiac disease, Crohn’s disease, or cystic fibrosis
• Taking certain medications, including some anticonvulsants and glucocorticoids
• Having chronic kidney or liver disease, which affects vitamin D activation

Exclusively breastfed infants are a special concern. Breast milk typically contains small amounts of vitamin D, and infants should not be exposed to direct sunlight. The American Academy of Pediatrics recommends that exclusively breastfed infants receive a vitamin D supplement of 400 IU per day starting soon after birth. This is an established recommendation, not a matter of debate.

How Is Vitamin D Deficiency Diagnosed and Treated?

Diagnosis is done through a blood test that measures 25-hydroxyvitamin D, often written as 25(OH)D. This is the main form of vitamin D circulating in the blood and the best indicator of overall status.

There is no universal agreement on the exact cutoff for deficiency. Many clinical guidelines consider a level below 20 ng/mL (50 nmol/L) to be deficient, and some define insufficiency as 20 to 29 ng/mL. The Endocrine Society has previously suggested that a level of 30 ng/mL or higher is sufficient for optimal bone health, but there is ongoing debate about whether this higher threshold is necessary for the general population. The evidence does not clearly show that raising levels above 20 ng/mL provides additional benefit for most healthy adults.

Treatment depends on how low the level is and what is causing the deficiency. A doctor may recommend:

• Oral vitamin D supplements, typically vitamin D3 (cholecalciferol)
• Higher doses for a period of time to correct severe deficiency, followed by a maintenance dose
• Calcium supplementation if dietary intake is low
• Addressing the underlying cause, such as a malabsorption condition

Doses for correcting deficiency are determined by a healthcare provider based on blood levels. Taking very high doses of vitamin D without medical supervision can be harmful. Vitamin D is fat-soluble, which means it can build up in the body. Excess vitamin D can cause high blood calcium, leading to nausea, kidney problems, and irregular heart rhythms. The tolerable upper intake level for adults established by the National Academies is 4,000 IU per day from all sources combined.

Routine screening for vitamin D deficiency in healthy adults without symptoms is not universally recommended. Some organizations advise testing only for people with risk factors or symptoms. If you are concerned about your level, talk to your doctor about whether testing makes sense for your situation.

Can You Prevent Vitamin D Deficiency?

Prevention focuses on getting enough vitamin D from sunlight, food, and supplements when needed. The amount of sun exposure needed varies widely depending on skin tone, latitude, season, and time of day. There is no single recommendation that works for everyone.

Food sources of vitamin D are limited. They include fatty fish like salmon and mackerel, egg yolks, and fortified foods such as milk, some plant-based milks, and certain breakfast cereals. In the United States, most milk is fortified with vitamin D, but other dairy products like cheese and yogurt usually are not.

For people who cannot get enough sun exposure or dietary vitamin D, supplements are a practical option. The recommended dietary allowance set by the National Academies is 600 IU per day for adults up to age 70 and 800 IU per day for adults over 70. These are general population recommendations, not treatment doses.

Some clinicians recommend higher intakes than these general guidelines, particularly for people with risk factors. But the evidence does not consistently show that higher doses provide extra benefit for most healthy adults, and the debate about optimal levels continues.