What Can Vitamin D Deficiency Cause To Your Body?

Vitamin D deficiency is more common than most people think, and it does not just affect your bones. When your body does not get enough vitamin D, it can change how your muscles, immune system, and even your mood function. Low levels are linked to bone pain, muscle weakness, a higher risk of falls in older adults, and a less effective immune response. In severe cases, it causes rickets in children and osteomalacia in adults, which is a softening of the bones that leads to pain and fractures.

How Does Vitamin D Actually Work in Your Body?

Vitamin D is not like most vitamins. It acts more like a hormone. Your body makes it when sunlight hits your skin, and it also comes from certain foods and supplements. But the version made in your skin is not active yet. It has to travel to your liver and then to your kidneys to become the form your body can use.

Once activated, vitamin D helps your intestines absorb calcium and phosphorus from food. Those two minerals are the building blocks of hard, healthy bone. Without enough vitamin D, your gut only absorbs about 10-15% of the calcium you eat instead of the normal 30-40%. That drop matters. Your body will not let blood calcium levels fall too low, so it starts pulling calcium out of your bones to keep your blood stable. Over time, that weakens the skeleton.

Vitamin D also attaches to receptors in many other tissues. Receptors are found in muscle cells, immune cells, and parts of the brain. That is why deficiency can affect more than just bone density.

What Can Vitamin D Deficiency Cause To Your Body?

Severe vitamin D deficiency causes a specific set of health problems. In children, it causes rickets. In rickets, the growing bones do not harden properly, leading to bowed legs, delayed growth, and sometimes bone pain. Rickets is rare in the United States now because of fortified milk, but it still happens, especially in breastfed infants who do not get supplements.

In adults, the same process causes a condition called osteomalacia. This is not the same as osteoporosis. Osteomalacia is a softening of the bone because new bone does not mineralize correctly. People with osteomalacia often describe a deep, aching bone pain that is worse with pressure. They may also feel muscle weakness, especially in the thighs and hips. Walking can become difficult, and some people develop a waddling gait.

Long-term, milder deficiency contributes to osteoporosis. When your body keeps pulling calcium from bones to maintain blood levels, bone density drops. Weaker bones break more easily. Hip fractures in older adults are strongly linked to low vitamin D status, and falls are more common because vitamin D deficiency also weakens muscles and impairs balance.

Can Low Vitamin D Cause Muscle Weakness and Falls?

Yes. Muscle weakness is one of the clearest signs of vitamin D deficiency. This is not vague fatigue. It is a measurable loss of muscle strength, particularly in the proximal muscles — the ones closest to the center of your body. People often notice difficulty climbing stairs, getting up from a low chair, or lifting objects overhead.

The mechanism is direct. Muscle cells have vitamin D receptors, and vitamin D is needed for normal muscle contraction and protein synthesis. Without it, muscle fibers shrink and weaken. This condition is called proximal myopathy.

In older adults, this weakness translates directly into falls. Falls are the leading cause of injury-related death in people over 65. Studies have found that correcting vitamin D deficiency in older adults reduces the rate of falls. Some research suggests that taking a daily supplement can lower fall risk by about 20-30% in deficient older adults, though results vary depending on the dose and the population studied.

Does Vitamin D Deficiency Weaken Your Immune System?

Vitamin D plays a real role in immune function. Immune cells — including T cells and macrophages — have vitamin D receptors. Vitamin D helps these cells respond to infections properly. It also helps regulate inflammation, keeping the immune response from becoming excessive.

People with very low vitamin D levels tend to have more respiratory infections. Some clinical trials have tested whether vitamin D supplements prevent colds and flu. The results show that supplementation helps most in people who are genuinely deficient. For people with normal levels, taking extra vitamin D does not provide much additional protection against infections.

There is also an established link between vitamin D deficiency and autoimmune diseases like multiple sclerosis, rheumatoid arthritis, and type 1 diabetes. The evidence here is mostly observational, meaning it shows an association but does not prove cause and effect. People with higher vitamin D levels appear to have lower rates of these diseases, but no large trial has proven that supplementation prevents them.

Can Low Vitamin D Affect Your Mood or Mental Health?

Some research suggests a connection between vitamin D and depression. People with depression often have lower vitamin D levels than people without depression. But the direction of that relationship is not clear. Depression can reduce appetite, time outdoors, and self-care, all of which lower vitamin D. So it may be that depression causes low vitamin D, not the other way around.

Clinical trials testing vitamin D supplements as a depression treatment have produced mixed results. Some show modest improvement in people with severe deficiency. Others show no benefit over placebo. The current evidence does not support using vitamin D as a standalone treatment for depression. If you have low mood, it is reasonable to get your vitamin D level checked, but do not expect a supplement to fix depression on its own.

There is also some early research looking at vitamin D and cognitive decline in older adults. Low levels are associated with faster cognitive decline and a higher risk of dementia. Again, this is association, not proven cause. The same caution applies: we do not yet know if correcting the deficiency prevents or slows dementia.

What Are the Less Common Effects of Severe Deficiency?

Severe, prolonged deficiency can cause problems beyond bone and muscle. One is hypocalcemia — low blood calcium. Symptoms include numbness and tingling in the hands and feet, muscle spasms, and in severe cases, seizures. This is more common in infants and people with conditions that block vitamin D absorption, such as celiac disease or Crohn’s disease.

Vitamin D deficiency has also been associated with high blood pressure and cardiovascular disease. The evidence is observational, and clinical trials have not consistently shown that supplementation lowers blood pressure or prevents heart attacks. Vitamin D deficiency may be a marker of poor overall health rather than a direct cause of heart disease.

Some research has linked low vitamin D to a higher risk of certain cancers, particularly colorectal cancer. The evidence is suggestive but not conclusive. No major health organization currently recommends vitamin D supplementation specifically to prevent cancer.

Who Is Most at Risk for Vitamin D Deficiency?

Certain groups are much more likely to have low vitamin D. Older adults produce less vitamin D in their skin when exposed to sunlight, and their kidneys are less efficient at converting it to the active form.

People with darker skin have higher levels of melanin, which reduces the skin’s ability to produce vitamin D from sunlight. In the United States, Black and Hispanic populations have significantly higher rates of deficiency than white populations.

People who live in northern latitudes or who spend little time outdoors are also at risk. So are people who always wear sunscreen or protective clothing when outside. Sunscreen does block vitamin D production, but dermatologists still recommend sunscreen because the skin cancer risk from unprotected sun exposure outweighs the benefit of vitamin D synthesis.

People with obesity are at higher risk because vitamin D is fat-soluble. It gets sequestered in fat tissue and does not circulate freely in the blood. People with digestive disorders that impair fat absorption — like celiac disease, Crohn’s disease, or after gastric bypass surgery — also absorb less vitamin D.

How Do You Know If You Are Deficient?

The only reliable way to know is a blood test. The test measures 25-hydroxyvitamin D, which is the storage form of the vitamin. Most laboratories define deficiency as a level below 20 ng/mL (50 nmol/L). Levels between 20 and 30 ng/mL are considered insufficient. Levels above 30 ng/mL are generally considered adequate.

Symptoms alone are not reliable. Bone pain and muscle weakness can have many causes. Many people with mild deficiency have no symptoms at all. That is why testing is important if you are in a high-risk group or if you have unexplained bone pain or muscle weakness.

If you are deficient, the standard treatment is a daily supplement. The dose depends on your starting level and your doctor’s recommendation. Do not self-prescribe very high doses. Vitamin D is fat-soluble, meaning your body stores it. Taking extremely high doses for months can cause vitamin D toxicity, which leads to dangerously high blood calcium levels.