What Happens When You're Severely Low on Vitamin D?

October 22, 2025 •

3 min read

According to research from the National Health and Nutrition Examination Survey (NHANES), nearly a quarter of U.S. adults are considered low in vitamin D. When you're severely low on vitamin D, your body can no longer properly absorb calcium, leading to serious health complications beyond simple tiredness, including weakened bones and impaired immune function.

Quick Summary

This article explores the severe consequences of a profound vitamin D deficiency, detailing the debilitating effects on bone health, the increased risk of infections, and potential links to mental health issues like depression. It also covers diagnosis methods and recommended treatment protocols.

Key Points

Skeletal Damage: A severe vitamin D deficiency leads to weakened bones and conditions like rickets in children and osteomalacia in adults, increasing fracture risk.
Muscle Weakness: Profoundly low levels can cause chronic muscle aches, cramps, and weakness, impacting mobility and increasing the risk of falls, especially in older adults.
Impaired Immune Function: Vitamin D is essential for a healthy immune response, and a deficiency can lead to an increased susceptibility to frequent infections, particularly respiratory illnesses.
Mental Health Concerns: Evidence links a severe deficiency to mood disorders such as depression and anxiety, and some studies show supplementation may improve symptoms.
Diagnosis is Key: A healthcare provider can diagnose a deficiency with a simple 25-hydroxyvitamin D blood test, which is the most accurate measure of your body's vitamin D stores.
Treatment is Effective: High-dose vitamin D supplementation, under medical supervision, can effectively correct a severe deficiency, followed by a lower maintenance dose to prevent recurrence.

Bone Health and Skeletal System

One of vitamin D's primary roles is regulating the absorption of calcium and phosphorus, which are vital for bone formation and strength. When you're severely low on vitamin D, your body struggles to absorb these minerals from your diet, triggering a chain reaction that compromises your entire skeletal system. This leads to bone demineralization, a process where bones break down faster than they can be rebuilt, causing them to soften and weaken.

Rickets in Children

For children, a severe and prolonged vitamin D deficiency can cause rickets, a condition where developing bones fail to mineralize properly. This can result in painful, soft, and misshapen bones, leading to physical deformities such as bowed legs, stunted growth, and enlarged or malformed joints. In very young infants, it can even cause muscle spasms and seizures due to extremely low calcium levels in the blood.

Osteomalacia in Adults

Adults with a severe vitamin D deficiency develop osteomalacia, a painful condition characterized by weak, soft bones. Unlike osteoporosis, which involves a loss of bone mass, osteomalacia is a defect in the bone mineralization process itself. Symptoms include chronic bone pain, often in the lower back, hips, and legs, and significant muscle weakness. This muscle weakness can lead to a waddling gait and an increased risk of falls and fractures.

Immune System Dysfunction

Beyond its well-known role in bone health, vitamin D is also critical for a properly functioning immune system. It directly interacts with the cells that fight infections, and a severe deficiency can significantly impair your body's ability to ward off illness.

Increased Susceptibility to Infections

Research shows a clear link between low vitamin D levels and an increased risk of respiratory tract infections, including the common cold, bronchitis, and pneumonia. Some studies also associate severe deficiency with a higher likelihood of other illnesses, such as tuberculosis. By supporting the function of T-cells and other immune components, adequate vitamin D is essential for a robust defense against pathogens.

Mental Health and Neurological Effects

Vitamin D's influence extends to the brain, and a severe deficiency can have a notable impact on mental health and cognitive function. The vitamin is involved in serotonin synthesis and brain plasticity, processes crucial for mood regulation.

Depression and Anxiety

Multiple studies have documented a correlation between low vitamin D levels and symptoms of depression and anxiety. While the exact nature of this relationship is still being researched, some trials suggest that supplementation can help improve mood in deficient individuals. In severe cases, profound vitamin D deficiency has been linked to severe depressive symptoms and anxiety.

Comparison of Vitamin D Deficiency Levels


Health Status	Serum Level (ng/mL)	Key Health Effects
Adequate	≥20	Optimal bone and overall health.
Insufficient	12 to <20	Often asymptomatic, but can be associated with increased risk of falls and bone loss.
Deficient (Mild-Moderate)	10 to <12	Increased risk of chronic fatigue, muscle aches, and general unwellness.
Severely Deficient	<10	High risk for osteomalacia (adults) and rickets (children), chronic pain, and severe muscle weakness.

Diagnosis and Treatment of Severe Vitamin D Deficiency

Diagnosing a severe vitamin D deficiency typically involves a blood test to measure the level of 25-hydroxyvitamin D. Treatment focuses on rapidly restoring vitamin D levels to a healthy range through high-dose supplementation, often with prescription-strength vitamin D2 or D3. The initial high-dose therapy is followed by a lower, daily maintenance dose. While supplementation is the most effective treatment, dietary changes and increased sun exposure are also part of a comprehensive strategy. It is crucial to work with a healthcare provider to determine the appropriate dosage and duration of treatment to avoid complications from over-supplementation, such as hypercalcemia.

Conclusion

Being severely low on vitamin D is far more than just a minor inconvenience. It leads to profound and debilitating health problems that affect the musculoskeletal, immune, and nervous systems. From the risk of developing conditions like rickets and osteomalacia to an increased susceptibility to infections and a potential impact on mental health, the consequences are significant. Early diagnosis through a blood test and proper, medically supervised treatment with supplements are critical for reversing the effects and preventing long-term complications. Don't ignore the subtle signs; addressing a deficiency promptly is key to protecting your overall well-being. For more detailed clinical guidelines, you can consult the National Institutes of Health Fact Sheet on Vitamin D.

Frequently Asked Questions

Initial symptoms can be vague and easy to dismiss, often including fatigue, general tiredness, and mild bone or muscle aches. As the deficiency worsens, symptoms become more pronounced, with chronic pain and muscle weakness being common indicators.

Yes. Severe and prolonged vitamin D deficiency leads to a loss of bone density and poor mineralization, which can contribute to osteoporosis and significantly increase the risk of fractures.

Studies have shown an association between low vitamin D levels and an increased risk of depression and anxiety. Vitamin D influences serotonin synthesis and brain function, so correcting a deficiency may help improve mood in some individuals.

Diagnosis is made through a blood test that measures your serum 25-hydroxyvitamin D (25[OH]D) level. This is the most accurate way to assess your body's vitamin D status.

Treatment involves high-dose vitamin D supplementation, typically with vitamin D2 or D3, to rapidly restore levels. A healthcare provider determines the appropriate dosage and duration, which is then followed by a lower, daily maintenance dose.

Individuals with limited sun exposure, older adults, people with darker skin, and those with malabsorption disorders (like Crohn's or celiac disease) are at higher risk. Breastfed infants and people with obesity are also susceptible.

Yes. While rare, vitamin D toxicity can occur from excessive oral intake of supplements, leading to a build-up of calcium in the blood (hypercalcemia). This can cause symptoms like nausea, weakness, and, in severe cases, kidney damage.