What Happens if Vitamin D Levels Are Really Low?

October 8, 2025 •

3 min read

According to the NIH Office of Dietary Supplements, almost one in four Americans have vitamin D blood levels considered inadequate for bone and overall health, while even lower levels define a deficiency. Knowing what happens if vitamin D levels are really low is crucial for understanding the widespread impact this 'sunshine vitamin' has on the entire body, not just our bones.

Quick Summary

Severe vitamin D deficiency affects bone mineralization, causing rickets in children and osteomalacia in adults, and can impact the immune system, increase infection risk, and lead to fatigue, muscle weakness, and mood disturbances. Timely treatment is crucial to prevent serious long-term complications, though some individuals may not experience noticeable symptoms initially.

Key Points

Severe Deficiency Risks: Low vitamin D impairs calcium absorption, leading to bone demineralization and diseases like rickets in children and osteomalacia in adults.
Immune System Dysfunction: A deficiency weakens the immune response, increasing susceptibility to infections and being linked to higher rates of autoimmune diseases.
Neurological and Mental Symptoms: Really low levels are associated with fatigue, mood changes, depression, and cognitive decline.
Cardiovascular Health Impact: Chronic deficiency is linked to an increased risk of cardiovascular disease, including hypertension and heart failure.
Diagnosis and Treatment: A blood test for 25-hydroxyvitamin D is used for diagnosis, and treatment typically involves high-dose supplements under medical supervision.
Long-term Consequences: Unaddressed, chronic deficiency can lead to irreversible bone damage, increased fracture risk, and greater susceptibility to other serious health conditions.

The Core Function of Vitamin D

Vitamin D is a fat-soluble nutrient that acts more like a hormone, binding to receptors in almost every cell. Its main role is to regulate calcium and phosphorus levels in the blood. Insufficient vitamin D hinders the body's ability to absorb these minerals from food, leading to health issues.

The Breakdown: What Goes Wrong When Levels Plummet

Critically low vitamin D levels (below 12 ng/mL or 30 nmol/L) significantly impair bodily functions. Reduced calcium absorption results in hypocalcemia. The body reacts by increasing parathyroid hormone (PTH) to extract calcium from bones, maintaining blood calcium but weakening bones.

Skeletal Consequences: Rickets and Osteomalacia

The most prominent effects of very low vitamin D are bone diseases: rickets in children and osteomalacia in adults.

Rickets: Affects children's growing bones, causing softness, weakness, bowed legs, and joint deformities.
Osteomalacia: In adults, it softens and weakens existing bones, leading to pain, fragility, and increased fracture risk.

Impact on the Immune System

Vitamin D is vital for modulating the immune system. Really low levels compromise the immune response, increasing susceptibility to:

Increased Infections: Vitamin D helps immune cells produce antimicrobial peptides. Deficiency makes the body more vulnerable to severe respiratory infections like colds, bronchitis, and pneumonia.
Autoimmune Disease Risk: Low vitamin D is linked to a higher risk of conditions like multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.

Neurological and Mental Health Effects

Vitamin D receptors in brain areas linked to mood and cognition suggest a role in neurological and psychological health. Severe deficiency can contribute to:

Fatigue and Depression: Strongly associated with fatigue, tiredness, and mood changes, including depression.
Cognitive Decline: May be linked to impaired cognitive function, particularly in older adults, possibly increasing the risk of dementia.
Neuromuscular Issues: Muscle pain, weakness, and cramps are common due to mineral imbalance. In rare cases, especially in infants, it can cause muscle spasms and seizures.

Cardiovascular and Other Risks

Chronic vitamin D deficiency is also a risk factor for cardiovascular issues.

Cardiovascular Disease: Linked to increased risk of high blood pressure, heart disease, heart failure, and stroke.
Increased Mortality: Studies indicate a higher all-cause and cardiovascular mortality risk in those with severe deficiency.
Poor Wound Healing: May be slower due to vitamin D's role in inflammation and infection control.

Diagnosis and Management of Severe Deficiency

Diagnosis involves a blood test to measure serum 25-hydroxyvitamin D [25(OH)D]. Levels below 12 ng/mL are deficient and require specific treatment.


Level (ng/mL)	Status	Approach
<12	Deficient	Requires medical intervention to raise levels.
12-20	Insufficient	May benefit from supplementation and monitoring.
20-30	Insufficient	Levels should be monitored and supplementation considered.

Treatment typically involves supplementation under medical supervision to restore levels. This should be managed by a healthcare provider. Lifestyle changes, including safe sun exposure and fortified foods, also help maintain levels.

Conclusion

Persistently low vitamin D levels pose significant health risks, from bone conditions like osteomalacia and rickets to issues with the immune system, nervous system, and cardiovascular health. Symptoms can be subtle, making diagnosis challenging, but addressing a severe deficiency is crucial to prevent long-term damage. With proper medical care, this deficiency is treatable, leading to improved overall health and reduced risk of serious complications. If you experience persistent fatigue, bone pain, or frequent infections, a vitamin D blood test is advisable.

NIH Office of Dietary Supplements provides additional resources on vitamin D and health.

Frequently Asked Questions

While medical organizations differ slightly, a serum 25-hydroxyvitamin D level below 12 nanograms per milliliter (ng/mL), or 30 nanomoles per liter (nmol/L), is generally considered a severe deficiency.

Symptoms can be vague, but early signs may include fatigue, bone pain, and muscle weakness or aches. Some individuals may experience no noticeable symptoms at all, even with a severe deficiency.

Yes, really low vitamin D levels have been associated with changes in mood and an increased risk of depression. The vitamin interacts with brain receptors that influence emotional processing.

A doctor will typically recommend a treatment plan involving supplementation to raise vitamin D levels to a healthier range. This plan is tailored to the individual's specific needs and the severity of the deficiency.

For a severe deficiency, diet alone is typically not enough to correct the problem. Supplements are necessary to quickly and effectively raise levels. However, a diet rich in vitamin D can help maintain healthy levels once restored.

In children, very low vitamin D levels cause rickets, a condition that leads to soft, weak, and deformed bones during their growth stage. Bowed legs and joint deformities are classic signs.

Yes, excessive intake from supplements can lead to vitamin D toxicity, a condition called hypervitaminosis D. Symptoms can include nausea, increased thirst, and poor appetite. You cannot get too much from sun exposure.