What Does Low Vitamin D Do to Calcium?

January 1, 2026 •

4 min read

According to research, inadequate vitamin D levels lead to a drastic reduction in the body's ability to absorb dietary calcium, causing a cascade of negative health consequences. Understanding what low vitamin D does to calcium is crucial for preventing long-term damage to your bones and overall health.

Quick Summary

Low vitamin D drastically reduces the body's absorption of calcium, forcing the body to pull mineral reserves from bones to maintain critical functions. This process leads to secondary hyperparathyroidism, which can cause significant bone weakening, osteoporosis, and related fractures over time.

Key Points

Impaired Absorption: Low vitamin D drastically reduces intestinal calcium absorption, forcing the body to seek calcium from internal sources.
Secondary Hyperparathyroidism: The body’s response to low blood calcium is to increase parathyroid hormone (PTH), which triggers the release of calcium from your bones.
Increased Bone Demineralization: Persistent PTH stimulation causes accelerated bone turnover and demineralization, weakening the skeletal structure over time.
Elevated Fracture Risk: The resulting low bone density from this chronic process significantly increases the risk of developing osteoporosis and suffering fractures.
Systemic Symptoms: Beyond bone health, low vitamin D's effect on calcium can cause muscle weakness, cramps, neurological symptoms, and can even influence heart function.
Need for Supplementation: Adequate intake through diet, sun exposure, and often supplementation is crucial for maintaining proper calcium absorption and preventing long-term health issues.

The Essential Partnership: Vitamin D and Calcium

Calcium is a vital mineral, not only for building and maintaining strong bones, but also for muscle function, nerve transmission, and blood clotting. However, consuming enough calcium is only half the battle; without adequate vitamin D, the body cannot effectively absorb and utilize this mineral. Vitamin D acts as a crucial facilitator, ensuring that the calcium you consume is properly moved from your digestive tract into your bloodstream. When vitamin D levels are sufficient, intestinal calcium absorption increases significantly, but a deficiency can reduce this to as low as 10–15%. This critical relationship is a key aspect of maintaining what is known as calcium homeostasis, or the stable balance of calcium within the body.

The Body’s Emergency Compensation for Low Vitamin D

When vitamin D levels drop, the body's ability to absorb dietary calcium plummets. This creates a state of low blood calcium, known as hypocalcemia. To correct this, the body initiates a complex hormonal response. Specialized glands in the neck, called the parathyroid glands, detect the drop in serum calcium and release a hormone called parathyroid hormone (PTH).

PTH has several mechanisms to increase blood calcium levels:

Stimulating bone resorption: This is the most significant effect. PTH activates cells called osteoclasts, which break down existing bone tissue to release stored calcium and phosphorus into the bloodstream. This is the body's emergency maneuver to prioritize vital functions over bone integrity.
Enhancing kidney reabsorption: PTH signals the kidneys to conserve calcium that would otherwise be excreted in urine.
Activating remaining vitamin D: The hormone prompts the kidneys to increase the production of the active form of vitamin D, which further aids in whatever limited intestinal calcium absorption is possible.

This protective, short-term mechanism becomes highly destructive over the long run. The constant mobilization of calcium from the skeleton leads to progressively weaker, more porous bones.

The Health Consequences of Compromised Calcium

Prolonged low vitamin D and the resulting calcium depletion can lead to severe health issues affecting the skeletal system and beyond.

Bone and Skeletal Disorders

Rickets (in children): With inadequate mineralization, growing bones fail to harden properly. This can cause bone deformities, such as bowed legs, and stunted growth. While less common today, it is a clear example of the severe impact of vitamin D and calcium deficiency.
Osteomalacia (in adults): This condition, characterized by soft bones, results from the failure of new bone tissue to mineralize properly. It can cause persistent bone pain, muscle weakness, and an increased risk of fractures.
Osteoporosis: A persistent state of secondary hyperparathyroidism due to low vitamin D accelerates bone loss, leading to osteoporosis. The continual draining of calcium from bones leaves them brittle and fragile, dramatically increasing the risk of painful and debilitating fractures, especially in older adults.

Non-Skeletal Health Problems

Muscle Weakness and Cramps: Proper muscle contraction relies on a precise balance of calcium. When hypocalcemia occurs, neuromuscular function is compromised, leading to weakness, involuntary twitching, and painful muscle cramps. In severe cases, this can lead to tetany, which involves severe and uncontrollable muscle spasms.
Neurological Symptoms: The nervous system also depends on calcium for proper functioning. Low blood calcium can cause neurological symptoms like confusion, fatigue, and depression.
Cardiovascular Issues: Severe hypocalcemia can even affect heart function. It can prolong the QT interval in an electrocardiogram, increasing the risk of abnormal heart rhythms.

Comparison: Vitamin D Sufficiency vs. Deficiency


Feature	Optimal Vitamin D Levels	Low Vitamin D Levels
Intestinal Calcium Absorption	Efficient and robust (30-40% of dietary intake).	Severely limited (10-15% of dietary intake).
Parathyroid Hormone (PTH)	Maintained within a normal, stable range.	Elevated to trigger calcium release from bones.
Bone Health	Optimized mineralization and strong bone density.	Accelerated bone resorption and demineralization.
Calcium Source	Primarily from diet and supplements.	Primarily from bone reserves.
Associated Risk	Low risk of rickets, osteomalacia, and osteoporosis.	High risk of rickets, osteomalacia, and osteoporosis.
Neuromuscular Function	Normal and healthy nerve and muscle activity.	Increased risk of muscle weakness, cramps, and tetany.

Prevention and Treatment Strategies

Preventing the negative effects of low vitamin D and calcium is a proactive effort that involves several key areas:

Adequate Sunlight Exposure: The skin naturally produces vitamin D when exposed to sunlight. For many, especially those at higher latitudes or with darker skin, sun exposure may not be sufficient year-round.
Dietary Sources: Incorporating foods rich in calcium and vitamin D can help. Good sources of vitamin D include fatty fish, fortified milk, cereals, and orange juice. Dairy products, leafy greens, and fortified juices are excellent sources of calcium.
Supplementation: For many, dietary intake and sun exposure are not enough. Health professionals often recommend supplements containing both vitamin D and calcium to ensure adequate levels. The recommended daily intake varies by age, with older adults often needing higher amounts.
Medical Supervision: Regular monitoring of vitamin D levels, especially for high-risk individuals, is essential. This includes breastfed infants, older adults, people with conditions causing malabsorption, and individuals with obesity.

Conclusion

The question of what does low vitamin D do to calcium has a clear answer: it disrupts the body’s fundamental ability to manage its mineral stores, with severe consequences. By impairing calcium absorption, a vitamin D deficiency forces the body to cannibalize its own bone tissue to maintain essential blood calcium levels. This leads to weakened bones, increasing the risk of debilitating conditions like osteoporosis and fractures. The impact extends beyond the skeleton, affecting muscle and nerve function. Proactive strategies including diet, sunlight, and targeted supplementation can help prevent this damaging cycle, protecting your long-term bone health and overall well-being.

For more in-depth information, you can read about the diagnosis and management of hypocalcaemia from the Canadian Family Physician journal.

Frequently Asked Questions

No, even if your diet is rich in calcium, a vitamin D deficiency will prevent your body from absorbing a significant portion of it. The body can only absorb about 10-15% of dietary calcium with low vitamin D.

The body compensates by increasing the production of parathyroid hormone (PTH). PTH then pulls calcium from your bones to restore normal blood calcium levels, a process called bone resorption.

Early signs can include muscle cramps, spasms, or weakness, particularly in the legs. You might also experience tingling or numbness around the mouth.

The chronic release of parathyroid hormone causes the body to continuously draw calcium from your bones. This leads to a gradual reduction in bone density, ultimately increasing your risk of osteoporosis and fractures.

Yes. Without sufficient vitamin D, your body can't absorb and utilize the calcium you consume, regardless of intake. This still puts you at risk for conditions caused by low blood calcium, like bone weakening.

Good sources of calcium include dairy products, leafy greens, and fortified foods. Vitamin D can be found in fatty fish, egg yolks, and fortified products like milk and cereal.

Many healthcare providers recommend a combined supplement, especially for those at risk of deficiency, to ensure both nutrients are replenished simultaneously and correctly absorbed. Always consult a healthcare provider for personalized recommendations.

Osteomalacia is the softening of bones due to a mineralization defect, often caused by vitamin D deficiency. Osteoporosis is a condition of fragile, porous bones caused by excessive bone loss over time, which low vitamin D can contribute to.