How does vitamin D affect calcium levels?

Vitamin D's Role in Calcium Regulation

Vitamin D directly controls calcium absorption in the small intestines, interacts with parathyroid hormone to maintain calcium homeostasis between blood and bones, and is essential for bone growth and maintaining bone density. 1

Mechanism of Vitamin D and Calcium Relationship

• Vitamin D helps bones absorb calcium, acting as a critical regulator of calcium and bone homeostasis 1, 2
• When vitamin D is insufficient, calcium absorption is impaired, leading to inadequate calcium levels for the body's needs 3
• The active form of vitamin D (calcitriol) controls calcium absorption in the small intestines through a vitamin-D-regulated saturable pathway 1, 2
• Vitamin D interacts with parathyroid hormone to maintain calcium homeostasis between blood and bones 1
• In vitamin D deficiency, parathyroid hormone production increases, causing calcium to be mobilized from bones and reabsorbed in the kidneys to maintain normal serum calcium levels - a condition known as secondary hyperparathyroidism 3

Sources and Metabolism of Vitamin D

• There are two main sources of vitamin D in the human body 1:
  • Ergocalciferol (vitamin D2) - consumed in the diet, mainly from fatty fish
  • Cholecalciferol (vitamin D3) - synthesized in the skin through ultraviolet B rays from the sun
• Vitamin D3 is converted to its active form through enzymatic processes in the liver and kidney 1
• Most cells contain specific receptors for the active form of vitamin D 1

Clinical Implications

• Insufficient vitamin D leads to impaired dietary calcium absorption 1
• When vitamin D is inadequate, the body uses calcium from skeletal stores, which can weaken existing bones 1
• Vitamin D supplementation increases calcium absorption, though the effect is modest (6% increase) when serum 25OHD levels are above 10 ng/ml 4
• Calcium absorption is more significantly related to serum 25OHD levels than to vitamin D dose 4
• The Institute of Medicine defines adequate vitamin D levels as >50 nM (>20 ng/ml) of 25-OH-vitamin D 1

Vitamin D and Calcium Beyond Bone Health

• Most organs, including the gut, brain, heart, pancreas, skin, kidneys, and immune system have receptors for active vitamin D 3
• Vitamin D has beneficial effects on muscle strength and balance 1
• Vitamin D deficiency can lead to neuromuscular dysfunction and increased risk of falls 3
• Current evidence indicates that calcium with or without vitamin D intake from food or supplements has no relationship (beneficial or harmful) with cardiovascular disease risk 1

Recommendations for Supplementation

• Calcium intake from food sources is preferred over supplements 1
• Supplemental calcium can be safely used to correct shortfalls in intake without cardiovascular concerns, as long as intake doesn't exceed 2000-2500 mg/day 1
• For those at risk of low calcium, consumption of 1500 mg/day is recommended to optimize bone health 1
• The gut cannot absorb more than 500 mg of calcium at once, so calcium intake should be spread throughout the day 1
• Daily maintenance supplementation of 1000-2000 IU vitamin D3 is reasonable for female athletes, depending on sun exposure 1

Pitfalls and Caveats

• High-dose vitamin D supplements (>4000 IU/day) have been associated with increased falls and fractures 5
• Calcium supplements can cause gastrointestinal side effects, particularly constipation, and increase the risk of kidney stones 5
• Correction of severe vitamin D deficiency (<25 nmol/L) is necessary before using potent anti-resorptive drugs to avoid hypocalcemia 5
• The increase in absorbed calcium from vitamin D supplementation is small (about 6%) and equivalent to consuming half a glass of milk or 100 mg of elemental calcium 4