Calcium absorption and vitamin D
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Vitamin D and Calcium Absorption: Key Mechanisms
Vitamin D plays a central role in regulating calcium absorption in the intestine. The active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], binds to specific receptors in intestinal cells, leading to the production of proteins, such as calcium-binding proteins, that facilitate the active transport of calcium from the gut into the bloodstream 135. This process is especially important when dietary calcium intake is low, as vitamin D enhances the efficiency of calcium absorption through a saturable, energy-dependent pathway 345.
Forms of Calcium Absorption: Active and Passive Pathways
Calcium is absorbed in the intestine through two main pathways: an active, vitamin D-dependent transcellular pathway and a passive, paracellular pathway. The active pathway is regulated by vitamin D and is most significant when calcium intake is low, while the passive pathway becomes more prominent when calcium intake is high 345. Vitamin D can also enhance passive calcium diffusion, although the mechanisms are less well understood .
Impact of Vitamin D Status on Calcium Absorption
Low vitamin D status, or deficiency, can limit the body's ability to absorb calcium efficiently. When vitamin D levels are very low, the production of 1,25(OH)2D3 drops, leading to reduced calcium absorption 459. However, studies show that calcium absorption remains relatively efficient until vitamin D levels become severely deficient (serum 25(OH)D below about 10 ng/mL or 25 nmol/L) 26910. In this severely deficient state, the body can no longer maintain adequate levels of the active vitamin D hormone, and calcium absorption decreases significantly .
Effects of Vitamin D Supplementation on Calcium Absorption
Research in both older and younger adults indicates that increasing vitamin D intake or serum vitamin D levels above the deficiency threshold does not lead to large increases in calcium absorption 2610. In postmenopausal women and young women with low vitamin D, supplementation raised serum vitamin D levels but resulted in only small, clinically insignificant increases in calcium absorption 2610. These increases are so minor that they could be matched by a small dietary increase in calcium, such as drinking half a glass of milk . There is no clear threshold above which additional vitamin D further boosts calcium absorption in people who are not severely deficient 2610.
Role of Other Factors in Calcium Absorption
Other hormones, such as parathyroid hormone (PTH), also influence calcium absorption, often by regulating vitamin D metabolism 14. Estrogen and insulin-like growth factor 1 (IGF-1) can affect calcium absorption independently of vitamin D, but most hormonal effects are mediated through changes in vitamin D activity .
Clinical Implications
Vitamin D is essential for maintaining normal calcium absorption, especially in states of low dietary calcium or severe vitamin D deficiency 13459. However, for most people with adequate or only mildly insufficient vitamin D levels, increasing vitamin D intake does not substantially increase calcium absorption 2610. This suggests that routine vitamin D supplementation for the sole purpose of increasing calcium absorption may not be necessary unless there is a clear deficiency.
Conclusion
Vitamin D is crucial for the active absorption of calcium in the intestine, particularly when vitamin D levels are very low. However, in individuals with adequate or mildly insufficient vitamin D status, increasing vitamin D intake does not significantly enhance calcium absorption. Maintaining sufficient vitamin D is important for bone health, but supplementation above the deficiency threshold offers little additional benefit for calcium absorption.
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