Understand the Role of Vitamin D for Bone Infrastructure

Urbanaveed
By Urbanaveed
7 Min Read

Vitamin D keeps our bones intact and robust by integrating calcium and other important elements into them. Sunlight instigates the conversion of a precursor molecule into an inactive form of vitamin D. Upon proper stimulation, this inactive form is converted into the active form (Calcitriol) in the liver and kidneys respectively. Calcitriol participates actively in the absorption and regulation of calcium and phosphorus, essential for bone health. Calcium is an important element that has a significant contribution to various physiological processes including bone health. Deficiencies in vitamin D lead to softening of bones in adults (Osteomalacia) and children(Rickets) due to poor calcium and phosphorus uptake.

There is also another point to note. Softened or weak bones are due to inadequate calcium and other needed elements supplied to bones. This can occur in two ways; either by low calcium in diet or lack of vitamin D synthesis in the body. If your diet lacks calcium and other important elements, then it’s another discussion. In this blog, we will discuss the importance of vitamin D in the absorption of calcium and its ultimate role in bone maintenance.

Role of Vitamin D in Calcium Absorption

The active form of vitamin D (Calcitriol) increases the uptake of calcium and phosphorus from the intestinal lumen into the bloodstream. This can be achieved in the following ways;

Role of vitamin D in calcium regulation.
Vitamin D is struggling to bring calcium into blood.
  1. Role of Calcium-binding protein: Calbindin is a calcium-binding protein, whose synthesis is stimulated by vitamin D. Calbindin facilitates the active transport of calcium across the intestinal cells into the bloodstream.
  2.  Role of Calcium channels: Vitamin D also regulates the expression of TRPV6 (transient receptor potential vanilloid 6), a calcium channel that allows calcium to pass through the intestinal lining easily.
  3.  Role of Phosphorus transporters: There are phosphorus transporters like calcium transporters and channels that assist the transport of phosphorus from the intestinal lumen into the bloodstream. Vitamin D regulates the expression of these transporters and facilitators to enhance the absorption of these elements from food.

Synergistic Effect of Vitamin D and PTH

Parathyroid glands secrete parathormone(PTH) that is stimulated by low blood calcium levels. PTH has a role in increasing blood calcium levels when needed. It does so by amplifying the absorption of calcium from the small intestine. For this purpose, PTH stimulates the conversion of the inactive form of vitamin D into its active form (Calcitriol) in the kidneys. Calcitriol then enhances the absorption of calcium and phosphorus from the intestinal lumen into the bloodstream. While PTH stimulates the release of calcium from the bone matrix (bone resorption), vitamin D ensures the availability of calcium for new bone formation. They both work synergistically to regulate the calcium levels in the body.

Bone Remodeling and Vitamin D

Bone cells (osteocytes) have two main types; osteoblasts and osteoclasts. Osteoblasts are the bone-forming cells while osteoclasts are the bone-dissolving cells. Vitamin D participates in bone formation by enhancing the number and functional ability of osteoblasts. Osteoblasts have specific receptors called VDRs (vitamin D receptors) to bind vitamin D. This attachment activates the differentiating and maturation process of osteoblasts. Osteoblasts cause the formation of new bone matrix in many ways;

  1. Osteocalcin synthesis: Osteocalcin is a calcium-regulating protein synthesized by osteoblasts. It stimulates this synthesis for bone mineralization. Osteocalcin binds with calcium ions and helps incorporate them into the bone matrix.
  2.  Collagen Synthesis: Osteoblasts can synthesize collagen, providing the organic framework for the bone matrix. Collagen provides flexibility and skeletal strength to the bones. Vitamin D plays an indirect role in this aspect. Although it does not directly produce collagen but helps in the mineralization process.
  3.  Hydroxyapatite crystals: Hydroxyapatite crystals are the integral inorganic components of the bone matrix. Hydroxyapatite is a type of calcium phosphate mineral with the chemical formula Ca₅(PO₄)₃(OH). It is a crystalline structure primarily composed of calcium, phosphorus, and hydroxide ions. Vitamin D plays an indirect role in the formation of these crystals by bringing calcium and phosphorus to the desired levels.
  4.  Osteoclast Inhibition: Vitamin D plays a role in maintaining the proper balance between osteoblasts and osteoclasts. Bone-dissolving cells (osteoclasts) inhibit bone synthesis and promote bone breaking or resorption. Vitamin D inhibits the osteoclast activity to some extent but proper balance is maintained between the two bone cells for normal bone integrity.

Conclusion

Although calcium is paramount for healthy bones, calcium homeostasis is also a factor. Proper balance should be maintained between the calcium levels in the blood and bones. PTH and vitamin D act synergistically for this purpose. Vitamin D has a role in bringing calcium into the bloodstream for bone mineralization. It’s a plus point that we don’t have to spend money to obtain this vital chemical, it can be gained naturally through the sun. As explained earlier, sunlight is the major source of vitamin D.

Although it does not directly synthesize vitamin D in our bodies but initiates the conversion process. It’s good that we know the real process of vitamin synthesis and its importance in our well-being. Apart from bone health, vitamin D also has an effective role in boosting mood and emotional health.

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Student of BSc MLT at NUMS, and Content Writer in Health, Medicine, and Wellness. Finding soothe in writing and spreading knowledge.
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