Calcium Citrate +
Vitamin D3 Tablets

Mechanism

Calcium carbonate: It is an inorganic salt used as an antacid. It is a basic compound that acts by neutralizing hydrochloric acid in gastric secretions. An increase in bicarbonate ions and prostaglandins may also confer cytoprotective effects. Calcium carbonate is a basic inorganic salt that acts by neutralizing hydrochloric acid in gastric secretions. It also inhibits the action of pepsin, it increases the pH. Cytoprotective effects may occur through increases in bicarbonate ion (HCO3-) and prostaglandins. Neutralization of hydrochloric acid results in the formation of calcium chloride, carbon dioxide and water. 90% of calcium chloride is converted to insoluble calcium salts.
Vitamin D3: The active form of vitamin D binds to intracellular receptors that then function as transcription factors to modulate gene expression. Like the receptors for other steroid hormones and thyroid hormones, the vitamin D receptor has hormone-binding and DNA-binding domains.

Pharmacodynamics

Calcium carbonate: Gastric-peptic disease occurs as a result of an imbalance between protective factors, such as mucus, bicarbonate, and prostaglandin secretion, and aggressive factors, such as hydrochloric acid, pepsin, and Helicobacter pylori . The acid-neutralizing capacity of calcium carbonate is 58 mEq/15 ml. When used as a nutritional supplement, calcium carbona te acts by directly increasing calcium stores within the body.
Vitamin D3: The in vivo synthesis of the predominant two biologically active metabolites of vitamin D occurs in two steps. The first hydroxylation of vitamin D3 cholecalciferol (or D2) occurs in the liver to yield 25-hydroxyvitamin D while the second hydroxylation happens in the kidneys to give 1, 25-dihydroxyvitamin D . These vitamin D metabolites subsequently facilitate the active absorption of calcium and phosphorus in the small intestine, serving to increase serum calcium and phosphate levels sufficiently to allow bone mineralization . There exists a period of 10 to 24 hours between the administration of cholecalciferol and the initiation of its action in the body due to the necessity of synthesis of the active vitamin D metabolites in the liver and kidneys . It is parathyroid hormone that is responsible for the regulation of such metabolism at the level of the kidneys.

Pharmacokinetics

Calcium carbonate:

• Absorption: Cholecalciferol is readily absorbed from the small intestine when fat absorption is normal. Bile is essential for its absorption. Studies show that (a) 25-hydroxyvitamin D is absorbed more efficiently than the non-hydroxylated form, (b) the amount of fat consumed does not significantly affect bioavailability, and (c) age does not significantly impact absorption.
• Protein Binding: Cholecalciferol exhibits 50–80% protein binding. In plasma, vitamin D3 (from diet or skin) binds to vitamin D-binding protein (DBP) produced in the liver for transport. The 25-hydroxycholecalciferol form circulates bound to DBP (α2-globulin).
• Metabolism: In the liver, cholecalciferol is hydroxylated by vitamin D-25-hydroxylase to form calcifediol (25-hydroxycholecalciferol). In the kidneys, calcifediol is further converted by 1-alpha-hydroxylase into calcitriol (1,25-dihydroxycholecalciferol), the active form of vitamin D3.
• Elimination: Cholecalciferol and its metabolites are primarily excreted through bile and feces.

Vitamin D3:

• Absorption: Maximum absorption occurs at doses of 500 mg or less when taken with food. Oral bioavailability is influenced by intestinal pH, presence of food, and dosage form.
• Distribution: Calcium is rapidly absorbed and distributed into extracellular fluids and skeletal tissues. Approximately 99% of total body calcium is stored in bones, while the remaining 1% is equally distributed between intracellular and extracellular compartments. Calcium also functions as a cofactor for numerous enzymatic reactions.
• Elimination: Calcium is primarily excreted through feces. Most of the filtered calcium in the kidneys is reabsorbed in the ascending limb of the loop of Henle and in the proximal and distal convoluted tubules. A small amount is also eliminated through sweat.

Side Effects

• Constipation
• Gastrointestinal discomfort
• Nausea
• Hypercalcemia (rare)

Contraindication

• Hypercalcemia
• Severe renal impairment
• Kidney stones
• Hypersensitivity to any component

Interaction

• Tetracyclines: Reduced absorption
• Iron supplements: Decreased bioavailability
• Thiazide diuretics: Increased calcium levels
• Vitamin D excess: Risk of hypercalcemia

Warning and Precautions

• Use under medical supervision
• Monitor calcium levels during long-term use
• Use cautiously in renal impairment
• Pregnancy and lactation: use only if prescribed
• Avoid overdose