Calcium Absorption || calcium absorption and vitamin d || calcium absorption and magnesium || what increase calcium absorption

Calcium absorption site

Calcium is actively absorbed from the duodenum and proximal jejunum against electrochemical gradients with the help of a Ca²⁺ ATPase and at least, a calcium-binding protein (CBP) of intestinal mucosal cells.

Transcellular route: Transcellular movements involves three sequential steps - 

• from the intestinal lumen to the intestinal mucosal cell across the brush border
• within the cell from the lumen to the serosal site (between the blood vessels and intestinal mucosal cell)
• finally extrusion from the cell into the blood circulation

            These three steps are regulated by calcitriol, the biologically active form of vitamin D.

Paracellular route: It involves passive calcium transport through the tight junction between the mucosal cells. The salient features of paracellular transport include - 

• It is dependent on the nutritional and physiological regulation
• It is concentration-dependent and occurs when there is an increased intake or a person taking supplements
• It occurs throughout the intestinal wall, ileum being the important site

RDA of calcium

Age group	Category of work	Calcium (mg/d)
Men	Sedentary	1000
	Moderate
	Heavy
Women	Sedentary	1000
	Moderate
	Heavy
	Pregnant women	1000
	Lactation   0-6 month   7-12 month	1200

Factors affecting calcium absorption:

Factors	Affect on calcium absorption	Calcium absorption mechanism
Physiological factors
1. Pregnancy and lactation	Increases	Increase requirement stimulates calcium absorption. Intestinal absorption of calcium increases during pregnancy for supplying a sufficient amount of calcium to the fetus.
2. Acidity	Increases	Calcium gets solubilized in an acidic medium and increases its absorption
3. Alkalinity	Decreases	Calcium becomes insoluble in an alkaline medium, therefore, reduces the absorption
4. Aging	Decreases	With advancing age efficiency of renal calcitriol in response to parathyroid hormone (PTH) reduces, therefore, reduces calcium absorption
5. Menopause	Decreases	In females higher levels of estrogen before menopause causes calcium absorption and reduces its excretion.
6. Gastrointestinal motility	Decreases	Laxative increases the rate of passage of food through the intestinal tract and thus decreases the absorption. The fiber-rich component also decreases the rate of absorption by increasing gastrointestinal motility.
7. Lack of exercise	Decreases	Sedentary life and lack of exercise can lead to bone resorption and decreases absorption
Dietary factors
1. Lactose	Increases	Lactose promotes calcium uptake by intestinal cells. Lactose lowers the pH and stimulates the intestinal flora.
2. Sugar	Decreases	It increases the urinary excretion of calcium and decreases the intestinal absorption of calcium.
3. Protein	Increases	Amino acids increase calcium solubility thereby increases calcium absorption.
4. Vitamin D	Increases	Synthesis of calcium-binding protein is induced by vitamin D in the epithelial cells of the intestine and this promotes calcium absorption.
5. Phosphorus	Increases	There is an inverse relationship between dietary phosphorus and urinary calcium. It has observed that calcium : phosphorus should be 2 : 1 for better calcium absorption.
6. Phytate	Decreases	Phytate combines with calcium to form an insoluble complex, thereby decreases absorption.
7. Oxalate	Decreases	Insoluble calcium oxalate form with calcium interacts with oxalate and this cause decrease in absorption.
8. Fermentable fiber	Decreases	By reducing the intestinal transit time fermentable fibers reduce calcium absorption.
9. Caffeine	Decreases	High intake of caffeine affects the bioavailability6 of calcium by increasing the loss of calcium in the urine and stimulating the secretion of calcium into the gastrointestinal tract.
10. Fat	Decreases	Fatty acids especially saturated fatty acids form insoluble soap with calcium and thus associate with increase losses of calcium through feces.
Psychological factors
1. Stress and anxiety	Decreases	Studies revealed that decreased efficiency of absorption and increased excretion of calcium occur when a person is stressed such as during examination.
2. Drugs	Decreases	Different drugs interfere with calcium absorption and decrease absorption.

Conclusion: 

The unabsorbed components of calcium that appear in feces together with the unabsorbed component of the digestive matter are known as endogenous fecal calcium. So, the net absorbed calcium is the difference between calcium intake and fecal calcium.