How calcium is absorbed by the body
The absorption of dietary calcium occurs mainly in the upper tract of the small intestine, thanks to two distinct mechanisms. The first is an active, saturable transport present in the duodenum and in the first part of the jejunum. This mechanism involves a series of calcium-binding transporter proteins, the synthesis of which is stimulated by calcitriol, i.e. by vitamin D activated in the kidney (1,25- (OH) 2-cholecalciferol).
- from the homeostasis of the mineral in the organism, which increases it in case of deficiency or increased need, such as during adolescence, pregnancy and breastfeeding;
- from age, being higher in the child than in the adult and even more so in the elderly.
The second mode of absorption of dietary calcium exploits a passive transport by simple paracellular diffusion. As such, it is non-saturable and independent of age, vitamin D and the need for the mineral; according to this route, calcium is absorbed throughout the small intestine, mainly in the ileum, and to a small extent also in the colon.
Active absorption becomes prevalent when the meal is low in calcium, while the fraction absorbed by passive transport increases as the calcium content of the chyme increases.
What factors influence the absorption of dietary calcium?
There is a great adaptability of the organism as regards the intestinal absorption of calcium. As an indication, children absorb 60% of dietary calcium, about double that of adults (30%) and triple that of the elderly (20-25%). The amount absorbed in the various ages is further influenced by dietary and non-dietary factors. Among the latter, the most important influence is given by the sun exposure which, also depending on the latitude, covers about 80% of the requirement of vitamin D, synthesizing it starting from a derivative of cholesterol. Severe liver and kidney diseases, syndromes from malabsorption, hypoparathyroidism, intestinal resection surgery, antibiotic and laxative intake are other factors that can significantly decrease the absorption of dietary calcium. As regards the diet, however, it must first be clarified that the influence of the elements we are going to examine becomes significant, to the point of creating imbalances, only in extreme cases. For example, the reduced absorption of calcium induced by phytates, oxalates and some dietary fiber, can generate calcium deficiencies only when the diet is extremely high in fiber and low in calcium (as occurs in many vegan diets).
How to improve calcium absorption
They facilitate the absorption of calcium (within the same meal or a close meal):
• the presence of proteins (calcium binds to amino acids and is thus absorbed more easily), in particular those of dairy products;
• intestinal acidity (symbiotic acidophilic bacterial flora), which solubilizes calcium salts (in this sense prebiotics and probiotics tend to increase the absorption of the mineral);
• the calcium / phosphorus ratio in the diet close to one;• lactose and other simple sugars.
They hinder the absorption of calcium (within the same meal or a close meal):
• a Calcium / Phosphorus imbalance, a condition that occurs following the excessive intake of animal proteins;
• intestinal alkalinity, which makes calcium salts less soluble;
• a defect in the absorption of fats (steatorrhea), in which case calcium forms insoluble soaps with fatty acids;
• a lack of vitamin D;
• phytic acid (bran, fiber, whole grains), oxalic acid (spinach, rhubarb, cocoa) and tannins (tea), coffee and alcohol.
Finally, remember that a good intestinal absorption of calcium does not necessarily reflect a good bioavailability of the mineral. Once absorbed and released into the blood, in fact, calcium can be eliminated in the urine or deposited in the bones (in addition to participating in the numerous other functions it is responsible for); in this sense, the greatest stimulus to the deposition of calcium in the skeleton is given by physical activity and in particular by that which involves important but not excessive stresses, such as dancing, walking, jogging and toning exercises in the gym. Urinary losses of calcium are instead reduced by phosphorus and enhanced by sodium (30-40 mg for every two extra grams of food intake) and by a diet rich in animal proteins.
