How to increase calcium absorption?

Calcium is one of the essential minerals for humans and you must get it through your diet. In addition to its well-known structural role as part of the woven bone and teeth, calcium participates in the activation of many enzymes, in blood coagulation, in muscle contraction, in the release of neurotransmitters and in the transmembrane transport of ions, very important, among others, for the transmission of nerve impulse and the regulation of the heartbeat.

The importance of calcium is evident in the Dietary Reference Intake (DRI), which ranges from 800 mg to 1,200 mg, one of the highest of all micronutrients and often elusive. In addition, it is one of the minerals that less is used in the dietsince his absorption has an efficiency that can range between 25 and 75%depending on numerous factors, such as age, diet, physical activity, relationship with other micronutrients or health status.

How is calcium absorbed?

Calcium is absorbed in its ionic form (Ca2+), so it must be detached from the substances that contain it and solubilized. For this to occur, the pH must be acidic, hence most of the calcium is absorbed in the duodenum (pH < 7). In the rest of the intestine, absorption is lower due to the higher pH, which favors the formation of calcium salts, most of which are insoluble.

Calcium absorption can occur in two ways:

  1. Paracellular pathway: follows a non-saturable passive diffusion absorption mechanism. It occurs throughout the entire intestine, mainly the jejunum and ileum, and is apparently independent of vitamin D.
  2. transcellular route: follows an active transport mechanism in enterocytes, the epithelial cells of the intestine, but it is a saturable pathway. Calcium is taken up by the enterocyte through specific calcium channels and is transported through the cell cytoplasm to the bloodstream. This route of absorption occurs mainly in the duodenum and proximal jejunum and is calcitriol dependentthe active form of vitamin D.

active absorption is much more efficient than passive absorption but has a maximum absorption limit. Given relatively high intakes of calcium, all the resources of the enterocytes can become occupied. At that moment, this pathway becomes saturated and is not able to absorb all the calcium that may be in the intestine.

The paracellular pathway, on the other hand, is not saturable and predominates at high doses of calcium when the transcellular pathway has been saturated, but it is very inefficient and is not capable of absorbing all the calcium that may be in the lumen.

From these characteristics it can be deduced that to increase calcium absorption, it is necessary to stimulate absorption via the transcellular route, which is the most efficient. In this way, the body could be able to assimilate the amount of calcium it needs even with relatively low intakes.

The role of vitamin D

The main factor regulating absorption of calcium via the transcellular route is the calcitriol (1,25-dihydroxycholecalciferol), a derivative of cholecalciferol (vitamin D3). The amount of vitamin D3 in food is usually very low. Its main source comes from cholesterol, which in the skin is transformed into vitamin D3 by action of UVB rays from solar radiation.

Subsequently, vitamin D3 is transformed into calcitriol in the kidneys, a process that is stimulated by parathormone (PTH)a hormone secreted by the thyroid.

Estrogenic hormones also stimulate calcitriol synthesis while glucocorticoids (cortisol, aldosterone) and thyroid hormone have the opposite effect. The estrogen drop after menopause is the main cause of osteoporosis in postmenopausal women.

Due to the effect of these hormones and the two absorption mechanisms, our body is equipped to ensure the absorption of calcium, regardless of whether the intake is high or low, but with a slight nuance: maximum absorption efficiency occurs when the intake of calcium is not very high but the demand is high (growth, pregnancy, lactation, physical exercise), what is when hormonal regulation stimulates the vitamin D-dependent absorption pathway.

Furthermore, calcium has a close relation to the metabolism of phosphorus and magnesium. All three minerals are regulated by PTH and if there is an excess or deficiency of any of them, the absorption of all three can be affected and even calcium metabolism can be compromised.

What dietary factors affect absorption?

The main dietary sources of calcium are in the milk and milk productsfish (especially those eaten with fishbone), whole grains, nuts and legumes. But having high amounts of calcium is not synonymous with good absorption or a positive effect on calcium metabolism, especially with regard to bone mineralization.

As we have seen, the active absorption of calcium is increased by vitamin D, which we synthesize thanks to exposure to solar radiation and which in the diet is very scarce. In northern countries, with times of the year with little solar incidence, it is usual to recommend supplementation with vitamin D, especially in special stages such as pregnancy or childhood.

Calcium absorption is also stimulated by factors that favor its solubilization in the intestinal lumen, such as acidic pH, and healthy lifestyle habits that increase the demand for calcium and activate hormonal regulation, such as constant physical exercise and regular.

On the contrary, factors that decrease its dissolution, such as the presence of oxalates and phytates in foods (green leafy vegetables, whole grains), practices that decrease the demand for calcium, such as sedentary lifestyleor situations that decrease estrogen levels (for example in menopause), interfere with effective absorption. The abuse of alcohol, caffeine, tobacco, diets rich in sodium and a high consumption of sugars also decrease absorption.

milk and milk products

Milk and its derivatives have always been the most recommended to achieve a correct calcium intake. In addition to containing calcium in high amounts, it also contains lactose and caseins that favor its absorption. Casein by forming soluble complexes and lactose is believed to increase the permeability of the intestinal mucosa.

Fermented milks, for example yogurt, have high amounts of lactic acid that also favor absorption due to the change in pH that it causes in the lumen. However, in fatty cheeses calcium is found forming a complex with saturated fats that prevents its use.

Despite being one of the foods with the highest amount of calcium and in good conditions to be absorbed, there are studies in which a increased calciuria (calcium in urine) and bone demineralization favored by milk proteins.

There are also epidemiological studies in which a higher incidence rate of osteoporosis is observed in countries with high consumption of dairy products and with high calcium intakes, which contrast with countries in which the incidence of osteoporosis is relatively low but so is the calcium intake (even less than 500 mg daily) and the consumption of dairy products.

Although the debate on the effect of milk, and animal proteins in general, on bone health is far from over, if you want to be sure of getting the necessary amounts of calcium, you should look for alternatives to dairy, even if it is because of the doubts it raises. There are already clearly opposed medical associations, such as the Physicians Committee for Responsible Medicine, and academic institutions, such as the Harvard University School of Public Health, that no longer consider products to be the best source of calcium for all people.

The role of dietary fiber

Excessive fiber intake (more than 35 g/day) can decrease calcium absorption, not because of the fiber itself, but because many high-fiber foods often contain oxalates and phytates. Oxalic acid, present in high amounts in green leafy vegetables, cocoa or beets, among others, forms calcium oxalate, an insoluble non-absorbable salt. Phytic acid also forms insoluble salts, which we can find in the husk of cereals.

On the other hand, certain types of fiber, such as inulin and other fermentable soluble fibers, improve calcium absorption due to the formation of short chain fatty acids when they are fermented by the intestinal flora. Some foods rich in inulin are garlic, onion, leek and other vegetables whose roots are consumed, mainly tubers.

How to get a correct calcium intake?

With what is currently known, achieving a correct intake of calcium and good bone health seems to be more related to abandoning a sedentary lifestyle and adopting healthy habitsincluding the regular and consistent physical activitypreferably outdoors to promote the synthesis of vitamin D, and a rich and varied diet that includes a high quantity of fruits and vegetables that ensures an adequate intake of calcium with a low proportion of animal proteins.

With a lifestyle that stimulates the formation of bone mass, the efficiency of calcium absorption would be maximum, allowing assimilate more quantity even with relatively low intakes. In fact, based on epidemiological data, some scientists believe that actual calcium needs may be lower than recommended, as long as hormonal stimulation of active calcium absorption is maintained, which for an adult it inevitably goes through maintaining an active lifestyle.

In the opposite case, bone mass can be lost even with high levels of calcium in the diet, hence the calcium and vitamin D supplementation as pharmacological treatment of osteoporosis low efficiency to reduce the risk of fractures, which is why the combination with antiresorptive drugs that inhibit bone resorption is becoming more frequent.

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