Calcium in the Human Body
In the human body there are about 1000 g of calcium, distributed:
- in bone tissue with structural function (99%);
- in muscle tissue (0.3%);
- in plasma, extracellular fluid and other cells (0.7%).
The calcium present in the plasma is represented, at 50%, by free calcium ions, 40%, is bound to proteins and, for 10%, is complexed to anions. Among these three, the most important fraction is represented by ionized calcium (50%), as it is physiologically active, therefore strictly controlled.
Calcium is defined as the concentration of calcium ions in the blood. Under normal conditions this parameter is maintained within a narrow range of values, which ranges from 9 to 10 mg per deciliter of blood. Both its lowering (hypocalcemia) and its excessive increase (hypercalcemia) cause serious functional alterations to the striated and smooth muscles.
In fact, extraosseo football performs many functions:
- it is necessary for the transmission of the nervous signal;
- is involved in the molecular mechanism of muscle contraction;
- works as an intracellular signal for certain hormones, such as insulin;
- it is necessary for the functioning of different enzymes thanks to which it intervenes, for example, in the coagulation cascade;
- it is part of the intercellular cement that holds the cells together at the level of the tight junctions;
Effects of hypocalcemia: tetany, cardiac hyperexcitability, bronchial, bladder, intestinal and vascular spasms.
Effects of hypercalcaemia: reduction of muscular and nervous excitability.
To avoid the onset of these conditions, calcemia is continuously kept under control thanks to the combined action of various hormones, such as calcitonin and parathormone.
Bones: what they are made of and how they are renewed
Bone is a highly specialized connective tissue and, as such, composed of cells, fibers and fundamental amorphous substance. The latter, together with the fibers, constitutes the so-called extracellular matrix, formed in turn by a mineral component and an organic fraction.
The organic component of the extracellular matrix, also called osteoid, is made up of collagen fibers (95%) and a fundamental amorphous substance (5%), in turn composed of proteoglycans.
Bone is a dynamic structure, subjected to a remodeling process that continues throughout life. The extent of this process is considerable (about 1/5 of the skeleton is remodeled every 12 months) and, as such, requires a good supply of energy. Furthermore, to support bone remodeling, it is essential to associate good calories, especially calcium, with caloric intake.
Responsible for bone renewal are two types of cells, called osteoclasts and osteoblasts, respectively. The former, polynuclear and rich in microvilli, secrete proteolytic acids and enzymes which, by destroying the bone matrix, release the minerals contained in it. Thanks to this process, about 500 mg of calcium are removed daily from the bone (0.05% of the total calcium). Following this process of bone erosion, osteoblasts intervene, cells with diametrically opposed functions compared to the previous ones. The osteoblasts, in fact, guarantee the formation and deposition of organic matrix in the cavities generated by the catabolic action of the osteoclasts. As soon as this matrix reaches a sufficient thickness, it is readily mineralized, thanks to the interposition of calcium. This mineralization process goes on for months, during which the density of the new bone progressively increases.
Most bone mass is accumulated within 18-20 years; after this period mineralization continues to increase, albeit slowly, until it reaches the peak around thirty years. For this reason it is very important to promote regular physical activity and adequate nutrition at a young age.
After the age of 40, bone mass undergoes a physiological reduction in the organic and mineral components. This absolutely physiological and therefore inevitable process is called senile osteoatrophy. On the contrary, if the loss of bone mass is such as to compromise the performance of normal bone functions, we speak of osteoporosis. The difference between osteoatrophy and osteoporosis, therefore, is only quantitative. The two conditions are instead equal from a qualitative point of view, because they share a reduction in bone mass for the organic and mineral components.
Risk factors for osteoporosis
Many risk factors predispose to osteoporosis. Some of these are congenital and, as such, not modifiable (female sex, white race, slender build, familiarity, age and menopause). For environmental or behavioral factors, however, much can be done:
- forced immobility (plastering of a limb, astronauts etc.) there are specific therapies to accelerate bone remineralization);
- Poor supply of calcium, Vitamin C (intervenes in the process of collagen maturation) and D (increases the intestinal absorption of the mineral).
- Sedentariness (movement facilitates the deposition of calcium in the bones);
- Excessive physical exercise (especially if not accompanied by an adequate intake of macro and micronutrients, can accelerate bone decalcification);
- High-protein diet (too many proteins promote hypecalciuria, that is an excessive elimination of calcium with urine); it should be pointed out, however, that in several studies the high-protein diets have been shown to increase the intestinal absorption of calcium, compensating for the increased urinary losses of the mineral; moreover, a diet rich in proteins seems to favor the synthesis of hormones with an anabolic effect on the bone (such as IGF-1), reducing the synthesis of parathormone; at present, therefore, high-protein diets are NOT considered harmful to bone health; even a low-protein diet, on the other hand, could represent a risk factor for osteoporosis.
- Alcohol and coffee abuse
- Smoke
- Prolonged use of some drugs (such as cortisone drugs)
The cessation of estrogen production increases the risk of osteoporosis in postmenopausal women, since the stimulatory effect of these hormones on osteoblastic proliferation is lost. The loss of bone mass is particularly high in the first five years after the climacteric. Even in this delicate period of life, physical exercise has proved particularly effective in mitigating bone loss.
| Recommended intake levels of calcium in the Italian population | |
| Age | mg / day |
| 0-1 | 500 |
| 1-6 | 800 |
| 7-10 | 1000 |
| 11-19 | 1200 |
| 20-29 | 1000 |
| 30-60 | 800 |
| > 60 | 1000 |
pregnancy and breastfeeding | +400 |
| for 5 years after menopause | 1500 |
Calcium and Vitamin D
The presence of vitamin D is essential for intestinal absorption of food calcium. This substance can be taken with certain foods (liver, fish and fish oils, eggs, butter milk and a few other foods) or be synthesized in the skin.
Starting from cholesterol, 7-dehydrocholesterol is formed which, due to the action of UV rays in the skin, gives rise to vitamin D3. In turn, this vitamin must be activated, first passing into the liver, where it is hydroxylated, and finally to the kidney, where it is completely activated. A deficiency of vitamin D can therefore depend on insufficient dietary intake and / or insufficient exposure to sunlight. Furthermore, this deficiency can be linked to the presence of serious hepatic and / or renal pathologies, which inhibit the activation of the vitamin.
Being fat-soluble, vitamin D is stored in adipose tissue. This substance promotes intestinal calcium absorption with the same mechanism as steroid hormones. As such, it enters the nucleus of enterocytes and induces the coding for the synthesis of a protein, called calcium binding protein (CaBP). This protein is able to transport calcium ions into enterocytes.
In essence, therefore, vitamin D is essential to increase the intestinal absorption of calcium taken with food. However, the amount of calcium ions that is absorbed also depends on other constituents of the diet. The bioavailability of calcium is in fact limited by the presence of intestinal oxalates (contained in cocoa and green leafy vegetables such as spinach and chard), phytates (bran, legumes, whole wheat bread) and by the presence of too many lipids.
Given the importance of vitamin D for the intestinal absorption of calcium, its deficiency is reflected in an inadequate mineralization of the newly formed bone matrix. When this condition becomes chronic, it causes rickets in children and osteomalacia in adults.
