SUMMARY
Calcium (Ca) is the most abundant mineral element in our body, since it is an important part of the skeleton and teeth. It supposes around 2% of the corporal weight. The functions of calcium are: a) skeletal functions and b) regulatory functions.
The bone is formed by a protein matrix that is mostly mineralized with calcium (the most abundant), phosphate and magnesium; For this, a correct dietary contribution of Ca, phosphorus and vitamin D is essential. Ca ionic (Ca 2+ ) is an indispensable cellular component to maintain and / or perform the different specialized functions of practically all the cells of the organism. Due to its important functions, Ca 2+ must be tightly regulated, keeping its plasma concentrations within narrow ranges. For this there is an accurate response to hypocalcemia or hypercalcemia, in which parathormone, calcitriol, calcitonin and vitamin K are involved.
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The intakes of Ca in the Spanish population are low in a significant percentage of the population, especially in older adults, especially in women. The main source of Ca in the diet is milk and all its derivatives. Green leafy vegetables, fruits and legumes may be important as sources of Ca in a Mediterranean food pattern. The bioavailability of Ca from the diet depends on physiological and dietary factors. The physiological ones include age, physiological situation (gestation and lactation), the status of Ca and vitamin D and the disease. Several studies relate the intake of Ca in the diet and various diseases, such as osteoporosis, cancer, cardiovascular diseases and obesity.
ABSTRACT
Calcium (Ca) is the most abundant mineral element in our body. It accounts for about 2% of body weight. The functions of calcium are: a) skeletal functions and b) regulatory functions.
Bone consists of a protein matrix that mineralizes mainly with calcium (the most abundant), phosphate and magnesium, for it is essential an adequate dietary intake of Ca, phosphorus and vitamin D. The ionic Ca (Ca 2+ ) is essential to maintain and / or perform different specialized functions of, virtually, all body cells. Because of its important functions Ca 2+ must be closely regulated, keeping plasma concentrations within narrow ranges. For this reason there is an accurate response against hypocalcemia or hypercalcemia in which the parathormone, calcitriol, calcitonin and vitamin K are involved.
Ca intakes in the Spanish population are low in a significant percentage of the older adult's population, especially in women. The main source of Ca in the diet is milk and milk derivatives. Green leafy vegetables, fruits and vegetables can be important sources of Cain a Mediterranean dietary pattern. The bioavailability of dietary Ca depends on physiological and dietary factors. Physiological include age, physiological status (gestation and lactation) Ca and vitamin D status and disease. Several studies relate Ca intake in the diet and various diseases, such as osteoporosis, cancer, cardiovascular disease and obesity.
Introduction
Calcium (Ca) is a micronutrient from the group of minerals that must always be part of our diet. It is the most abundant mineral element in our body, since it is an important part of the skeleton and teeth. It supposes around 2% of the corporal weight; in absolute figures, approximately 1,200 g (1,2 kg). Of all body calcium, 99% is found in the skeleton and teeth in the form of hydroxyapatite, a crystalline compound that includes phosphorus (Ca 10 [PO 4 ] 6 [OH] 2 ). The rest (1%) is found in the soft tissues and body fluids (1).
This particular body distribution justifies its essential functions in the body, the mineralization of bones and teeth and the regulation of cellular functions in practically all body tissues. As examples, Ca is essential for muscle contraction and nervous system function (1,2).
Like all nutrients, Ca must be provided in the diet and the main source of this mineral is milk and milk products, which contribute around 40% of the recommended daily intakes (IDR).
In this article we will include the functions of Ca in our organism, the homeostasis of this mineral, the dietary sources, its bioavailability and the recommended intakes. Finally, we will include the relationship between Ca and bone health and its role in the maintenance of body weight, in cardiovascular diseases and cancer.
Calcium functions
The functions of calcium, as we have already pointed out, can be specified in two: a) skeletal functions and b) regulatory functions (1).
SKELETAL FUNCTION
Ca is a fundamental part of our skeleton (bones) and teeth. The bone is formed by a protein matrix that is mostly mineralized with calcium (the most abundant), phosphate and magnesium; For this, a correct dietary contribution of Ca, phosphorus and vitamin D is essential.
The bone tissue is formed by two different types, the compact bone (cortical) (80%), whose function is to give hardness to the skeleton and exercise structural function, and trabecular bone (20%), whose function is metabolic. Despite its compact appearance, bone is a dynamic structure that is constantly remodeling, destroying (resorption) and forming (formation) continuously (1).
The relative rate of resorption and bone formation will depend on age. From 20-30 years, where a peak of mineralization is reached, the formation predominates over the resorption, and from 30-35 begins to prevail the resorption against the formation, with a loss of bone density. This last process is especially relevant in women after menopause, where it is accelerated and can compromise bone health (osteoporosis) if there is an adequate supply of Ca, P, Mg and vitamin D, mainly along with lifestyles healthy, among which regular physical activity is included (1,3).
The tooth is made up of three types of tissues: enamel, dentine and cement. Enamel and dentin are formed by hydroxyapatite.
NON-SKELETAL FUNCTION
Ca (Ca ion: Ca 2+ ) is a cellular component essential to maintain and / or perform the different specialized functions of practically all the cells of the organism. These functions, non-skeletal, we can divide into structural and properly regulatory. Within the former, Ca is involved in the maintenance of cellular structures (organelles), secretory granules, cellular and subcellular membranes and nuclear structures (such as chromosomes) (1,4).
In relation to its regulatory function, this mineral can exercise its function passively or actively. Passively, plasma calcium levels regulate enzymatic reactions. The active regulatory function is exerted by the intracellular concentration of Ca 2+ . The changes in its intracellular concentration, in response to a stimulus (hormone, neurotransmitter, etc.), modify the behavior, the functional response, of that cell. These functional responses include cell division, secretion, aggregation, muscle contraction, transformation and metabolism. The maintenance of an adequate concentration of cytoplasmic Ca 2+ (of the order of 0.1 μmol / l), with respect to the extracellular one (of the order of 1,1 mmol / l), can maintain an optimal function of the cell; On the other hand, an unregulated increase in the cytoplasm can initiate a process of cell damage and death (4,5).
Due to its role as a second intracellular messenger, calcium intervenes in intracellular proteolysis, apoptosis and autophagy, enzymatic activation / deactivation (by phosphorylation / dephosphorylation), secretion (including that of neurotransmitters and neuromodulators in the nervous system), muscular contraction, aggregation platelet, cellular bioenergetics, gene transcription, etc. (4.6).
Calcium homeostasis
Due to its important functions, Ca 2+ must be tightly regulated, maintaining its plasma concentrations within narrow ranges (1,1 and 1,3 mmol / l). For this there is an accurate response to hypocalcemia or hypercalcemia involving parathormone and 1,25 dihydroxycholecalciferol (1,25 [OH] 2 vitamin D 3 ), and calcitonin, and recently the vitamin has been involved K for its relationship with osteocalcin. These humoral regulators act at a bone, renal and intestinal level, affecting the mobilization and deposit of calcium in the bone, its intestinal absorption and its renal excretion. Calcium homeostasis is closely related to that of phosphates through fibroblast growth factor 23 (FGF23) and Klotho factor (7).
Recommended calcium intakes
The recommended intakes of Ca are related to age, due to the different needs of this mineral in different stages of life. Thus, in active growth stages (first years of life and pubertal growth spurt) the demands are greater to meet the growth in length of the long bones. Subsequently, especially in women after menopause, the greater bone loss of calcium increases the requirements to maintain an adequate bone density that avoids the higher incidence of stress bone fractures, more frequent at this stage.
Table I lists the recommended intakes of Ca for the Spanish population taken from various sources and compared with those of the Institute of Medicine (IOM) of the United States, including, in the latter, the Estimated Average Requirements (EAR).
Calcium intakes in Spain
There are several studies that collect the intakes of Ca from the Spanish population. The ENIDE study (2011) (12) shows an observed calcium intake of 886 mg / day in men and 834 in women. The adequacy ranges from 65% to 98%, depending on the recommended daily intakes (IDR) considered. However, if in this same study we consider the distribution of usual intakes taking into account the intra and interindividual variations and the EAR ( Table I ), the percentages of population at risk of low intakes of Ca ranges from 20-30% in men and 35-82% in women, depending on the age segments. The main source of this mineral is milk and milk products (44%), followed by fish, molluscs and crustaceans and pulses, seeds and nuts.
The data on Ca intake from the FEN-MAGRAMA food consumption panel are similar to the ENIDE study and show a slight downward trend from 2003 to 2008 (889 mg / day versus 874 mg / day) (13).
In studies of specific groups, such as menopausal women, Ortega et al. they present calcium intakes of 956.1 mg / day, slightly higher than those found in the two previous studies of the adult Spanish population. However, the risk percentages of marginal calcium intakes, compared to the EAR, is 41% of menopausal women. Other studies show intakes of Ca that range between 626 and 644 mg / day in postmenopausal women and men over 50 years of age, or the INDICAP study in the Spanish adult population, which observes intakes of 991 mg / day (14-17).
Globally, the intakes of Ca are low in a significant percentage of the Spanish population, especially in older adults, and among these, women have lower intakes.
Dietary sources of calcium and bioavailability
The main source of Ca in the diet is milk and all its derivatives. Some cheeses contain more than 1 g of Ca per 100 g of food, with butter having a lower content, 15 mg / 100 g. The liquid milk presents an average of 124 mg / 100 g. Two thirds of Ca in milk are bound to casein and the rest free (1).
Then there are green leafy vegetables, fruits and legumes, which can be important in a Mediterranean food pattern, which can provide up to 400 mg / day. Cereals are not a rich source of Ca unless they are supplemented with it, as in some countries such as the United Kingdom. Finally, we must not forget water and food supplements.
The bioavailability of Ca from the diet depends on physiological and dietary factors. The physiological ones include the age, physiological situation (gestation and lactation), the status of Ca and vitamin D, and the disease.
The dietary factors depend on each food and its composition, which can affect positively or negatively their intestinal absorption. Thus, the physical form of Ca (relative solubility and presence of oxalates, phytates and uronates), the presence in the diet of foods rich or not in vitamin D, the content of fat and protein and other factors (caffeine, alcohol, etc.) .) can affect the absorption of the mineral and, therefore, modify its bioavailability. For example, Ca present in milk has a high bioavailability, better than that of cereals and vegetables and similar to inorganic sources (CO 3 Ca). Thus, the bioavailability of milk is 30% compared to 5% of spinach. This high bioavailability is due to the absence of inhibitory factors and the presence of other components such as lactose, which prevents its precipitation, casein (phosphopeptides) and non-digestible oligosaccharides (inulin and oligofructose). A particular case is breast milk, with an even greater bioavailability (35%) based on the fatty acid profile and the Ca: P ratio (1).
On the other hand, the lower bioavailability of Ca present in vegetables is due to the presence of substances such as oxalates, uronates and especially phytates, which adversely affect the absorption of the mineral and consequently its bioavailability. Regarding the Ca present in water (tap or bottled minerals), its bioavailability is high; however, it has a great variability in the content of this mineral and, in general, has lower concentrations than milk and its derivatives (1).
Calcium and disease
Ca 2+ plays an essential role in innumerable functions of the organism, modifying its intracellular concentrations and initiating intracellular signaling pathways. However, when homeostasis fails, various pathological changes occur as a result of alterations in the cytoplasmic levels of this cation. This area includes musculoskeletal, neurological, neurodegenerative, cardiomyopathy, etc., as a consequence of the alteration of Ca 2+ homeostasis (18).
On the other hand, it is known by various studies the relationship between the intake of Ca in the diet and various diseases: bone, chronic (cancer and cardiovascular disease) and obesity.
CALCIUM AND BONE HEALTH. OSTEOPOROSIS
Osteoporosis is defined as a disease characterized by a low bone mass density (> 2.5 standard deviations from the mean values of young adults) together with a deterioration of the bone microarchitecture that leads to bone fragility and the consequent increased risk of fracture. This disease affects millions of people and WHO considers it one of the main health problems in the world. This is because this disease multiplies by 4 the risk of suffering a bone fracture. To know the density of bone mass, the most generalized technique is double energy X-ray radioabsorptiometry (DEXA). Osteoporosis can be primary or secondary to other pathological processes. The primary can be type I, of early establishment before menopause and type II, senile (1,18).
The cause of osteoporosis is an imbalance between the inputs and outputs of Ca from the body. The intake of Ca and its digestive and metabolic utilization are not enough to compensate for the losses required by faeces and urine. Normally under these circumstances and for type II osteoporosis there is a decrease in the synthesis of calcitriol, the active metabolite of vitamin D, by a problem in the renal function where it is formed, or an insensitivity of the intestinal epithelial cells to it, due to advanced age. This is compounded by low vitamin D status due to low intakes or low exposure to solar radiation (1,18,19).
The influence of the intake of Ca on osteoporosis is observed by low intakes that can not maintain the calcemia, so Ca is mobilized from the bone deposits. There are studies that relate the high intakes of milk and milk products during adolescence with higher bone density and lower risk of fractures in the postmenopausal period. However, other studies find no association between calcium intake at a given time and bone mass. Therefore, several prospective studies have not been able to establish an association between Ca intake and reduction of the risk of osteporotic fractures (1).
On the other hand, Ca supplements at doses higher than those we consume with the diet, more than 500 mg / day, seem to reduce the risk of fractures in men and women at advanced ages, and when dietary Ca intakes were Very low. An important aspect is the time to administer supplements of this mineral, being more effective in late stages of menopause than immediately after its establishment (1).
CALCIUM AND CANCER
Several studies show a relationship between the intake of Ca and colorectal cancer. The consumption of less than 400 mg / day of Ca is associated with a higher incidence of this type of neoplasm compared to intakes higher than 800 mg / day. Other studies have shown that high intakes of Ca also reduce the formation of adenomatous colon polyps, a precursor lesion of cancer. Also, supplementation with Ca reduces the recurrence of colon adenomas. According to several studies, this effect of Ca is related to the ability of this cation to bind fatty acids and bile acids in the intestinal lumen, preventing both, in their ionized form, from exerting a proliferative effect on the colonocytes, increasing the probability of cell formation with genome errors (20).
Ca action has also been proposed through the calcium-sensitive receptor present in colon epithelial cells both normal and neoplastic. This receptor detects changes in the concentrations of Ca in the diet. Its binding to the receptor causes elevations in intracellular Ca that result in growth inhibition, increased apoptosis and promote cell differentiation (20-23).
There are studies on the relationship of Ca with other types of cancer such as breast cancer, although there is no such evident evidence, since the studies used milk and its derivatives as a source of the mineral (1).
CALCIUM AND CARDIOVASCULAR DISEASES
Epidemiological studies have shown that there is an inverse relationship between the intakes of Ca and the risk of suffering from cardiovascular diseases. These effects seem related to plasma lipid levels. Thus, mineral supplementation decreases total cholesterol and LDL-cholesterol in plasma, while increasing HDL-cholesterol. On the other hand, low intakes of Ca have been related to hypertension, a risk factor for cardiovascular diseases (24).
It is necessary to point out that some studies have shown that the use of Ca supplements in high doses can have a negative impact on cardiovascular health. In fact, it has been reported that with an intake of Ca below 500 mg / day and above 1,200 mg / day, mortality curves due to cardiovascular diseases and total mortality increase (25,26).
CALCIUM AND BODY WEIGHT
One of the recent findings is the relationship between the intake of Ca and body weight. Several studies show an inverse relationship between the intake of Ca and the body mass index (BMI). This relationship between cation and body weight is related to the role it plays in the regulation of body adiposity through a decrease in lipogenesis and an increase in lipolysis in adipose tissue. Along with this has been described a role of Ca in the balance of body energy, decreasing appetite and increasing thermogenesis. In addition, the calcium in the intestinal lumen can form soaps with the fat of the diet, favoring its precipitation and its fecal excretion, and consequently, diminishing its absorption. In these mechanisms, the calcium-sensitive adipocyte receptor appears to be involved. Its stimulation by low intakes of Ca decreases lipolysis and increases lipogenesis.