Calcium

Calcium from plant sources has lower bioavailability compared to calcium from dairy products. This is due to the presence of antinutrients such as:

• phytates (found in sesame, nuts, legumes, for example) — they bind minerals, including calcium, and hinder their absorption in the intestine;

• oxalates (in spinach, rhubarb, sorrel) — also interfere with calcium absorption.

In the case of sesame: despite its very high calcium content (over 900 mg per 100 g), only a small portion is absorbed — about 10–20%, or even less if the sesame has not been pre-processed (soaking, roasting, fermentation).

To enhance calcium absorption from plant sources, it is beneficial to:

• combine them with vitamin D and magnesium;

• minimize antinutrients (through soaking, sprouting, fermentation).

Thus, sesame is not the best source of calcium in terms of absorbability, despite its high content.

But there is an exception – leafy vegetables without oxalates, such as broccoli and kale. Calcium from these is absorbed at 30-35%.

Whole milk stands out among sources of calcium not so much for its quantity, but mainly for its bioavailability.

In the body, calcium absorption is determined not by the absolute content, but by the conditions under which it can pass through the intestinal barrier and enter the bloodstream.

Why calcium from milk is absorbed better:

Absence of antinutrients. In grains, nuts, and legumes, calcium is bound by phytic acid (phytates) and phenolic compounds. These complexes are insoluble and are hardly broken down in the gastrointestinal tract, making calcium from them poorly available.

In milk, there are no such bindings: calcium is in a soluble form, bound to proteins and lactate, so it easily dissociates and passes through the intestinal mucosa.

Milk proteins as carriers. Casein and whey form soluble complexes with calcium (casein-phosphopeptides), which protect it from precipitation in the intestine and help transport it through the epithelium. These protein fragments act as "transport molecules."

Lactose. Milk sugar is fermented in the intestine, producing organic acids that slightly acidify the environment and improve calcium solubility. In children and people with good lactase activity, this effect is particularly pronounced.

Fat-soluble vitamins and lipids. Whole milk contains vitamin D (albeit in small amounts, but it is better absorbed in combination with fats), as well as vitamin K2 (especially in milk from grass-fed cows). These vitamins regulate calcium transport: D increases absorption in the intestine, K2 directs calcium to bones and teeth rather than to blood vessels.

Optimal mineral ratio. Milk contains magnesium, phosphorus, and potassium in the right proportions. For example, excess phosphorus in grains hinders calcium absorption, but in milk, the Ca:P ratio is optimal for bone mineralization.

Physiological compatibility. Evolutionarily, calcium from milk is a "child" mechanism for ensuring skeletal mineralization. Therefore, humans have developed enzymes and receptors that actively absorb this form of calcium.

Thus, calcium absorption from milk is 2–3 times higher than from plant sources with the same calcium content.

Gallstone disease is closely related to mineral metabolism, and its basis often lies in the imbalance of calcium and magnesium.

Composition of stones. Stones in the gallbladder mostly consist of cholesterol (70–80%), less often of bilirubin or calcium salts.

But it is the calcium salts (carbonates, phosphates, calcium bilirubinates) that provide the "framework" for crystallization around which the stone forms.

Role of calcium. When there is too much calcium in the bile, it binds with bile acids or bilirubin and forms insoluble crystals.

Excess calcium can occur due to its disturbed metabolism, low stomach acidity (poor digestion of fats and calcium), and magnesium and vitamin D deficiency.

Role of magnesium. Magnesium is an antagonist of calcium: it prevents excessive crystallization of calcium salts and improves the solubility of bile.

Magnesium relaxes the smooth muscles of the gallbladder and ducts, improving bile flow and reducing its stagnation. Magnesium also participates in the activation of liver enzymes that regulate cholesterol metabolism.

What happens with magnesium deficiency:

• Calcium starts to "behave aggressively": more precipitates in soft tissues and bile, less goes to the bones.
• The function of bile acids is disrupted, and cholesterol precipitates, forming the basis for the stone.
• Spasm of the gallbladder increases, further hindering normal outflow.

Therapeutic significance of magnesium. Thus, increased intake of magnesium (through food or supplements) reduces the risk of stone formation. It helps normalize the function of the sphincter of Oddi and the motility of the gallbladder. Magnesium stabilizes calcium levels and reduces the formation of insoluble salts.

Therefore, gallstone disease can indeed be considered a consequence of a calcium-magnesium imbalance, where excess calcium with a deficiency of magnesium leads to crystallization and stone formation.

Calcium (Ca), strontium (Sr), and barium (Ba) belong to the same group in the periodic table (alkaline earth metals). They have similar ionic structures and can partially replace each other in biological processes. However, such substitution is not an "equivalent replacement," but rather a forced measure by the organism in the case of a deficiency of the primary element.

Why this happens

Similarity of ions. The ions Ca²⁺, Sr²⁺, and Ba²⁺ have similar radius and charge, which allows them to be incorporated into the mineral matrix of bones and teeth instead of calcium.

Mineral matrix of bones. The hydroxyapatite of bones (Ca₁₀(PO₄)₆(OH)₂) can partially include strontium or barium in the absence of calcium. The organism "does not differentiate" between them at the lattice structure level, but the biochemical properties change.

What are the dangers

Strontium (Sr). In small doses (for example, strontium ranelate in medicine) it can enhance bone mineralization and reduce the risk of fractures. However, with prolonged calcium deficiency and accumulation of Sr instead of Ca, the strength of bones changes: they may become more brittle, and the structure may be compromised.

Barium (Ba). Barium is biologically active but toxic. When deposited in bones or soft tissues, it disrupts the functioning of the nervous system, muscles, and heart (as it interferes with the normal transmission of calcium signals).

Overall impact on the body:

• Bones may become less elastic and more brittle.
• Problems with neuromuscular conductivity may arise (as calcium is a key element in nerve impulse transmission).
• With the accumulation of barium - toxic effects (arrhythmias, muscle spasms, neurological disorders).

Thus, the replacement of calcium with strontium or barium is an "emergency mode" of the organism's functioning, which maintains the structure of bones but deteriorates their quality and overall health condition.

Vitamin D plays a key role in regulating calcium and phosphorus metabolism. Its active form — calcitriol (1,25-dihydroxyvitamin D₃) — acts as a hormone, influencing three main systems: the intestine, bones, and kidneys.

How vitamin D affects calcium absorption

Activates calcium transport in the intestine. Vitamin D increases the synthesis of the protein calbindin in the cells of the intestinal mucosa. Calbindin binds calcium from food and transports it through the intestinal epithelium into the bloodstream.

Without sufficient vitamin D, this mechanism hardly works, and most of the calcium is simply excreted with the feces.

Regulates the function of calcium channels. Vitamin D stimulates the expression of calcium channels (TRPV6) on the surface of enterocytes, allowing calcium to enter the intestinal cells.

Maintains calcium balance in the blood through bones and kidneys. Vitamin D, together with parathyroid hormone (PTH), regulates how much calcium will be retained in the blood and how much will be deposited in bone tissue.

What happens with vitamin D deficiency

Calcium from food is hardly absorbed. Even with a calcium-rich diet (dairy products, fish with bones, nuts), it will pass through the system.

The body compensates for the deficiency by drawing from the bones. To maintain calcium levels in the blood (vital for the functioning of the heart and nerves), PTH increases the resorption of bone tissue.

Osteopenia and osteoporosis develop. Prolonged deficiency leads to brittle bones, increased risk of fractures, and deformities (rickets in children, osteomalacia in adults).

Conclusion

Without sufficient levels of vitamin D, calcium is indeed hardly absorbed from the gastrointestinal tract and begins to leach from the bones, which is one of the key reasons for osteoporosis and muscle weakness.

Normally, about 99% of calcium is found in bone tissue as part of hydroxyapatite, while only about 1% of calcium circulates in the bloodstream. The body tightly regulates this level, maintaining it even with insufficient calcium intake from food.

If there is not enough calcium in the diet, the body "maintains the norm" in the blood by breaking down bones.

When blood tests show calcium levels above the average norm, it may indicate:

• Active resorption of bone tissue. Parathyroid hormones (PTH) stimulate osteoclasts, the cells that break down bone tissue, to release calcium and maintain its level in the blood. This is a typical mechanism in osteoporosis or hyperparathyroidism.
• Calcium deficiency in tissues with "normal" or high blood levels. In the bones, there is depletion of reserves, while in the blood, there is a "normal" or elevated value due to the mobilization of calcium from deposits. This is similar to a situation where the body withdraws money from a savings account to keep the balance on a card unchanged.
• Increased breakdown of bone tissue (osteoporosis, osteomalacia, postmenopausal period). In these conditions, bone minerals enter the bloodstream, masking the actual calcium deficiency in the skeleton.
• Disorders of vitamin D metabolism. A deficiency of the active form of vitamin D (calcitriol) hinders the normal absorption of calcium from food, forcing the body to use bone calcium.

Thus, high calcium levels in the blood are often a signal of bone calcium loss, which over time leads to bone thinning and osteoporosis.

The use of corticosteroids is associated with the leaching of calcium from bone tissue and an increase in blood calcium levels, but this does not always occur simultaneously or to the same extent.

How corticosteroids affect calcium metabolism

Suppression of osteoblasts and activation of osteoclasts. Corticosteroids suppress the activity of osteoblasts – the cells that form bone tissue – while simultaneously stimulating osteoclasts, which break down bone. This leads to decalcification of bones: calcium leaves the bones into the blood, reducing bone density.

Decreased calcium absorption in the intestine. Glucocorticoids reduce the expression of calcium transporter proteins in the intestine, including by suppressing the action of vitamin D. This decreases calcium absorption from food, leading to systemic calcium deficiency, despite its possible excessive presence in the blood.

Increased calcium excretion through the kidneys. Under the influence of corticosteroids, calciuria – the loss of calcium in urine – is intensified. This is another way the body loses calcium, worsening the balance.

Consequences of intake

To compensate for the decrease in blood calcium (due to poor absorption and excretion), the body activates parathyroid hormone (PTH). This hormone mobilizes calcium from the bones – and a pathological cycle begins: bones are destroyed → calcium enters the blood → calcium is excreted in urine → calcium levels drop again → PTH stimulates even more bone destruction.

This is called secondary hyperparathyroidism due to prolonged corticosteroid use.

Conclusion

Corticosteroids leach calcium from bones. They also lead to decreased calcium absorption and loss through urine. This may cause a temporary increase in blood calcium levels, but more often leads to a general deficiency of calcium in tissues, including bone. This increases the risk of osteoporosis, fractures, muscle weakness, and in the long term – arrhythmias and kidney stone disease.

Prevention

When corticosteroids are used for a long time, doctors often prescribe:

• Calcium + vitamin D (to compensate for losses),
• Bisphosphonates (bone resorption inhibitors),
• Regular monitoring of bone density (densitometry).