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.
