Calcium is one of the basic elements necessary for the proper functioning of the body. It is the main component that builds bones and teeth. But that's not all, because it is involved in blood clotting. Its deficiencies are relatively common in the human population, so it's important to be aware of substances that can reduce its bioavailability—that is, its ability to be absorbed by the body.
Many factors influence the body's ability to absorb calcium, including our age, diet, and the source of the calcium. Some of these are beyond our control, while others are within our control. Today, I will focus on describing the dietary ingredients that affect its bioavailability.
Vitamin D
Vitamin D plays an important role in calcium absorption. By stimulating the synthesis of calcium-rich protein by erythrocytes, it enables its transport from the intestine to the corresponding body cells. In addition, vitamin D promotes the reabsorption of calcium, thereby preventing excessive calcium loss in the urine. To provide the correct amount of vitamin D (30-50 µg/ml 25-hydroxycholecalciferol in the blood), you should spend between 10-15 minutes in the sun with 18% body exposure without applying sunscreen. It is extremely difficult to obtain it through food. Too little vitamin D causes a decrease in calcium absorption of about 30-50%, up to a maximum of 15%. In the long term, this can lead to mineralization disorders of the bone matrix, bone osteomalacia, rickets, and osteoporosis [1] [2].
Vitamin C
Sources of vitamin C include rose hip, acerola, black pepper, black currant, oranges, and kiwi. Thanks to it, calcium is better absorbed, but not only calcium is worth mentioning, because the company of vitamin C increases the absorption of iron and magnesium [3].
magnesium
Magnesium influences the transport of calcium and potassium across plasma membranes and the transport of calcium to and from bones. Magnesium salts encase a mineral called hydroxyapatite in bones. It is composed of calcium and phosphorus and forms the framework of connective tissue, which influences bone strength. Magnesium is essential for its formation. Magnesium-rich foods include sesame, poppy seeds, almonds, pistachios, pumpkin seeds, whole-grain bread, bran, sprouts, buckwheat, and many others [4].
phosphorus
While calcium absorption is not affected, long-term use of antacids such as magnesium hydroxide and aluminum hydroxide produces insoluble phosphorus-bound salts that cause hypophosphatemia (phosphorus deficiency), ultimately leading to increased calcium absorption but simultaneously increased urinary excretion. All in all, this ultimately leads to a decrease in calcium levels in the body. Therefore, to ensure the correct concentration of calcium in the body, it is important to pay attention to the calcium to phosphorus ratio. Under optimal conditions, it should be 1:1. [4].
protein
Protein can affect calcium absorption in two ways. Too little protein disrupts calcium transport through the intestinal wall to the appropriate body parts. The calcium transporter, in the form of a protein, is composed primarily of the amino acids lysine and arginine, which is why more attention should be paid to these during periods of increased calcium demand. Too much protein can simultaneously increase diuresis and increase the excretion of calcium in the urine. The optimal calcium-to-protein ratio is 16 mg Ca to 1 g protein [1] [4].
Fats
Fatty acids are components of fat that influence its properties. There are saturated and unsaturated fatty acids, the latter being further divided into monounsaturated and polyunsaturated. Studies have observed greater calcium absorption in the presence of unsaturated fatty acids. Furthermore, people with fat malabsorption have reduced calcium absorption (absorption). The mechanism of this relationship is currently unknown [1] [4].
Oxalates (oxalic and phytic acid)
Oxalates, or more accurately oxalic acid, combine with calcium to form insoluble salts—calcium oxalates—which not only reduce calcium availability but, in excess, can also lead to the formation of kidney stones. Therefore, if you have an increased calcium need, it's worth avoiding products rich in oxalic acid or keeping a greater distance between calcium-rich products or supplements. Oxalic acid is found in large quantities in foods such as rhubarb, sorrel, spinach, chard, beetroot, coffee, cocoa, and tea [4].
Fructans (inulin, oligofructose, fructooligosaccharides)
Currently, there are no human studies confirming the link between fructans and calcium, although preliminary research is relatively promising, particularly in the case of inulin. It is thought that this link is caused by a decrease in pH following fermentation, which in turn increases the passive absorption of calcium through the intestinal wall [1] [9].
Fiber, resistant starch, residue-rich diet
Large amounts of fiber reduce calcium absorption, and this relationship is observed even when calcium intake is adequate. However, studies have shown that resistant starch (part of insoluble fiber) does not fulfill this condition. The presence of resistant starch increases fiber absorption. Further research is needed to elucidate the exact mechanisms of the observed effects. However, due to the many associations related to calcium absorption, some researchers recommend avoiding calcium-rich products with a high amount of fiber, as is the case with calcium supplementation. It may also be beneficial to maintain a gap between taking the supplement and eating a meal [4] [10].
SUMMARY
Vitamins D and C, magnesium, lactose, casein, and inulin increase calcium availability. Phosphorus and protein, when taken in appropriate doses, also have a positive effect on calcium bioavailability. On the other hand, oxalates, saturated fatty acids, alcohol, high fiber content in meals, and insufficient concentrations of vitamin D and magnesium reduce calcium absorption. Because of so many dependencies, some sources suggest that it's best to supplement calcium alone between meals or try to supplement it only in the presence of products that increase its bioavailability.
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