How do we absorb calcium?

Does not liking to drink milk mean that you are calcium deficient? Do you need to take calcium tablets every day? Will calcium supplementation make your bones healthy? The question of calcium supplementation is not that simple. There are a series of scientific principles behind it.

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From "eating calcium" to "excreting calcium"

The calcium we take in through our diet usually exists in the form of compounds. They pass through the esophagus together with protein, water, fat, lactose, etc. and reach the "harsh environment" of the stomach, where they face the first test from the human body.

Gastric cells soften food by secreting hydrochloric acid, and the hydrogen ions in the hydrochloric acid will free calcium ions from inorganic calcium compounds. The calcium in ionic state continues to move forward and soon reaches the small intestine (organic calcium such as calcium caseinate will not be converted into calcium ions, but enter the small intestine in a bound state). Here, a large number of wrinkles and tiny villi allow the small intestine to have the largest surface area in the smallest space to fully contact the chyme. On the intestinal mucosal cells, "calcium-binding proteins" can easily "form partners" with calcium ions in the intestinal cavity, acting as carriers of calcium ions (calcium ion channels) to promote the absorption of calcium ions. When calcium ions enter the cytoplasm of mucosal cells, they are hidden in the mitochondria, and calcium-binding proteins transport them to the serosal surface of the cells, and through this "calcium ion exchange system" the calcium ions are expelled from the cells and enter the blood.

After that, the blood carries calcium ions and is transported to various tissues and cells throughout the body for calcium exchange. After this exchange, the remaining blood calcium will be transported to the bones and stored, which not only ensures the strength of the bones, but also prepares for the need of "calcium deficiency". In this process, osteoblasts guide bone salts (inorganic salts in bones, the main component of which is calcium) and bone matrix to shape bones; osteoclasts guide bone absorption and dissolve and metabolize aging and damaged bones. After that, the precipitated calcium flows through the kidneys with the blood circulation, and is continuously filtered and screened. Most of the blood calcium is recycled and reused by the kidneys, and a small part of the "waste calcium" that is no longer used is excreted from the body with urine. From "taking in calcium" to "excreting calcium" is the calcium metabolism process of the human body.

Who directs calcium absorption?

A considerable part of the calcium we take from our diet is not absorbed, but is directly excreted from the body. What factors influence the absorption of calcium? First of all, after calcium reaches the small intestine, there are generally two absorption channels. One is active transport through the channels on the surface of the small intestinal cells. The carrying capacity of the ion channels is limited, and the absorption rate will decrease if you eat too much calcium. The other is passive transport. In this case, calcium ions diffuse through the cell membrane along the concentration gradient to ensure the balance on both sides of the cell membrane. In the process, the cell does not need to provide additional energy. In other words, taking more calcium will promote absorption to a certain extent. Secondly, the active chemical properties of calcium itself also more or less influence the body's absorption of it. The dietary composition of Chinese residents is mainly plant food, which contains a large number of factors that affect the absorption of calcium, iron, and zinc, such as phytic acid, oxalic acid, cellulose, etc. Oxalic acid and phytic acid in plants combine with calcium and are not easily absorbed. In addition, the body's absorption of calcium is also regulated by factors such as hormones, among which vitamin D, calcitonin, and parathyroid hormone are very important.

Under the irradiation of ultraviolet rays, 7-dehydrocholesterol under the human skin will directly turn into vitamin D3. Subsequently, vitamin D3 is stored in the liver, combined with a special transport globulin in 25-hydroxycalciferol, and circulated throughout the body with the blood. The main effect of vitamin D3 on calcium absorption is to regulate active absorption and establish channels for active absorption. When vitamin D3 is reduced, the channels for active absorption will be reduced. Too much vitamin D will not be useful, and too little may have side effects.

Calcitonin can reduce the concentration of calcium ions in plasma and deposit calcium ions in bones. On the other hand, calcitonin can also inhibit the dissolution of bone salts, reduce the amount of calcium released into the blood by bone tissue, and lead to a decrease in blood calcium concentration. The parathyroid gland secretes a hormone whose main function is to mobilize calcium in bones to move outside cells. It promotes the reabsorption of calcium by the renal tubules and increases the excretion of phosphorus. The secretion of parathyroid hormone is also regulated by the blood calcium concentration, and the two are inversely proportional. When the blood calcium concentration is high, its secretion is inhibited; when the blood calcium concentration is low, its secretion is hyperactive. The action of calcitonin antagonizes the action of parathyroid hormone. In the human body, these three hormones coordinate with each other to maintain the concentration of blood calcium.

Calcium isn’t just for bones and teeth

Calcium absorbed by the small intestine enters the blood and maintains a relatively balanced blood calcium level to keep the heart beating and nerve signals transmitted. The human body controls the calcium in the blood, including the liquid calcium between cells, very finely. There is only a 10% change. A little higher or lower will cause problems.

The calcium content in our diet changes every day, but the blood calcium concentration can remain constant. Why is this? The human body is much smarter than we think. When the gastrointestinal tract absorbs a large amount of calcium, the blood calcium concentration can temporarily increase. At this time, the kidney's ability to excrete calcium increases, calcium absorption decreases, urine calcium increases significantly, and blood calcium concentration decreases. On the other hand, when blood calcium decreases, gastrointestinal absorption of calcium increases, and when blood calcium increases, gastrointestinal absorption of calcium decreases.

Does calcium supplementation mean strong bones?

In addition to ensuring the calcium content in the blood, most of the calcium absorbed by the human body is stored in bones and teeth. This is the "storage" of calcium. This "storage" is not static, but is constantly metabolizing. Because bones are under stress for a long time, they will age just like metals will fatigue. In order to maintain bone strength, bones need to metabolize, just like an asphalt road. Once it is damaged, people from the municipal department will be responsible for filling and repairing it with asphalt. The role of bone remodeling is to pave the "small pits" on the bones. It usually takes about 20 years to metabolize all the bones in the body.

Our bodies themselves also have economic considerations, and we build as much as we need. Modern urban dwellers sit still all day long and spend less time outdoors. As a result, our bones lack the necessary external force and sunlight to make them strong, and their strength is worrying. What's more frightening is that we worry about malnutrition and take excessive calcium supplements, which may cause ossification in places that should not ossify, such as muscles and ligaments. Especially for children, if they only rely on eating to supplement calcium and do not exercise, their bones will be relatively weak and not as strong as we expected. And many archaeological evidences also show that Asian women living 1,200 years ago had 20% more bone mass than women today, although they did not take calcium tablets and may not be able to supplement milk regularly.

How much calcium should we supplement?

With the improvement of living standards, the 2022 version of the "Dietary Guidelines for Chinese Residents" places special emphasis on milk intake, from the original 300 grams/day of milk and dairy products recommended in the 2016 version to 300-500 grams/day in the new version. According to the current nutritional dietary guidelines, the recommended calcium intake (RNI) for adults is 800 mg per day. This standard is based on Western research. At present, for the Chinese, it is still a seemingly distant goal. The "Report on the Nutrition and Chronic Disease Status of Chinese Residents (2020)" shows that the average daily calcium intake of Chinese residents is 356.2 mg. Not only in our country, but also in many countries in East Asia and Southeast Asia, residents' calcium intake is relatively low, which is related to our eating habits.

Adult calcium RDA varies from country to country. The World Health Organization (WHO) recommends 400-500 mg of calcium for adults in developing countries. The reason why it is lower than that in developed countries is probably because the protein content in the diet of these countries is lower. Protein has an important effect on urinary calcium excretion, and eating high-protein foods can increase urinary calcium excretion.

Calcium supplementation is not a simple issue. If you want to maintain bone health, calcium supplementation alone is not enough. Exercise and other factors are also very important. Calcium supplementation is not the more the better. For example, when the daily dietary calcium intake is higher than 1200 mg, cardiovascular mortality begins to increase.

When it comes to calcium supplementation, we should not focus just on the numbers, but the series of scientific principles behind the numbers.

The article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.

Author: Zhu Xinna, popular science author, independent book planner, and excellent reading promoter in Beijing

Reviewer: Chen Ran, Associate Research Librarian (Science Communication) / Senior Engineer, COFCO Nutrition and Health Research Institute