Calcium is an essential dietary mineral and a major component of bones and teeth. An adult human body contains more than 1 kg of calcium, most of which provides for structural functions. It is mainly needed for the firmness of bones, but movement would not be possible without it either. Many other biological processes also require calcium, such as the contraction of muscles and blood vessels and the functioning of the nervous system.  Calcium also affects hormone production: if it is deficient, levels of certain hormones rise. (1) Because so much calcium is stored in our bones, our body can regulate to make a continual supply available. Yet for this very reason, if you consume too little for a sustained period, your body will make up the shortfall from your bones, weakening them over time, and leading to osteoporosis. Supplementing with calcium alone has led to mixed and disappointing results, but when combined with vitamin D it has been effective in increasing bone density. 

How much calcium do we need? 

The daily calcium intake for adults is officially set at 1000 mg, while for teenagers still in development a slightly higher intake of 1200 mg is recommended. Pregnant and menopausal women are also recommended to take more calcium to provide additional calcium to the fetus and to make up for less efficient absorption of calcium after menopause. (2,3) 

To some extent, our body can regulate how much calcium it gleans from food: absorption is more efficient at low levels of intake, while higher calcium intake leads to lower rates of absorption. (4) Vitamin D also greatly influences absorption, so with an adequate supply of vitamin D, the body can make do with less calcium. 

The combined intake of vitamins D and K is very important, as they are not only necessary for calcium to be absorbed but are also paramount for its transport to the right destination in the body. Adequate calcium intake notwithstanding, if vitamins D and K are deficient, calcium can be deposited in the vascular walls and other soft tissues, leading to complications. (5,6) 

How to meet our calcium requirements? 

Upon hearing the word calcium, most people think of dairy products, and for good reason. Dairy products are indeed a good source of calcium, and the calcium they contain is well absorbed. (7) Far fewer people know that another very good source is bone itself. Small fish, such as sardines, which are eaten with the bone, are an excellent source of calcium and have been the main source of calcium for many traditional peoples. (8) 

Vegetables, particularly cruciferous vegetables such as broccoli, as well as seeds and nuts also contain calcium, but the absorbancy is highly variable. Some vegetables, such as broccoli or certain salads, are utilized even better than dairy products, but spinach, for example, is not a good source of calcium at all because of its high oxalate content. Vegetables and seeds are much more difficult to get the necessary amount because most people do not want to eat several kilograms of vegetables or 10-20 dkg of nuts per day. (9) Cereals and meat alone are not good sources of calcium either, so it is worth accompanying them with other food groups. 

A balanced diet based on natural foods, including both dairy products and vegetables, is almost guaranteed to provide enough calcium, but without them, our daily intake is likely to be too low.  

Is calcium supplementation a good idea? 

Calcium supplementation used to be very popular but has become less so recently, as it has not lived up to expectations in most studies. In these studies,  bone strength was not increased, nor risk of fracture decreased. In a 2015 review study, neither a higher intake of calcium nor calcium supplementation significantly reduced the risk of bone fractures, which would be the main goal of calcium supplementation. (10) However, when combined with vitamin D, the effect was much greater: this combination actually strengthens bones and reduces the likelihood of fractures. (11,12) 

Obviously, calcium supplementation is most effective for those with too low a calcium intake. In this case, supplementation makes sense, but optimizing vitamin D and K intake may also be a solution, in and of itself. If someone is already consuming enough calcium, supplementing with additional calcium can, in fact, have a negative effect. The excess can be deposited in the wrong places, at no additional benefit. This is probably the reason why several meta-analyses have found that calcium supplementation increased the risk of certain cardiovascular diseases. (13,14,15)  

Interesting facts 

In addition to animal bones, eggshells are another good source of calcium. It may sound strange at first, but several studies have found it to be a good source, with excellent absorption and utilization. (16,17,18) Just half an egg is enough to meet an adult person's total daily calcium requirements. Eggshells should be thoroughly cleaned, then heat-treated in the oven, and finely ground with a coffee grinder. They can then be added to various drinks, soups or even other foods if one's calcium intake is too low. 

[The cover photo of the note shows the mineral form of calcium]