Calcium supplements increase the risk of atherosclerosis, blood clots, and other side effects

Many people take calcium supplements for bone health. However, this does not significantly reduce their risk of fractures and may even be associated with a range of serious side effects and premature death because calcium from supplements can accumulate in the blood vessel walls and increase the risk of atherosclerosis and heart attacks. A new review article published in Current Osteoporosis Reports highlights this connection. It is also important to be aware that the body’s calcium absorption, distribution, and bone health depend on vitamins C, D, and K2, as well as magnesium.

Our bone tissues contain about 99 percent of the body’s calcium, which is why calcium supplements have been prescribed for decades to prevent osteoporosis. Shortly after medical science learned how to measure bone mineral density (BMD), studies from the 1990s (Chapuy et al.) showed that supplementation with calcium and vitamin D slightly reduced fractures in elderly women living in nursing homes. It also turned out that these women were severely deficient in vitamin D, which is necessary for calcium absorption. This study paved the way for future standard treatment for osteoporosis, where fairly high doses of calcium were prescribed together with vitamin D.
Since then, many studies have shown that taking large amounts of calcium comes with certain risks. Firstly, calcium supplements can increase the risk of constipation and inhibit the absorption of iron, zinc, and magnesium. Secondly, they increase the risk of kidney stones. Thirdly, meta-analyses have revealed that calcium supplements increase the risk of heart attacks by 10-20 percent. This is not the case when calcium is obtained through diet, though.
Under normal circumstances, blood calcium levels must be carefully controlled. However, ingestion of large calcium supplements raises blood calcium levels for the next eight hours, thereby increasing the risk of atherosclerosis and high blood pressure. According to the review article, several Mendelian randomization studies have shown that elevated blood calcium levels increase the risk of cardiovascular diseases and premature death.
It also appears that the large calcium doses that are often combined with small amounts of vitamin D, do not have a major effect on preventing osteoporosis. This is because magnesium and vitamin K2 play a more important role in the body’s calcium distribution.
It is also important to note that nerve cells, muscle cells, and other cells in soft tissues ideally contain very little calcium – more specifically less than one percent of the body’s total calcium reserves. Calcium ions specifically ensure nerve cell signaling, muscle contractions, and the ability of the heart to beat, which are all vital functions. If these cells are flooded by calcium ions, they risk cramping and stress. This can also trigger inflammatory conditions and cell death.

How much calcium do we need?

The majority of people get their calcium from green vegetables, nuts, seeds, almonds, whole grains, eggs, and water. Studies from Africa and Asia have shown that daily intake of a few hundred milligrams of calcium is sufficient for bone health. Consuming less than 200 mg, however, can cause rickets in children.
In 1974, the WHO and Food and Agriculture Organization recommended 400-500 mg of calcium per day for adults.
Later studies of peri-menopausal nuns showed their calcium requirement was 990 mg daily, and post-menopausal women needed 1,500 mg daily. Although these studies form the basis of current official recommendations in the Western world, later studies suggest there is no biological basis for these high recommendations, simply because the body’s calcium balance involves much more than taking calcium supplements.
A 2016 calcium balance analysis of Chinese women over 60 showed that daily intake of 300 mg of calcium from a typical Chinese diet was enough to maintain a positive calcium balance and healthy bones.
According to an older study, Japanese women consume about 400-500 mg of dietary calcium daily, yet they even have a lower rate of osteoporosis than European and American women do. However, these Asian women get more magnesium from vegetables and more vitamin K2 from fermented products.
In Denmark, the official daily calcium recommendation (RI) for adults calls for 800 mg. For those with osteoporosis, the Danish Endocrinological Society recommends 1,000 - 1,200 mg calcium daily along with vitamin D. As the new review article and numerous studies show, however, large calcium supplements may do more harm than good by increasing the risk of cardiovascular diseases, blood clots, inflammation, and other serious side effects.

• Many researchers advise against taking calcium as a mono supplement.

What effects do vitamins C, D, and K2 and magnesium have on bone health?

As noted, healthy bones and bone mineral density depend on much more than calcium.
Vitamin C is important for making collagen, which is crucial for the structure of connective tissues like bones, blood vessels, and other tissues. According to a Polish study (Alicja Ewa Tetaczak et al.), vitamin C deficiency can cause changes in the extracellular matrix of bones as well as blood vessels and tendons, potentially leading to fragile bones.
Vitamin D determines, among other things, how much calcium is absorbed from the intestines. However, if a meal or supplement contains too much calcium, it can leach vitamin D from the body. Additionally, vitamin D deficiencies are already widespread, and there is no guarantee that official recommendations are able to optimize blood levels of the nutrient.
Magnesium serves as a gatekeeper in the calcium channels of cell membranes. It ensures that nearly all calcium is channeled into bone cells that contain 60 percent of the body’s magnesium, but it also has other roles in bone health. Magnesium also keeps calcium concentrations very low in cells of soft tissues such as nerves and muscles. Magnesium deficiency, which is common, disrupts the body’s calcium distribution and increases the risk of osteoporosis, cardiovascular disease, cell stress, and inflammation because calcium ends up in the wrong tissues. Moreover, the body requires less calcium when magnesium levels are optimal.
Vitamin K2 is important for the formation of two proteins that are crucial for bone health. In blood vessels, vitamin K2 activates MGP (matrix Gla protein), which removes calcium from the bloodstream and effectively prevents atherosclerosis. Vitamin K2 also plays a role in forming osteocalcin, which strengthens bone tissue through a process called carboxylation.
In the past, we obtained more vitamin K2 from fermented products. Vitamin K2 is also found in fatty foods such as egg yolk, butter, and aged cheese, preferably from grass-fed animals. It is also possible to convert vitamin K1 into K2, but this requires a sufficient intake of vitamin K1 from dietary sources and a well-functioning gut flora.

Note: Beware of ”bone thieves” and remember to exercise

• Soda, sugar, and alcohol can interfere negatively with bone-building processes
• Cholesterol-lowering statins inhibit vitamins D and K2, which are also synthesized from cholesterol
• Antacids, diuretics, blood thinners (e.g., Marevan), antidepressants, and corticosteroids can increase calcium excretion or otherwise disrupt calcium metabolism
• Your bones benefit from daily weight-bearing exercise.

References:

Ian R Reid. Calcium Supplementation – Efficacy and safety. Current Osteoporosis Reports 2025

Alicja Eea Rataczak et al. Vitamin C Deficiency and the Risk of Osteoporosis in Patients with an Inflammatory Bowel Disease. Nutrients 2020

Andrea Rosanoff et al. Essential Nutrient Interactions: Does Low or Suboptimal Magnesium Interact with Vitamin D and/or Calcium status. Advances in Nutrition 2016

Nutrition insight. Role of Vitamin K in Bone Health Underlined in New Study. Jun 2017

Fujita T, Fukase M. Comparison of osteoporosis and calcium intake between Japan and United States. PubMed. 2002