Calcium is the most abundant mineral in the human body. It plays a vital role in body structure, but it has other critical functions in human physiology. Ninety-nine percent of whole-body calcium is incorporated into the structure of bones and teeth. The remaining one percent is distributed throughout the body. Many cells contain calcium channels in their membranes. Calcium ions moving through these channels trigger nerve impulses, muscle contraction (including in the heart), blood vessel contraction and relaxation, and secretion of endocrine hormones. Calcium is also part of the clotting cascade, interacting with vitamin K in the activation of clotting factors.

Calcium from foods and some types of dietary supplements must become ionized in an acid medium in order to be absorbed in the small intestine. It is believed that calcium must be in an acid medium for approximately one hour for this to occur. Some forms of calcium bound to highly soluble ligands can be absorbed in non-ionized forms.

Calcium is absorbed by more than one mechanism throughout the small intestine. Active transport occurs in the duodenum and proximal jejunum and is dependent on both dietary intake and the needs of the body at the time of intake. This process is highly vitamin D-dependent.¹ Transport in the duodenum is thought to be most efficient. Passive diffusion of calcium can occur anywhere in the small intestine, but the ileum is the most abundant area. It is completely dependent on the amount of ionized calcium present in the gut and is not dependent on vitamin D status. Some passive diffusion can also occur in the colon, and, under the right circumstances, this can account for up to four percent of calcium absorption.² There is some indication that individuals with less absorption of calcium in the proximal intestine will be able to absorb more calcium from the colon.

Dietary factors can have a great influence over calcium bioavailability. Dietary oxalate can significantly bind to calcium, making it unavailable for absorption. Oxalate is found in many vegetables, such as spinach, sweet potatoes, and beans, as well as berries and chocolate. Phytate, mostly found in grains, can impair absorption to a much lesser degree. Magnesium and calcium will compete for absorption if both are present in substantial amounts. Phosphorus at a level greater than calcium may impair calcium balance, though this is a complex relationship. Large amounts of sodium and protein both increase calcium loss. Caffeine can significantly increase calcium loss in the urine and gut. Steatorrhea (fatty stool) will cause decreased calcium absorption when undigested fats bind with calcium in the gut, creating soaps that cannot be absorbed. 

It is important to know that calcium within the body is tightly regulated. Parathyroid hormone (PTH), calcitonin, and vitamin D are the primary regulatory agents, but sex hormones, adrenal hormones, insulin-like growth factor 1 (IGF-1), and leptin have influence over bone mineralization or loss as well. Because of the vital functions that calcium regulates, any changes in blood calcium levels above or below the desired range signal the calcium regulatory system to remove calcium from bone, absorb more or less calcium from the gut, and excrete more or less calcium in the urine. Thus, when dietary calcium levels are low or absorption is poor (or both), the body will simply remove calcium from bone to keep blood levels in the desired range. In many cases, this is painless and causes few dramatic symptoms until bone loss is severe.

DEFICIENCY

In discussing calcium deficiency, it is important to consider both hypocalcemia and metabolic bone disease. Isolated hypocalcemia (too little calcium in the blood) tends to occur when disorders of calcium metabolism are present, the most common of which is hypoparathyroidism. Other causes of hypocalcemia include low calcium intake, low protein, malabsorption, hypo- or achlorhydria, hypomagnesemia, and vitamin D deficiency. Metabolic bone disease includes the conditions of osteoporosis, osteomalacia, and hyperparathyroidism.

Mild or even moderate hypocalcemia can be completely asymptomatic. Neuromuscular irritability can manifest with spontaneous muscle cramping, paresthesia (pins-and-needles) in the fingers and sometimes toes, numbness of the fingers and hands, and loss of sensation around the mouth. Individuals might also experience muscles that are stiff, painful, and cramp easily with minimal work, as well as gastrointestinal cramps. Another symptom is bronchospasm, which can lead to difficulty breathing, and those with asthma may report more frequent, hard to control attacks. Hypotension can result in faintness, increased spontaneous sweating, and difficulty swallowing. In people with a history of epilepsy, there may be an increased frequency of seizure activity.

Since bone is the storage pool for almost all calcium in the body, chronic deficiency leads to bone loss and can result in skeletal problems, including fractures. Fractures are often associated with a major trauma, such as falls, but the incident may also be minor and unmemorable. Years-long bone loss can lead to loss of height, thoracic kyphosis (“dowager’s hump”), and lumbar flattening. Other chronic manifestations of low calcium include dry skin, brittle hair and nails, poor dental health, psoriatic skin lesions, itching, anxiety, agitation, depression, hypertension, and increased risk for colon cancer. Chronic or mild hypocalcemia should be treated with oral calcium, and, when necessary, underlying causes of deficiency, such as low protein, must be addressed. Treatment of acute or severe hypocalcemia may require intravenous (IV) repletion in an inpatient setting.

RECOMMENDED INTAKE AND SOURCES

The recommended daily allowance (RDA) for calcium is 1,000mg for adults 19 to 50 years of age; for women aged 51 to 70 years, this increases to 1,200mg daily. Adolescents aged 9 to 18 years should have 1,300mg per day. Dairy products (e.g., milk, yogurt, cheese), tofu, and fortified drinks (e.g., fruit and vegetable juices, soy milk, rice milk, almond milk) are the best sources of calcium. Beans and dark green vegetables, such as kale and broccoli, also have substantial calcium, but it is much less bioavailable due to phytates and oxalates. Fish in which the bones are edible (e.g., sardines, anchovies, canned salmon) are excellent sources of calcium, with high bioavailability.³

TOXICITY

There is very little toxicity caused by oral calcium. The Institute of Medicine set the tolerable upper limit (UL) at 2,500mg. The most common problems associated with excess calcium are gastrointestinal in nature and include constipation, nausea, vomiting, dry mouth, and loss of appetite. There is also increased risk of kidney stone formation in individuals who are prone to such condition. Those with a history of kidney stones should be instructed to take calcium with food rather than between meals.

INTERACTIONS

Calcium has many known interactions with nutrients and drugs. Iron absorption can be significantly impaired by large doses of calcium or by consumption of calcium-rich foods. Co-consumption of iron with a high calcium load can decrease iron absorption by up to 62 percent.⁴ Phosphorus can both cause increased calcium excretion in the gut and reduce calcium loss in the urine. A chronic high-phosphorus, low calcium diet can result in elevated PTH and overall net bone loss. Excessive intakes of sodium and caffeine also increase calcium loss from the body. Zinc and calcium also have reported interactions. High calcium levels taken concurrently with zinc can decrease zinc absorption by as much as 50 percent,⁵ yet in individuals with low dietary intake of calcium, zinc supplementation has been shown to decrease calcium uptake.⁶ Ultimately, when using supplemental calcium for long-term risk reduction, it is important to ensure that other minerals are adequately represented. 

Some medications are not adequately absorbed in the presence of calcium. These include many antibiotics (e.g., tetracycline, doxycycline, minocycline, quinolones) and thyroid hormones. Some cardiac medications, such as propranolol and its derivatives and calcium channel blockers, may be less effective in those taking calcium supplements. There are also many drugs that contribute to calcium loss. Individuals who need to be on calcium supplements and are also taking prescription medication should consult with their pharmacists.

Editor’s note. Consult with your physician or a certified dietitian/nutritionist to determine a diet best suited to your individual needs.

This article was adapted with permission from Jacques J. Calcium. Micronutrition For The Weight Loss Surgery Patient. Edgemont, PA: Matrix Medical Communications; 2005: 85–92.

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3. National Institutes of Health. Calcium: fact sheet for consumers. Updated 6 Oct 2022. https://ods.od.nih.gov/factsheets/Calcium-Consumer/. Accessed 26 Apr 2023.
4. Wood RJ, Zheng JJ. High dietary calcium intakes reduce zinc absorption and balance in humans. Am J Clin Nutr. 1997;65(6):1803–1809.
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6. van Meurs JB, Dhonukshe-Rutten RA, Pluijm SM, et al. Homocysteine levels and the risk of osteoporotic fracture. N Engl J Med. 2004;350(20):2033–2041.