Key Takeaways (expand)
- Calcium is the most abundant mineral in the human body, composing up to 2% of our total body weight!
- Along with being a major structural component of bones, calcium helps build dental enamel (the hard outer shell of teeth).
- Calcium also functions as an electrolyte—giving it a role in regulating nerve impulses, muscle contraction, heartbeat, blood clotting, fluid balance, and blood pH.
- Too much or too little calcium in the blood can is harmful (and sometimes fatal!), so the body works hard to keep levels with in very a specific range—mobilizing it out of bone tissue when levels get too low, and altering its absorption and excretion when levels get too high.
- Calcium has important interactions with other nutrients: it’s dependent on vitamin D for proper intestinal absorption, can be excreted in excess when phosphorus intake is too high, and requires magnesium for mobilization to and from bone tissue.
- One of the best-studied benefits of calcium is on bone health: numerous studies show that calcium is protective against osteoporosis, can lower the risk of fractures in older adults (especially when combined with vitamin D), and reduces bone loss following menopause.
- Dietary calcium can also reduce kidney stone formation, due to binding with oxalate (a component of kidney stones) in the intestine and reducing its absorption!
- Calcium can also protect against high blood pressure—especially during pregnancy.
- Many studies have linked calcium intake with lower risk of colorectal cancer, as well as a lower risk of death following colorectal cancer diagnosis.
- As a result of its cell signaling duties, calcium is involved in a number of processes that help support healthy body weight—including insulin secretion, glycogen metabolism, lipolysis (lipid breakdown), fat oxidation, hormone signaling, and appetite regulation.
- There’s some evidence that calcium can make PMS symptoms less severe!
- Although more research is needed, a few studies have linked higher calcium intake with an increased risk of heart attack and prostate cancer—but these associations may be driven by other components of calcium-rich foods (especially dairy).
- Calcium insufficiency can lead to loss of bone density, increased risk of dental decay, and (in children and adolescents) a lack of reaching peak bone mass.
- Great sources of calcium include dairy products (particularly milk, cheese, yogurt, and kefir), bone-in sardines, green vegetables (especially Brassica vegetables like collard greens, kale, bok choy, turnip greens, broccoli, and cabbage), spinach, and seaweed.
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Calcium is the most abundant mineral in the human body, composing up to 2% of our total body weight—the vast majority of which is located in our bones and teeth. Along with its famous role in building strong bones, calcium serves as a second messenger in cell-signaling pathways.
Importantly, this nutrient functions as an electrolyte—a class of minerals that dissociate into charged particles (called ions) when dissolved in solution, making them capable of conducting electricity. On the whole, electrolytes help regulate fluid balance within the body, regulate nerve and muscle function (including the heart!), maintain a normal blood pH, and transmit nerve signals!
Calcium was first isolated in 1808 by Sir Humphry Davy (Britain’s leading scientist at the time, and also the inventor of the miner’s safety lamp!), who used electrolysis to isolate calcium from a mixture of lime and mercury oxide. But, humans have been using calcium long before we knew its chemical makeup! The Ancient Romans mixed calcium oxide with water to use as mortar for stone structures, and literature from the 10th century AD references plaster of paris (a mixture of calcium sulphate and dehydrated gypsum) to use for setting broken bones. The word calcium derives from the Latin calx, meaning “lime.”
Along with dairy products (particularly low-fat items), good sources of calcium are bone-in sardines, green vegetables (especially Brassica vegetables like kale, collard greens, turnip greens, bok choy, broccoli, and cabbage), and seaweed. Some other plant foods like spinach, rhubarb, and beans are also relatively rich in calcium, but also contain high levels of oxalate or phytate, which bind to calcium and inhibit its absorption. Other food sources include fortified foods like orange juice, soy milk, and certain cereals.
The Biological Roles of Calcium
Calcium’s two main functions are structural and regulatory: one, supporting skeletal health, and two, helping regulate cellular communication and protein functioning. And, under those two umbrellas are some pretty impressive activities!
Calcium is indispensable for building bone tissue. Along with serving as a major structural component of the skeleton (bone mineral is comprised of calcium phosphate crystals), calcium is a component of dental enamel—the hard outer shell of teeth. Cells called osteoblasts are responsible for depositing calcium to synthesize new bone. Without this mineral, we wouldn’t have the building blocks for the robust inner framework that supports our entire body!
Keeping circulating calcium levels within a very specific range is critical for physiological functioning. So critical, in fact, that the parathyroid hormone (PTH) is in charge of tightly regulating the amount of calcium in our blood, sometimes by “borrowing” calcium from bones (as well as from kidney and intestinal cells) if there’s not enough of it in the blood. More specifically, when the parathyroid glands sense calcium levels dropping, they begin secreting PTH, which stimulates bone resorption (the breakdown of bone tissue to release calcium and other minerals) and prompts the conversion of vitamin D into its active form (which in turn reduces the excretion of calcium and increases its intestinal absorption). Indeed, it’s less dangerous for the body to borrow calcium from skeletal tissue than to let blood calcium levels fall out of range: if the latter happens to an extreme degree, it can be deadly!
The importance of calcium homeostasis is due to the many functions calcium is essential for, particularly when it comes to cell signaling. This mineral helps regulate the constriction and relaxation of blood vessels, nerve impulses, muscle contraction, glycogen metabolism, cellular differentiation and motility, the secretion of hormones like insulin, and coagulation—AKA blood clotting (the binding of calcium ions is needed for activating seven vitamin K-dependent clotting factors). These activities are all controlled by differences in electrochemical signaling inside and outside of cells, as well as the flow through cellular membranes that can change these gradients. And, calcium is required to stabilize many proteins, including enzymes. So, the concentration of calcium in different tissues impacts extremely important functions like communication in the nervous system or contractility of our heart.
What’s more, calcium ions can also be toxic to cells. So, the body has to strike a balance between calcium-mediated cell function and calcium-mediated cell death by not only preventing blood calcium levels from dropping too low, but also preventing them from getting too high. To maintain calcium levels within a healthy upper bound, the thyroid gland begins secreting a peptide hormone called calcitonin as soon as blood calcium concentrations start rising; this hormone then inhibits PTH secretion, increases urinary calcium secretion, and decreases both intestinal calcium absorption and bone resorption.
Interactions with Other Nutrients
Calcium has some important interactions with other nutrients, too. It’s dependent on the active form of vitamin D (1,25-dihydroxyvitamin D) for optimal absorption from the intestines, and can be excreted in excess when sodium intake is high (for every gram of sodium excreted by the kidneys, about 26 mg of calcium gets drawn into the urine). However, some trials have found that increasing potassium intake can prevent this sodium-induced calcium loss. Phosphorus, too, increases the urinary excretion of calcium, particularly in diets with low calcium-to-phosphorus ratios or with large quantities of soft drinks containing phosphoric acid. Magnesium deficiency can impair PTH excretion by the parathyroid glands, altering the movement of calcium from bone tissue to blood (and vice versa). Calcium may even interact with the heavy metal lead, acting protectively against lead toxicity by decreasing how much of it gets absorbed in the intestine, and preventing lead from being released from the skeleton during bone demineralization. And, despite the myth that high-protein diets cause a negative calcium balance and can increase bone loss, most studies have actually found that protein enhances intestinal calcium absorption and is either neutral or beneficial for bone health (as measured by bone mineral density and fracture risk).
Drug Interactions with Calcium
Calcium (particularly supplements) can also interact with certain drugs in ways that are important to be aware of. In people taking digoxin, high-dose calcium supplementation can increase the risk of abnormal heart rhythms, and it can also reduce the absorption of bisphosphonates, the beta-blocker sotalol, tetracycline, quinolone antibiotics, and levothyroxine (so, it’s important to take any calcium supplements at least two hours apart from taking these medications!).
Calcium in Health and Disease
Getting enough calcium is vital for protecting against osteoporosis and bone fractures, especially in conjunction with vitamin D and magnesium. But, it can also help reduce your risk of kidney stones, protect against pregnancy-related high blood pressure, lower your risk of colorectal cancer (and improve survival following diagnosis), help you maintain a healthy body weight, and even reduce PMS symptoms!
Calcium and Bone Health
Scientists have studied the effects of calcium from food and supplements on a number of diseases—one of the most well-researched being osteoporosis. Across a variety of studies, including randomized controlled trials, increasing dietary calcium intake appears to modestly improve bone density in older adults, and calcium supplementation (especially combined with vitamin D) is associated with a significant reduction in fracture risk over time—including hip fractures. In other trials, the bone-building benefits of increased exercise only happened when calcium intake was adequate (1000 mg or more), suggesting this nutrient is also important for supporting the anti-osteoporosis effects of other interventions. Similarly, calcium intake has been shown to help protect against the increased rate of bone loss following menopause.
Calcium and Kidney Stones
Calcium also may affect the risk of kidney stones—hard deposits that form from minerals and salts (usually calcium and oxalate) in the urine. Although high urinary calcium is a risk factor for kidney stones, dietary calcium actually appears protective of kidney stone formation, with a number of prospective cohort studies showing a significantly lower risk for individuals with the highest (versus lowest) dietary calcium intake. And, intervention studies involving low-calcium diets (400 mg per day) for the treatment of high urinary calcium led to over a 50% increased risk of kidney stone recurrence compared to normal-calcium diets (1200 mg per day)! These effects may be due to calcium binding with oxalate in the intestine to form insoluble calcium oxalate salts, in turn reducing oxalate absorption and helping prevent high urinary oxalate levels (which is another risk factor for kidney stones). In fact, some small intervention studies have shown that as long as calcium intake is adequate (1000 mg per day), even relatively large intakes of oxalate don’t affect kidney stone risk.
That being said, a small number of trials have suggested that calcium supplementation—opposed to dietary calcium—does increase kidney stone risk, especially when it’s taken with vitamin D. But, this finding hasn’t been consistent across studies, so we need more research to know for sure!
Calcium and Blood Pressure
Calcium may also help protect against high blood pressure, especially pregnancy-related hypertensive disorders. In observational studies of expecting mothers, greater calcium intake is associated with a lower risk of high blood pressure, preeclampsia, and eclampsia, and placebo-controlled studies have shown that supplementing with calcium (1000 to 2000 mg per day, beginning anywhere from 20 to 34 weeks of pregnancy) lowers the risk of high blood pressure, preeclampsia, and preterm birth—especially among individuals who had low dietary calcium intakes to begin with.
And although the research here is less robust, calcium is sometimes used to treat hypertension in the general population; this makes sense considering its role in the contraction and relaxing of blood vessels.
Calcium and Colorectal Cancer
In prospective cohort studies, higher calcium intakes (especially from dairy foods) have been linked to lower risk of colorectal cancer, and randomized controlled trials have found that consuming 1200 mg of elemental calcium daily for four years reduces colorectal cancer incidence by 26%. (However, trials using people that already had fairly high calcium intakes when the study began didn’t show any anti-cancer benefits of additional calcium supplementation—suggesting that baseline intake is an important factor here!) And, dose-response analyses have found that for every 300 mg increase in daily calcium intake, the odds of getting colorectal cancer drop by 5%, while odds of high-risk adenoma drop by 11%. There’s even evidence that higher calcium intake after colorectal cancer diagnosis is associated with a lower risk of death.
These protective effects are probably due to calcium’s ability to bind to harmful substances in the colon and prevent cancer cells from growing.
Calcium and Obesity
Some evidence indicates that high calcium intake (especially from dairy) is associated with lower body weight and abdominal obesity, and some clinical trials have demonstrated that consuming 1200 mg per day of dietary calcium can aid in weight loss more effectively than lower-calcium diets. This suggests that calcium plays a role in weight regulation, possibly by reducing lipogenesis (fat accumulation in fat cells), stimulating lipolysis (the breakdown of lipids) driving fat oxidation, improving hormone signaling, increasing muscle efficiency, regulating appetite, or reducing dietary fat absorption in the intestine (calcium can trap dietary fat into calcium soaps, which are insoluble and hence excreted instead of absorbed by the body).
However, it’s worth noting that some of the calcium-obesity research is confounded by dairy intake, which has non-calcium components that could promote weight loss through other mechanisms (like conjugated linoleic acid and branched chain amino acids).
Calcium and PMS
Interestingly, calcium supplementation has been shown to make PMS symptoms less severe, and people with high dietary calcium intakes (1200 to 1300 mg daily) have a 30% lower risk of developing PMS compared to people with low calcium intakes (around 500 mg daily). This may be due to improved nervous system function (better mood) and adequate muscular signaling (reduced cramping).
Health Effects of Calcium Deficiency
Data suggests that upwards of 40% of adults aren’t getting enough calcium in their diet. This can be particularly true for people with lactose intolerance, or other individuals who avoid dairy products without replacing them with other calcium-rich foods. But, the symptoms of dietary deficiency or insufficiency aren’t outwardly obvious at first, and won’t show up on blood tests as low serum values (as happens with some nutrient deficiencies) due to our body’s ability to keep blood calcium levels stable regardless of dietary intake.
Because our bodies tightly regulate the amount of calcium in our blood by borrowing it from bone tissue as needed, insufficient calcium intake can lead to excessive bone resorption—ultimately leading to osteopenia, osteoporosis, and increased risk of fracture, as well as increased risk of dental decay. And in children and adolescents, inadequate calcium during growing years can prevent them from reaching optimal peak bone mass. In extreme cases, ongoing calcium deficiency (especially if combined with other conditions affecting calcium absorption or metabolism, like hypoparathyroidism) can lead to symptoms like confusion, muscle cramps and spasms, memory loss, brittle nails, hallucinations, numbness, and tingling hands or feet.
When calcium levels do get too low in the blood, it’s usually a consequence of abnormal parathyroid function, vitamin D deficiency, kidney failure, or alcoholism-induced magnesium deficiency, which can all happen regardless of actual calcium intake. Testing whether there has been adequate intake of calcium over time is best done with a bone scan, because this is a reliable measure of where calcium is stored.
Problems From Too Much Calcium
Excess calcium carries some unique risks, such as hypercalcemia (abnormally high calcium levels in the blood). Although this condition is typically due to overactive parathyroid glands or certain cancers, it can sometimes be caused by taking high-dose calcium supplements with absorbable alkali (such as over-the-counter calcium carbonate products, like Tums or Rolaids). This risk is particularly high among postmenopausal women, and is seen when taking calcium supplements ranging from 1.5 to 16.5 grams per day for at least two years. When hypercalcemia occurs, it triggers a shift in the body’s acid-base balance towards the alkaline state, eventually damaging kidney function and becoming fatal if left untreated.
Some evidence has also linked calcium supplements to an increased risk of heart attacks. This may be due to excess calcium from dietary supplements becoming incorporated into arterial plaque, instead of depositing into skeletal tissue or being excreted. However, the evidence isn’t totally consistent here, and dietary calcium (versus calcium in supplement form) is generally associated with less calcification in the arteries and better overall cardiovascular health. So, while food sources of calcium are unlikely to negatively impact heart health, it may be worth seeking medical advice from your health care provider before beginning calcium supplementation—especially if you have a family history of heart disease or other cardiovascular risk factors.
Some forms of calcium supplements (such as calcium carbonate and calcium lactate) can cause side effects like constipation, bloating, and abdominal pain. In these cases, calcium citrate can be an effective alternative, since it produces fewer gastrointestinal effects. What’s more, calcium carbonate can be poorly absorbed among people taking acid-reducing medications, since this form of calcium requires stomach acid for absorption; other forms of calcium may be more effective under these circumstances.
Additionally, in observational and intervention studies, high calcium intakes from dairy have occasionally been linked with a greater incidence of prostate cancer, and a few studies have found that calcium supplementation is associated with a greater risk of cardiovascular disease. More research is needed to fully understand these links, whether they’re specific to calcium or to certain components of dairy (such as insulin-like growth factor-I), and who might be most at risk. In general, calcium appears to be safe for these conditions as long as total intake doesn’t exceed the recommended upper limit of 2000 to 2500 mg per day.
How Much Calcium Do We Need?
As with most nutrients, the body needs varying amounts of calcium as we age or go through different health experiences, with the highest requirement being during adolescence and pregnancy (1300 mg per day). The recommended dietary allowance (RDA) for adults 19 to 50 years old is 1000 mg per day, after which the recommended amount increases to 1200 mg per day—especially after menopause. When estimating intake, though, it’s important to keep in mind that calcium bioavailability varies among different foods, and can also be inhibited by some drugs that reduce calcium absorption!
Good Food Sources of Calcium
The following foods are also excellent or good sources of calcium, containing at least 10% (and up to 50%) of the daily value per serving.
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