Calcium – For Strong Bones

Calcium is the most abundant mineral in the human body — about 99% of it is stored in bones and teeth, where it provides structural strength and rigidity. The remaining 1% circulates in the blood and soft tissues, where it plays critical roles in muscle contraction, nerve transmission, hormone secretion, and blood clotting. Your body tightly regulates blood calcium levels. If dietary intake falls short, it draws calcium from your bones to keep blood levels normal — a process that works in the short term but gradually weakens bone density over time.

How the Body Regulates Calcium

The body keeps blood calcium within a narrow range, mainly through parathyroid hormone (PTH) and vitamin D. When levels drop, the parathyroid glands (usually four small glands in the neck) release PTH. PTH signals the kidneys to excrete less calcium, triggers the kidneys to activate vitamin D (which increases calcium absorption in the gut), and stimulates bone to release calcium into the blood. When calcium levels rise, PTH falls and these effects reverse.

A third hormone, calcitonin, made by the thyroid gland, can lower blood calcium by slowing the breakdown of bone — but in adults its role is minor: people who’ve had their thyroid removed keep normal calcium levels. The constant balancing act is why adequate dietary intake matters: when the diet consistently falls short, PTH keeps drawing calcium from bone, and bone density pays the price.

RDA (Recommended Dietary Allowance)

* Adequate Intake (AI) for infants. Source: US National Academies, via the NIH Office of Dietary Supplements. The daily value used on food labels is 1,300mg. Recommendations vary between countries: the UK, for example, sets 700mg a day for adults.

ODA (Optimal Daily Allowance)

Meet the RDA — 1,000–1,200mg a day for most adults — preferably mostly from food. There’s no good evidence that going above the RDA builds stronger bones or prevents more fractures, and very high intakes, particularly from supplements, raise the risk of kidney stones. If your diet falls short, use a supplement only to fill the gap — often 500–600mg a day rather than a full 1,000mg or more — and pair it with adequate vitamin D. Teenagers (1,300mg) and women over 50 (1,200mg) need more than other adults.

Found In (per 100g)

How much you absorb matters as much as how much a food contains. The body absorbs roughly 30% of the calcium in dairy foods, and around half of the calcium in low-oxalate greens such as kale, broccoli, and bok choy. But spinach is a poor source despite its numbers: its oxalates bind calcium so tightly that only about 5% is absorbed. Whole (unhulled) sesame seeds are often listed among the richest sources, but most of their calcium sits in the hull, bound by oxalates, so it’s poorly absorbed — and hulled seeds and tahini contain much less.

Milk is one of the easiest everyday sources, with a typical 250ml glass providing roughly 300mg — about a quarter of the daily value. Calcium-fortified plant milks, juices, and cereals can match it — shake fortified plant milks well, as the calcium can settle at the bottom. Tofu varies enormously with how it’s made: tofu set with calcium sulfate is very high, while tofu set with nigari (magnesium chloride) contains less, and silken tofu contains very little — check the label. Greek yogurt contains a little less calcium than regular yogurt, because some is lost in the liquid strained off.

Helps With

• Building and maintaining strong bones and teeth
• Muscle contraction and relaxation (including the heart muscle)
• Nerve signal transmission
• Blood clotting
• Regulating heart rhythm
• Hormone secretion and enzyme activation
• Reducing the risk of osteoporosis — adequate calcium is essential for building and keeping bone, though in healthy adults living independently, calcium supplements have shown little or no reduction in fractures; benefits have been clearest in frail older people with low intakes, taking calcium together with vitamin D

For Athletes and Active People

Weight-bearing exercise is one of the strongest stimuli for bone density — but exercise alone isn’t enough if calcium intake is inadequate. The two work together: training gives bones a reason to get denser, and calcium gives them the material to do it with.

Sweat losses and long sessions: calcium is lost in sweat, and during long endurance sessions the resulting dip in blood calcium can trigger a rise in PTH and a short-term increase in bone breakdown. In studies of cyclists, taking about 1,000mg of calcium, or eating a calcium-rich meal, before long rides reduced this response. It’s a promising, low-risk strategy for endurance athletes, though its long-term effect on bone density hasn’t been established yet.

Restricted diets and energy availability: athletes who limit dairy or restrict food to control weight are at higher risk of low calcium intake. In female athletes, the combination of low energy availability, disrupted menstrual cycles, and low bone density is known as the female athlete triad, and the broader condition, which affects male athletes too, is called Relative Energy Deficiency in Sport (RED-S). Bone lost this way can be hard to fully regain — especially in teenagers and young adults, who are still building their peak bone mass — so eating enough overall matters as much as calcium itself.

Muscle function: calcium is the trigger for muscle contraction — it’s released inside muscle cells when a nerve signal arrives. Low dietary calcium doesn’t usually cause performance problems directly, because blood levels are tightly maintained, but long-term low intake undermines the bones that training depends on.

Insufficient Doses

Low intake is more likely in:

• People avoiding dairy (vegans, and those with lactose intolerance or milk allergy) who don’t use fortified alternatives
• People with low vitamin D — calcium absorption depends heavily on vitamin D, so even a good calcium intake is poorly absorbed without it
• Post-menopausal women — the drop in oestrogen speeds up bone loss and reduces calcium absorption
• People on long-term corticosteroids — these drugs reduce calcium absorption and increase its loss
• People with malabsorption conditions (coeliac disease, Crohn’s disease, inflammatory bowel disease)
• People with very high sodium intake, which increases calcium loss in urine. Caffeine has a smaller effect, easily offset by a little extra calcium, such as milk in coffee

What low intake does:

Low calcium intake usually causes no symptoms at all, because the body keeps blood calcium normal by taking it from bone. The damage is silent and gradual:

• Lower bone density over time — osteopenia, and eventually osteoporosis
• A higher risk of fractures, sometimes from minor falls
• In children, lower peak bone mass, and in some parts of the world, very low calcium intake contributes to rickets (softening and weakening of growing bones), usually together with low vitamin D

Low blood calcium (hypocalcaemia) is different, and is usually caused by medical problems — such as underactive parathyroid glands, severe vitamin D deficiency, kidney disease, or low magnesium — rather than by diet alone. Its symptoms include:

• Numbness or tingling around the mouth and in the fingers and toes
• Muscle cramps and spasms
• Fatigue
• In severe cases: abnormal heart rhythms, seizures, and confusion — a medical emergency

Excessive Doses

Upper limit (UL): 2,500mg a day for adults aged 19–50, and 2,000mg a day for adults 51 and over, from all sources. The limits for children are:

Source: US National Academies, via the NIH Office of Dietary Supplements.

Too much calcium almost always comes from supplements rather than food, and can cause:

• Kidney stones: in the large Women’s Health Initiative trial, women taking calcium (1,000mg a day) with vitamin D had a 17% higher risk of kidney stones. Interestingly, calcium from food has the opposite effect — eaten with meals, it binds oxalate in the gut, and higher dietary calcium is linked with fewer kidney stones.
• High blood calcium (hypercalcaemia): nausea, vomiting, constipation, confusion, fatigue, and excessive thirst. Very large amounts of calcium carbonate — for example, from heavy antacid use — can cause “milk-alkali syndrome,” which can damage the kidneys.
• Reduced absorption of other minerals: high-dose calcium can reduce iron absorption from the same meal, and may modestly affect zinc and magnesium.
• Possible heart risk — still debated: some analyses of trials linked calcium supplements (not calcium from food) with a modestly higher risk of heart attacks, possibly because supplements cause brief spikes in blood calcium. Other analyses found no link. It’s a reason to get calcium from food where you can, and to use supplements only to fill a shortfall.

Some observational studies have also linked very high calcium intakes with a higher risk of prostate cancer, though the evidence is mixed. Calcium from food isn’t associated with toxicity at normal intakes.

Different Types

• Calcium Carbonate — the most common and cheapest form, containing 40% elemental calcium by weight. Needs stomach acid to absorb well, so take it with food. Can cause constipation and bloating. Not ideal for people with low stomach acid or those taking acid-reducing medicines.
• Calcium Citrate — contains 21% elemental calcium, so you need larger tablets for the same dose, but it doesn’t need stomach acid and is absorbed well with or without food — making it the better choice for older people and those taking acid-reducing medicines. Less likely to cause constipation.
• Calcium Hydroxyapatite (microcrystalline hydroxyapatite) — derived from bone, containing calcium in the same mineral structure as human bone, along with collagen, phosphorus, and other compounds. A few small studies suggest benefits for bone density compared with calcium carbonate, but the evidence is limited and of low quality. More expensive; choose third-party tested products, since bone-derived supplements can contain traces of lead.
• Calcium Gluconate — low elemental calcium content (9%); mainly used medically, given intravenously to treat severe hypocalcaemia. Not practical as a general supplement.
• Calcium Lactate — 13% elemental calcium; gentle on digestion but rarely used in supplements. The white crystals sometimes seen on aged cheeses are often calcium lactate.
• Coral Calcium — essentially calcium carbonate from coral. It has no meaningful advantage over standard calcium carbonate, despite marketing claims (in the US, regulators took action against some sellers for false health claims), and it raises environmental concerns.

Which form for your goal:

How to Take It

The body absorbs calcium best in doses of about 500mg or less at a time — above that, the percentage absorbed falls. If your total supplemental dose is more than 500mg, split it into two or more servings through the day.

Take calcium carbonate with food so it absorbs properly. Calcium citrate can be taken with or without food. Take calcium supplements a few hours apart from iron supplements, as calcium reduces iron absorption.

Vitamin D is essential for calcium absorption — without enough vitamin D, even a well-planned calcium intake will be poorly absorbed. Some people pair calcium supplements with vitamin K2, on the theory that K2 helps direct calcium into bone rather than arteries. The idea is plausible, but it hasn’t been proven in trials — and anyone taking warfarin shouldn’t take K2 without medical advice.

Avoid taking calcium at the same time as high-dose magnesium, zinc, or iron supplements, and check the timing with your medicines (see below).

Interactions

Calcium supplements can reduce the absorption of several medicines, so they need to be taken at different times: thyroid medication (levothyroxine) — usually at least four hours apart; osteoporosis drugs (bisphosphonates, such as alendronate) — take the bisphosphonate first, as directed; and tetracycline and fluoroquinolone antibiotics (such as ciprofloxacin) — a few hours apart. Long-term corticosteroids, acid-reducing medicines (which reduce calcium carbonate absorption), and some anti-seizure medicines can lower calcium absorption or increase its loss. Thiazide diuretics reduce calcium loss in urine, so combining them with high-dose calcium supplements raises the risk of high blood calcium. High blood calcium also increases the risk of side effects from digoxin, a heart medicine.

Synergy

• Works with vitamin D, which enables calcium absorption in the intestines.
• Often paired with vitamin K2, which activates proteins that help bind calcium in bone and keep it out of artery walls — a plausible partnership, though the combination’s benefits haven’t been proven in trials.
• Works with magnesium and phosphorus to build bone structure and strength.

Antagonistic Interactions

• High sodium intake increases calcium loss in urine.
• Heavy cola drinking has been linked with lower bone density in women in one large study — possibly because of its phosphoric acid, or because cola often replaces milk.
• High-dose calcium reduces iron absorption, and may modestly reduce zinc and magnesium absorption — avoid taking them together.
• Oxalates (in spinach, rhubarb, and beet greens) and phytates (in whole grains and legumes) bind calcium and reduce how much is absorbed from those foods.

Food values: USDA FoodData Central — parmesan cheese, hard (SR Legacy, FDC ID 170848); Swiss cheese (Foundation Foods, 746767); cheddar cheese (Foundation Foods, 328637); tofu, firm, prepared with calcium sulfate (SR Legacy, 172475); chia seeds (Foundation Foods, 2710819); mozzarella, whole milk (SR Legacy, 170845); sardines, canned in oil, with bones (SR Legacy, 175139); pink salmon, canned (SR Legacy, 175175); almonds, raw (Foundation Foods, 2346393); yogurt, plain, low-fat (SR Legacy, 170886); spinach, boiled (SR Legacy, 168463); plain nonfat Greek yogurt (Foundation Foods, 330137); edamame, prepared (SR Legacy, 168411); broccoli, raw (Foundation Foods, 747447). RDA and upper limits from the US National Academies, via the NIH Office of Dietary Supplements. % Daily Values use the FDA’s current Daily Value of 1,300mg.