Calcium

What it is

Calcium makes up most of the mineral in your bones and teeth and also plays a role in muscle contraction, nerve signalling and blood clotting. Good sources are dairy, fortified plant milks, tinned fish with bones, tofu set with calcium and some green vegetables.

The adult UK reference intake is around 700 mg a day. Most people who eat dairy or fortified alternatives meet this, but people avoiding dairy, older adults and those with restrictive diets may fall short.

Calcium supplements are often paired with vitamin D for bone health. Recent research has been less enthusiastic about their benefits in healthy people than older advice suggested.

How it works

Bones constantly rebuild, and calcium is the raw material. Absorption depends on vitamin D and stomach acid. Your body keeps blood calcium in a tight range by pulling from or depositing into bone, so a low intake over years can weaken bone.

What the evidence says

• Strong Meeting daily needs when diet is low
  If your diet really is short on calcium, a supplement fills the gap.
• Moderate Slowing bone loss in older adults
  Effects on fracture risk are small and best seen in people with low intake or vitamin D deficiency.
• Weak Fracture prevention in the general population
  Large reviews find little or no reduction in healthy adults taking calcium on top of adequate diets.
• Limited Premenstrual symptoms
  Some trials show mild symptom relief; research is not strong.
• Moderate Heartburn relief (carbonate antacids)
  Works quickly as an antacid, not as a long-term supplement.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Work out your food intake first; many people need only 200-500 mg more.
• Take no more than about 500 mg at one sitting for best absorption.
• Carbonate needs stomach acid, so take with food; citrate is fine with or without.
• Pairing with vitamin D is typical, discuss doses with a pharmacist.

Forms and what to look for

• Calcium carbonate: cheapest, highest elemental calcium, take with meals.
• Calcium citrate: absorbed regardless of stomach acid, better for people on acid blockers.
• Calcium gluconate and lactate: low in elemental calcium per tablet.
• Coral or oyster-shell calcium: marketing claims outpace evidence.
• Check the elemental calcium listed, not the compound weight.

What to expect, and when

• You will not feel anything.
• Changes in bone density, if any, take a year or more to measure.
• Constipation or bloating can show up within days.

Side effects

• Constipation, gas and bloating
• Nausea
• Kidney stones in susceptible people at high intakes
• High blood calcium (hypercalcaemia) with excess, causing thirst, nausea, confusion

Interactions

• Reduces absorption of levothyroxine, quinolone and tetracycline antibiotics, bisphosphonates and iron
• Raises calcium levels with thiazide diuretics
• Can interact with digoxin and some heart medicines
• Vitamin D increases calcium absorption, so combined high doses add up

Who should avoid it

• Anyone with hypercalcaemia or hyperparathyroidism
• People with a history of kidney stones, without advice
• People with sarcoidosis or some cancers
• Anyone on heart medication that interacts, unless supervised

Risks and safety

• Excess supplemental calcium has been linked in some research to kidney stones and, more tentatively, heart concerns. The evidence is debated.
• Not on the WADA list.
• Some antacid products and sources have contained lead in the past; choose reputable brands.
• Milk-alkali syndrome can occur with very high antacid use.

Buying well: quality and testing

• Look for third-party testing for heavy metals.
• Check elemental calcium per tablet.
• Be sceptical of oversized tablets claiming one-a-day dosing.
• Avoid unregulated natural coral products.

Bottom line

Aim to get calcium from food first. A small supplement makes sense if you avoid dairy and fortified foods or a clinician recommends it, but a big daily dose for everyone is not supported by current evidence.