Calcium

Calcium is the most abundant mineral in the body — about 1.2 kg in an adult, and 99% of it is in the skeleton. The remaining percent, found in the blood and in cells, is responsible for muscle contraction, nerve conduction, blood clotting and hormone secretion.

And here is the point that decides everything: the body keeps blood calcium within a very narrow range. When intake is low, it simply draws calcium out of bone — which means a normal blood test does not indicate adequate intake or the state of your bones.

In recent years another important distinction has been added: calcium from food and calcium from a supplement do not behave the same way — not in bone, not in the kidneys, and perhaps not in the blood vessels.

Dosage and sources

Note the last few rows: spinach has calcium on paper — but its oxalates bind it, and only about 5% is absorbed. In kale, which is low in oxalates, the absorption rate is about 50% — the highest of any food. In practice spinach provides about 5 mg of calcium per 100 g, not 99.

Benefits (Pros)

1. Bone mass and osteoporosis

Calcium, together with vitamin D, vitamin K and protein, is the raw material of the skeleton. Adequate intake in adolescence and in your 20s sets your “peak bone mass” — the baseline from which decline begins in later life. This is the period when intake matters most, and precisely when it is low for many people.

2. Muscle contraction and nerve conduction

The release of calcium inside the muscle cell is what triggers the contraction mechanism. It is also needed for synaptic transmission and blood clotting.

3. Fewer kidney stones — from food

This is counterintuitive, and it is well established: higher dietary calcium intake lowers the risk of calcium-oxalate stones. The reason: calcium binds oxalate in the gut and is excreted in the stool, instead of the oxalate being absorbed and reaching the kidney. People prone to stones should eat calcium with meals, not avoid it.

4. Blood pressure and preeclampsia

Calcium supplementation in pregnancy has been shown to lower the risk of preeclampsia, especially in populations with low intake — one of the World Health Organization’s evidence-based recommendations.

Downsides and Side Effects (Cons)

1. Calcium supplements and the heart — the open debate

Several influential meta-analyses found a link between calcium supplements (mainly without vitamin D) and a slight increase in the risk of cardiac events. The finding is methodologically disputed, and not all studies support it.

What is agreed: the link has not been found with dietary calcium. The proposed explanation: food releases calcium slowly, whereas a 500-1,000 mg supplement creates a sharp spike in blood calcium. Hence the common recommendation: food first, a supplement only to close the gap.

2. Kidney stones — from a supplement, in the opposite direction

While dietary calcium lowers stone risk, a calcium supplement taken without a meal may raise it — it doesn’t meet oxalate in the gut, and the excess is excreted in the urine. If you take a supplement — always with food.

3. Blocking absorption of minerals and medications

Calcium competes for absorption with iron, zinc and magnesium. It also blocks the absorption of important medications — levothyroxine (for the thyroid), tetracycline and quinolone antibiotics, and bisphosphonates. Separate them by at least 4 hours.

4. Carbonate vs. citrate

• Calcium carbonate — cheap and concentrated (40% calcium), but requires stomach acid and so must be taken with food. Not suitable for people taking omeprazole and similar drugs.
• Calcium citrate — more expensive and less concentrated (21%), but absorbed even without acid, so it suits older adults and people taking acid blockers.

5. The per-dose absorption limit

The body efficiently absorbs up to about 500 mg of calcium at a time. A 1,000 mg supplement in a single dose wastes most of it. Split it into 500 mg doses.

6. Constipation and bloating

The most common side effect of calcium supplements, especially carbonate.

Summary

Calcium is essential for bone, muscle and nerves — and most people can reach their target from food: yogurt, sardines with bones, tahini, calcium-set tofu and kale. Three practical points get lost in most discussions: spinach is not a calcium source — oxalates block 95% of it, whereas kale is 50% absorbed; calcium from food lowers kidney stone risk, while a supplement taken without food may raise it; and it is supplements, not food, that have been linked to doubts about the heart. If you do take one — doses of up to 500 mg, with food, and separated from iron and medications.