Calcium

Calcium is an essential mineral; over 99% of the body’s calcium is stored in bones and teeth. It is the most abundant mineral in the human body and is needed not only for bone strength, but also for muscle contraction, nerve signaling, blood clotting, and hormone release. The most important practical point is that blood calcium can stay normal even when dietary intake is low, because the body can draw calcium from bone.

What it is

Calcium, often written as Ca, is a dietary mineral found in foods, some medicines such as antacids, and dietary supplements. In the body, most calcium is present as hydroxyapatite in bones and teeth, where it provides structural strength. A much smaller amount circulates in blood and tissues, but that small pool is tightly regulated because it is critical for normal cell function.

Common food sources include:

Source	Notes
Milk, curd, yogurt, cheese	Well-absorbed calcium sources
Ragi (finger millet)	Important non-dairy source in India
Soy products, especially calcium-set tofu	Amount varies by preparation
Small fish eaten with bones	Can provide substantial calcium
Green leafy vegetables	Useful, but absorption varies by plant
Fortified foods	Some plant milks, cereals, and foods may be fortified

In India, calcium intake can be low in some groups because of limited dairy intake, low dietary diversity, or higher needs during adolescence, pregnancy, and older age. National nutrition guidance in India generally emphasizes meeting needs through a balanced diet first, with supplements used when intake is inadequate or a clinician advises them.

How it works

Calcium balance depends on the gut, bones, kidneys, parathyroid hormone, and vitamin D.

• Absorption in the intestine: Calcium is absorbed by active transport and passive diffusion. Active transport becomes especially important when intake is lower.
• Vitamin D support: Vitamin D helps the intestine absorb calcium efficiently.
• Bone as a reservoir: Bone stores calcium and can release it when blood calcium needs to be maintained.
• Kidney regulation: The kidneys help conserve or excrete calcium depending on the body’s needs.

This is why calcium and vitamin D are often discussed together. If vitamin D status is poor, calcium absorption falls, and the body may compensate by increasing parathyroid hormone, which can increase bone turnover.

Evidence and uses

Calcium is essential for building and maintaining bone mass, especially during childhood, adolescence, and early adulthood. Adequate intake also helps reduce the risk of deficiency-related bone disease such as rickets in children when vitamin D is also low, and osteomalacia in adults.

For bone health, the evidence supports getting enough calcium, preferably from food. However, research does not show that taking more and more calcium provides unlimited benefit. Systematic reviews suggest that once intake is adequate, extra supplementation has only modest effects on bone density, and calcium supplements alone have a limited role in preventing fractures for many community-dwelling adults.

Key points on use:

1. Dietary adequacy matters most. Low habitual intake should be corrected.
2. Supplements can help when diet is insufficient. This is common in people who avoid dairy, have malabsorption, or have higher requirements.
3. Vitamin D status matters. Calcium may not be used well if vitamin D deficiency is present.
4. More is not always better. Excess intake can cause harm.

Common supplement forms include:

Form	Practical point
Calcium carbonate	Contains more elemental calcium; usually absorbed best with food
Calcium citrate	Often better tolerated and can be taken with or without food

Elemental calcium is the amount that counts toward intake, not the total tablet weight. This is a common source of confusion on supplement labels.

Safety and interactions

Calcium from food is generally safe. Problems are more likely with high-dose supplements or in people with certain medical conditions.

Possible side effects of supplements include:

• Constipation
• Bloating or gas
• Nausea
• Kidney stones in susceptible people

Very high calcium intake can contribute to hypercalcemia or milk-alkali syndrome, especially when combined with large amounts of absorbable alkali. People with kidney disease, a history of kidney stones, hyperparathyroidism, sarcoidosis, or certain cancers need medical guidance before using supplements.

Important interactions include:

Medicine or nutrient	Why spacing or caution matters
Levothyroxine	Calcium can reduce absorption
Iron supplements	Calcium can reduce iron absorption when taken together
Tetracycline and fluoroquinolone antibiotics	Calcium can bind the drug and reduce absorption
Bisphosphonates	Calcium can interfere with absorption if taken too close together

If you use any of these, ask a clinician or pharmacist how to separate doses. Do not start long-term calcium supplements just because they are sold over the counter.

Diagnosis / how it's measured

Calcium nutrition is not assessed well by a routine blood calcium test alone. Serum calcium is tightly controlled, so it may remain normal even when dietary intake is low.

Ways calcium status is considered include:

1. Diet history: The most useful first step for many people.
2. Bone mineral density testing: A DXA scan reflects long-term skeletal health, not just recent intake.
3. Blood tests when clinically indicated: Total calcium, ionized calcium, albumin, parathyroid hormone, vitamin D, phosphate, and kidney function may be checked when a disorder of calcium metabolism is suspected.

A low or high blood calcium level is a medical finding that can reflect hormone disorders, kidney disease, vitamin D problems, cancer, medicines, or acute illness. It should not be interpreted as a simple measure of dietary calcium intake.

When to see a clinician

Talk to a clinician if you:

• Have osteoporosis, osteopenia, or a fragility fracture
• Avoid dairy and think your intake may be low
• Have chronic diarrhea, celiac disease, inflammatory bowel disease, or prior bariatric surgery
• Have kidney stones or chronic kidney disease
• Are pregnant, breastfeeding, an older adult, or an adolescent with poor diet
• Plan to take calcium supplements regularly

Seek prompt care for symptoms that could suggest abnormal blood calcium, such as severe weakness, confusion, vomiting, muscle cramps, tingling, or an irregular heartbeat.

Limitations and open questions

Calcium is clearly essential, but several questions remain debated. Evidence is mixed on how much extra benefit supplements provide beyond an adequate diet, especially for fracture prevention in generally healthy adults. Research also continues on whether supplement timing, dose size, and coexisting vitamin D deficiency change outcomes.

Another limitation is that calcium needs are not identical for everyone. Absorption varies with age, vitamin D status, gut health, and the food matrix. Plant-based diets can meet calcium needs, but bioavailability differs across foods. For this reason, the best approach is usually individualized: estimate dietary intake, consider risk factors, and use supplements only when needed rather than assuming everyone benefits from routine high-dose calcium.