What calcium does in the body

Calcium is the most abundant mineral in the body. Nearly all of it is stored in bones and teeth, where it contributes structure and strength. The small amount circulating in blood and tissues supports muscle contraction, nerve signaling, blood-vessel function, hormone release, and blood clotting.

Bone is living tissue that is continuously remodeled. When blood calcium falls, hormonal systems can release calcium from bone to protect essential functions. Chronically inadequate intake can therefore contribute to reduced bone strength even when a routine blood calcium result appears normal.

How much calcium do adults need?

The NIH recommended dietary allowance is 1,000 mg per day for adults ages 19–50 and men ages 51–70. Women ages 51–70 and all adults age 71 and older are advised to obtain 1,200 mg daily. Pregnant and breastfeeding adults generally need 1,000 mg; teenagers have higher requirements.

These targets refer to total calcium from foods, fortified products, beverages, supplements, and calcium-containing medicines such as some antacids. They are not automatic supplement doses. Estimating food intake first helps prevent unnecessary duplication.

Best food sources of calcium

Dairy foods

Milk, yogurt, and cheese are concentrated, well-absorbed sources. Lactose-free dairy provides similar calcium for people with lactose intolerance. Yogurt and hard cheeses may be tolerated better than milk, but milk allergy requires avoidance rather than lactose reduction.

Fish, vegetables and legumes

Canned sardines and salmon with edible bones provide substantial calcium. Kale, bok choy, turnip greens, broccoli, white beans, tahini, and almonds contribute varying amounts. Spinach contains calcium, but oxalate binds much of it, so the absorbed amount is lower than from kale or bok choy.

Fortified foods and beverages

Many soy, pea, almond, and oat beverages, some juices, breakfast cereals, and tofu contain added calcium. Fortification varies widely. Shake plant-beverage cartons because calcium can settle, and check both serving size and percentage Daily Value. A product that looks like milk is not necessarily fortified.

Calcium absorption and vitamin D

Vitamin D supports active calcium absorption in the intestine and helps maintain blood calcium. Severe vitamin D deficiency can cause osteomalacia in adults and undermine bone mineralization even when calcium intake appears adequate. Sun exposure, food, supplements, skin pigmentation, age, season, location, and medical conditions all influence vitamin D status.

The body absorbs calcium most efficiently in amounts of about 500 mg or less at one time. Absorption also varies with age, vitamin D status, stomach acid, the calcium compound, and the meal. More calcium swallowed does not always mean proportionally more absorbed.

Protein, magnesium, phosphorus, and overall energy intake also matter for bone. Extremely restrictive eating, low body weight, eating disorders, smoking, heavy alcohol use, and prolonged inactivity can harm bone health regardless of calcium intake.

Exercise for bone health

Bone responds to mechanical load. Weight-bearing activities such as brisk walking, stair climbing, dancing, hiking, and court sports challenge the skeleton. Resistance training with weights, bands, or body weight strengthens muscles and applies site-specific loading to bone.

Balance, gait, and functional training reduce fall risk, which is crucial because supplements cannot prevent every fracture. The best program depends on current fitness, osteoporosis severity, joint health, prior fractures, and fall risk. People with vertebral fractures or advanced osteoporosis should obtain tailored guidance before high-impact exercise or loaded spinal flexion.

Bone density, osteopenia and osteoporosis

Peak bone mass is largely built during childhood and early adulthood. Bone loss accelerates after menopause as estrogen falls, and age-related loss occurs in men as well. Osteoporosis is defined by reduced bone strength and increased fracture risk; osteopenia describes bone density below normal but not in the osteoporosis range.

A dual-energy X-ray absorptiometry (DXA) scan measures bone mineral density. Screening decisions consider age, sex, menopause, fractures, medications such as long-term corticosteroids, low body weight, smoking, alcohol, family history, and medical conditions. Calcium intake alone cannot diagnose or exclude osteoporosis.

Someone with osteoporosis or a fragility fracture may need prescription treatment in addition to nutrition and exercise. Adequate calcium and vitamin D support therapy but do not replace medicines shown to reduce fracture risk in appropriate patients.

Do calcium supplements prevent fractures?

Evidence is mixed and depends on the population. Some studies show small improvements in bone mineral density, but fracture prevention in generally healthy community-dwelling adults is uncertain. A large 2025 systematic review found little to no clinically meaningful fracture or fall benefit from calcium, vitamin D, or combined supplementation in adults not receiving osteoporosis medicine; evidence was limited for high-risk or residential-care populations.

This does not mean calcium is unimportant for bone health. It means that giving a supplement to everyone is not equivalent to correcting inadequate intake in a person who needs it. Benefits and risks differ for deficiency, institutional care, malabsorption, osteoporosis treatment, kidney disease, and healthy adults with adequate diets.

Calcium carbonate versus calcium citrate

Calcium carbonate contains more elemental calcium per tablet and is usually less expensive. It is absorbed best with food and depends more on stomach acid. Calcium citrate can be taken with or without food and may be preferable for older adults, people using acid-suppressing medicines, or those with reduced stomach acid.

Check the Supplement Facts label for elemental calcium, not the total weight of the compound. Split supplemental amounts above 500 mg into separate doses. If constipation, gas, or bloating occurs, a smaller amount, different schedule, or different form may help.

Risks of excessive calcium

Too much supplemental calcium can cause constipation, nausea, abdominal discomfort, and interfere with absorption of other nutrients or medicines. Very high blood calcium can cause weakness, confusion, excessive thirst and urination, kidney problems, and abnormal heart rhythms, though it usually results from disease rather than ordinary food intake.

Some evidence links calcium plus vitamin D supplements to a small increase in kidney stones, while research on cardiovascular risk is inconsistent. People with kidney stones, kidney disease, hyperparathyroidism, sarcoidosis, cancer, or high blood calcium require individualized advice.

Medication interactions

Calcium can reduce absorption of levothyroxine, some quinolone antibiotics, bisphosphonates, and dolutegravir. Exact spacing depends on the medication; for example, levothyroxine labeling commonly requires at least four hours from calcium carbonate. Do not apply one spacing rule to every prescription—ask a pharmacist.

Lithium can raise blood calcium. Thiazide diuretics reduce urinary calcium loss and may contribute to high calcium in susceptible people. Long-term corticosteroids accelerate bone loss. Calcium-containing antacids count toward total intake and can become a hidden source when used frequently.

A practical calcium and bone health plan

• Estimate intake: list calcium-rich foods and fortified products consumed on a typical day.
• Fill gaps with food first: choose options that fit allergies, preferences, and budget.
• Use the smallest supplement needed: count calcium already present in multivitamins and antacids.
• Support vitamin D: review intake, risk factors, and testing with a clinician when appropriate.
• Train bones and muscles: combine weight-bearing, resistance, and balance activity.
• Reduce fracture risks: avoid smoking, limit heavy alcohol use, review fall hazards, and address vision or medication issues.

For more context on the nutrient that helps calcium absorption, read our vitamin D guide.

Frequently asked questions

References

1. Calcium: Health Professional Fact Sheet. NIH Office of Dietary Supplements.
2. Calcium: Consumer Fact Sheet. NIH Office of Dietary Supplements.
3. Vitamin D: Health Professional Fact Sheet. NIH Office of Dietary Supplements.
4. Calcium, Vitamin D, or Combined Supplementation to Prevent Fractures and Falls. Systematic review and meta-analysis, 2025.
5. Vitamin D, Calcium, or Combined Supplementation for Primary Fracture Prevention. Systematic review.