Calcium

Calcium is the most abundant mineral in the human body, with adult total calcium content approximately 1-1.2 kg. Approximately 99% is stored in bone as hydroxyapatite (Ca10(PO4)6(OH)2) that provides skeletal structure and serves as a reservoir. The remaining 1% is partitioned among extracellular fluid, intracellular cytosol, and membrane compartments, where it mediates neurotransmission, muscle contraction, blood coagulation, hormone secretion, and intracellular signaling.

Homeostatic regulation

Serum calcium is maintained in a narrow range (8.5-10.5 mg/dL total, ~1.1-1.3 mmol/L ionized) by the integrated action of parathyroid hormone (PTH), calcitriol (1,25-dihydroxyvitamin D), and calcitonin. When serum calcium falls, parathyroid chief cells detect the change through the calcium-sensing receptor (CaSR) and release PTH, which (1) stimulates renal calcium reabsorption and phosphate excretion, (2) activates renal CYP27B1 to produce calcitriol, (3) mobilizes bone calcium via osteoclast activation. Calcitriol enhances intestinal calcium absorption. Calcitonin, released from thyroid C-cells during hypercalcemia, inhibits osteoclast activity (though its physiological role in adult humans is modest).

Requirements

The 2011 NAM DRI set RDAs of 1000 mg/day for men 19-70 and women 19-50, 1200 mg/day for men 71+ and women 51+, and 1300 mg/day for adolescents and during pregnancy/lactation. Tolerable Upper Intake Level is 2500 mg/day for ages 19-50 and 2000 mg/day for ages 51+. These values have been criticized as possibly too high based on comparative international recommendations, which range from 500-1000 mg/day.

Dietary sources

Per USDA FoodData Central (mg Ca per serving): milk (8 oz) 300, yogurt (6 oz) 300, cheddar cheese (1.5 oz) 300, fortified soy or almond milk (8 oz) 300-450, fortified orange juice (8 oz) 300, tofu made with calcium sulfate (1/2 cup) 250-400, sardines with bones (3 oz) 325, salmon with bones (3 oz) 180, collard greens (cooked, 1 cup) 270, kale (cooked, 1 cup) 95, broccoli (cooked, 1 cup) 60, white beans (1 cup) 160, sesame seeds (1 oz) 280, almonds (1 oz) 75.

Bioavailability

Calcium absorption ranges from 20-40% of dietary calcium, depending on vitamin D status, concurrent meal composition, and age. Absorption efficiency declines with age. Oxalate-rich foods (spinach, Swiss chard, rhubarb) bind calcium in the intestinal lumen, producing very low bioavailability (5-10%) despite high total calcium content. Phytate in whole grains and legumes modestly reduces absorption. Calcium from dairy and calcium-set tofu is absorbed at 30-35%; calcium from fortified foods is comparable to dairy.

Calcium and bone outcomes

Adequate calcium intake supports peak bone mass acquisition through adolescence and early adulthood and limits bone loss in later adulthood. However, the magnitude of effect of calcium supplementation on fracture reduction in community-dwelling older adults has been modest in meta-analyses (Bolland et al. 2015), and the US Preventive Services Task Force has not recommended routine calcium-plus-vitamin-D supplementation for primary fracture prevention in postmenopausal women. Calcium is best obtained from food sources when possible.

Cardiovascular concerns

High-dose calcium supplementation (>1000 mg/day) has been associated with increased cardiovascular events in some meta-analyses (Bolland 2010, 2011), though the relationship is contested. Dietary calcium from food has not shown this signal. The practical response has been to prefer food-based calcium intake and reserve supplementation for documented deficiency or specific clinical indications.

Deficiency and excess

Symptomatic hypocalcemia is rare in ambulatory populations; it occurs in hypoparathyroidism, severe vitamin D deficiency, chronic kidney disease, and certain medications. Symptoms include perioral paresthesia, muscle cramps, tetany, and in severe cases seizures. Hypercalcemia from dietary sources is essentially unknown; it reflects hyperparathyroidism, malignancy, excessive vitamin D, or sarcoidosis. Chronic high-dose calcium supplementation can cause constipation, kidney stones (particularly in predisposed individuals), and the milk-alkali syndrome.