Calcium

What Is Calcium?

Calcium is the most abundant mineral in the human body. An adult body contains approximately 1,000-1,200 grams of calcium, with about 99 percent stored in bones and teeth as hydroxyapatite. The remaining 1 percent circulates in the blood and soft tissues, where it performs vital regulatory functions. The body tightly controls blood calcium within the narrow range of 8.5-10.5 mg/dL, withdrawing calcium from bone when dietary intake is insufficient.

Key Functions

• Bone and tooth structure: Calcium and phosphorus form the rigid mineral matrix that gives bones their strength and hardness.
• Muscle contraction: Calcium ions released from the sarcoplasmic reticulum trigger the interaction of actin and myosin filaments, enabling muscle contraction — including in the heart.
• Nerve transmission: Calcium influx at nerve terminals triggers neurotransmitter release.
• Blood clotting: Several clotting factors (II, VII, IX, X) require calcium for activation.
• Cell signaling: Calcium acts as a second messenger in numerous hormone and growth factor pathways.

Dietary Reference Values and Food Sources

The Recommended Dietary Allowance (RDA) for calcium is 1,000 mg/day for adults aged 19-50, rising to 1,200 mg/day for women over 50 and adults over 70. The Tolerable Upper Intake Level (UL) is 2,500 mg/day for adults. Key food sources:

Deficiency and Bone Health

Chronically low calcium intake leads to secondary hyperparathyroidism, where parathyroid hormone continuously mobilizes calcium from bone, eventually causing osteoporosis and increased fracture risk. Adequate calcium intake throughout life — especially during childhood, adolescence, and early adulthood — is critical for reaching peak bone mass. Vitamin D is required for efficient intestinal calcium absorption; deficiency of either nutrient compounds bone loss.