Calcium is the most abundant mineral in the body — 99% stored in bones and teeth. Adequate calcium throughout life is critical for preventing osteoporosis.

Functions:

• Bone and tooth structure
• Muscle contraction (including heart)
• Nerve signal transmission
• Blood clotting

Recommended intake:

• Adults 19–50: 1,000 mg/day
• Women 51+, Men 71+: 1,200 mg/day
• Teenagers: 1,300 mg/day (peak bone mass building years)

Best sources: Dairy (milk, yogurt, cheese), fortified plant milks, sardines (with bones), tofu (made with calcium sulfate), leafy greens (kale, bok choy — not spinach, which has oxalates that inhibit absorption).

Key insight: Calcium absorption requires vitamin D. Supplementing calcium without adequate vitamin D is largely ineffective.

Reference:

Wikipedia: Calcium in biology

https://en.wikipedia.org/wiki/Calcium_in_biology