Calcium and Bone

Big Picture

  • Proteins:

    • Comprise about 20% of the weight of lean tissues.

    • Most abundant proteins in the body are:

    • Actin-myosin: Found in skeletal muscle.

    • Collagen: Found in bone.

  • Bone:

    • The major structural compartment in the body.

    • Major elements in bone include:

    • Calcium

    • Phosphorus

Calcium and Bone Formation

  • Calcified Bone:

    • Formed by the combination of calcium and phosphate around collagen, which is the primary structural protein.

  • Calcium Balance:

    • Influences bone mass, strength, and the risk of fractures.

  • Vitamin D Deficiency:

    • Can lead to conditions such as rickets (in children) or osteomalacia (soft bone in adults) due to inadequate calcification of bone collagen.

Osteoporosis

  • Definition:

    • The most prevalent modern disease affecting bones.

    • Characterized by a reduction in bone mass (both collagen and calcium).

  • Prevalence:

    • Increases with age and is significantly more common in women.

  • Consequences:

    • Increased risk of bone fractures, including:

    • Vertebral Compression Fractures

    • Hip Fractures: Common and serious complication, especially in elderly women, potentially leading to death.

  • Notable Quote:

    • “We enter the world through the pelvis and leave it through the hip.” - Dr. Astley Cooper

Functions of Calcium in the Body

  • Nerve Conduction:

    • Involves calcium fluxes across cell membranes derived from body fluids.

  • Signaling Pathways:

    • Many intracellular signaling pathways depend on the uptake, release, or sequestration of calcium.

  • Protein Structure and Action:

    • Critical for processes such as blood clotting.

  • Muscle Contraction:

    • Essential for interactions between actin and myosin.

  • Blood Calcium Defense:

    • Blood calcium levels are tightly regulated despite various physiological challenges.

Calcium Homeostasis

Mechanisms of Regulation

  • Low Blood Calcium:

    • Stimulates the release of parathyroid hormone (PTH).

  • High Blood Calcium:

    • Reduces PTH secretion and increases calcitonin secretion.

  • Actions of PTH:

    • Stimulates:

    • Calcium release from bones.

    • Calcium retention by kidneys.

    • Calcium and phosphate absorption by the intestine to enhance dietary calcium and phosphate absorption.

    • Causes:

    • Loss of phosphate by the kidney.

    • Bone calcium loss to defend blood calcium levels.

  • Effects of Calcitonin:

    • Generally has opposite effects compared to PTH.

Vitamin D and Calcium Absorption

  • PTH and Vitamin D:

    • PTH also stimulates conversion of vitamin D to its active form, [1,25]-D.

  • Active Vitamin D:

    • Enhances calcium and phosphate absorption in the intestine, along with a variety of other functions.

  • Sources of Vitamin D:

    • Can be obtained from diet or synthesized via skin exposure to UV sunlight.

  • Deficiency Contexts:

    • More common in northern latitudes during winter, in shut-ins, in individuals with darker skin, or in those without dietary sources of vitamin D (e.g., fortified milk, oily fish).

  • Physician Awareness:

    • Consideration of vitamin D deficiency may be warranted in specific contexts including:

    • Winter months

    • Northern latitudes

    • Darker complexions

    • Nutritional compromise

    • Indoor confinement

Dietary Calcium and Its Absorption

  • Fractional Absorption:

    • Varies significantly based on the body's needs and hormonal environment.

    • Can range from <25% to 75% of dietary calcium absorbed, meaning that increasing dietary calcium could lead to diverse outcomes in calcium stores.

  • Recommended Daily Intake:

    • Adults (ages 19-50): approximately 1,000 mg/day.

    • Adolescents: 1,300 mg/day.

    • Adults over age 51: 1,200 mg/day.

  • Calcium Sources in Foods:

    • Dairy Products: ~300 mg/cup of milk.

    • Other sources: Beans and some green vegetables.

    • Co-factors impact absorption percentages.

  • Calcium Supplements:

    • Frequently prescribed to meet recommended intake for individuals at risk of osteoporosis or hypocalcemia.

Non-Dietary Factors Influencing Bone Mass

  • Sex Steroid Hormones:

    • Influential in bone mass regulation;

    • Testosterone and Estrogen: Promote bone mass increase.

    • Amenorrhea or Early Menopause: Can lead to reductions in bone mass.

    • Corticosteroids: Have a significantly decreasing effect on bone mass.

  • Weight Bearing Activities:

    • Stimulate bone deposition; individuals with higher body weight generally exhibit greater bone mass.

  • Paradox in Female Athletes:

    • Some experience osteopenia (loss of bone mass) despite engaging in weight-bearing activities, primarily due to amenorrhea and estrogen deficiency linked to low body weight.

  • Peak Bone Mass in Women:

    • Reaches its maximum in the 20s or 30s and subsequently declines.

  • Interventions for Bone Mass:

    • Can target increases in peak bone mass (e.g., ensuring adequate calcium intake during teenage years) or aim to slow subsequent bone loss (e.g., hormonal therapies for post-menopausal women).

Medical Consequences of Calcium and Phosphorus Levels

Hypercalcemia

  • Causes:

    • Diseases such as hyperparathyroid (e.g., endocrine tumors) or certain cancers, and hypervitaminosis D.

    • NOT due to excessive dietary calcium intake, as the body defends against it.

  • Consequences:

    • Severe fatigue, coma, and potential death.

Hypocalcemia

  • Causes:

    • Vitamin D deficiency, hypoparathyroidism, or kidney disease.

  • Consequences:

    • Muscle spasms and seizures.

Hyperphosphatemia

  • Causes:

    • Disease states such as hypervitaminosis D, hypoparathyroidism, or kidney disease; excessive phosphorus intake (e.g., through laxatives).

Hypophosphatemia

  • Causes:

    • Not typically due to dietary insufficiency; diseases include vitamin D deficiency, hyperparathyroidism, or phosphate-binding antacids.

  • Symptoms:

    • Muscle weakness and pain.

Summary of Calcium and Phosphate

Key Elements

  • Calcium (Ca):

    • Integral for numerous biochemical processes and essential for bone strength.

    • Sources mainly include dairy.

    • Calcium balances significantly influence bone mass and strength; imbalances are often disease-related rather than physiological fluctuations; closely defended by a complex regulatory system including Vitamin D, parathyroid hormone, kidney functionality, and others.

  • Phosphorus (P):

    • Also important for multiple biochemical and physiological processes and for bone integrity.

    • Defended by systems that are similar to those for calcium, albeit sometimes in opposing directions.