What causes bone calcium loss?

What Causes Bones to Lose Calcium

Bones lose calcium primarily through an imbalance between bone resorption and bone formation, driven by inadequate dietary calcium intake, hormonal changes (particularly estrogen loss at menopause and elevated parathyroid hormone), age-related impairments in calcium absorption, and increased urinary and dermal calcium losses.

Primary Mechanisms of Bone Calcium Loss

Inadequate Dietary Calcium Intake

• When dietary calcium intake falls below 700-800 mg/day, the skeleton becomes mobilized as a calcium source to maintain critical blood calcium levels, leading to progressive bone demineralization. 1
• The body prioritizes maintaining serum calcium concentration through tight hormonal regulation (parathyroid hormone, calcitonin, vitamin D), sacrificing skeletal calcium stores when intake is insufficient 1
• This process occurs because more than 98% of total body calcium resides in the skeleton, serving as both structural support and a reservoir that can be drawn upon during deficiency 2

Hormonal Dysregulation and Secondary Hyperparathyroidism

• Inadequate calcium intake triggers compensatory elevation of parathyroid hormone (PTH), which directly stimulates bone resorption to release calcium and phosphate into circulation. 1, 3
• Elderly women on usual calcium intake (815 mg/day) demonstrate 70% higher PTH levels and 30% higher bone resorption markers compared to younger women, whereas those consuming high calcium intake (2414 mg/day) normalize both parameters 4
• This secondary hyperparathyroidism develops as a direct consequence of the skeleton being mobilized when dietary intake fails to offset obligate calcium losses in urine, sweat, and stool 2, 4

Age-Related Impairments in Calcium Homeostasis

• Intestinal calcium absorption and the ability to adapt to low calcium diets become impaired in postmenopausal women and elderly persons due to decreased kidney production of active vitamin D (1,25-dihydroxyvitamin D). 5, 6
• Age-related changes include decreased physical activity, dietary inadequacies, and alterations in calcium-regulating hormones, with PTH increasing and active vitamin D metabolites decreasing 6
• These impairments can only be overcome by substantially increased calcium intakes of 1500-2500 mg/day in many postmenopausal and elderly individuals 5

Estrogen Deficiency at Menopause

• Perimenopausal bone loss occurs primarily due to estrogen loss, creating a net loss of skeletal calcium from imbalanced bone resorption and formation 1
• Women lose bone more rapidly around menopause and for approximately 10 years after, with trabecular bone particularly vulnerable 1, 5

Secondary Causes of Accelerated Calcium Loss

Corticosteroid-Induced Bone Loss

• Corticosteroids reduce calcium absorption from the gut and increase urinary calcium losses, leading to secondary hyperparathyroidism and subsequent bone resorption. 1
• All patients receiving corticosteroids should receive 800-1000 mg/day calcium and 800 IU/day vitamin D to prevent bone loss 1

Dermal and Urinary Calcium Losses

• Calcium is lost through sweat during prolonged exercise, reducing serum ionized calcium and triggering PTH elevation that stimulates bone resorption 1
• Significant dermal calcium losses occur alongside increased PTH following prolonged exercise, particularly in hot environments 1
• Urinary calcium losses contribute to obligate daily calcium requirements that must be replaced through dietary intake 2, 4

Chronic Kidney Disease

• In advanced CKD, phosphate retention, impaired calcitriol production, and hypocalcemia drive compensatory PTH elevation, accelerating high-turnover bone disease with excessive bone resorption. 3
• The hormonal cascade begins with elevated FGF-23, followed by decreased calcitriol, and ultimately elevated PTH, all triggered by progressive phosphate retention 3

Clinical Implications and Prevention

Critical Calcium Intake Thresholds

• Bone loss and fracture risk increase when calcium intake falls below 700-800 mg/day 1
• Recommended intake is 1000-1200 mg/day for adults, with higher requirements (1500 mg/day) for athletes and those at risk for bone loss 1, 7
• The upper safety limit is 2000-2500 mg/day total intake from all sources 2, 5

Common Pitfalls to Avoid

• Attempting to prevent bone loss through calcium supplementation alone without addressing estrogen deficiency in postmenopausal women is insufficient, as calcium cannot substitute for hormone replacement therapy. 5
• Calcium supplementation should always be combined with vitamin D (800 IU daily) for optimal bone protection, as combined supplementation reduces fracture risk more effectively than calcium alone 1, 7
• Dividing calcium doses to no more than 500-600 mg per dose optimizes absorption, as the gut cannot absorb more than 500 mg at once 7, 2
• Excessive dietary protein and fiber may induce negative calcium balance and increase dietary calcium requirements 5