Disorders of Calcium Homeostasis and Vitamin D Metabolism

Chemical Forms of Vitamin D and Hormonal Regulation

• Vitamin D Nomenclature:   - Forms: Known as calcitriol or calcidiol.   - Sources: $D_2$ is the plant version; $D_3$ is the animal version.   - States: $25$-hydroxy vitamin D is the storage form. $1,25$-dihydroxy vitamin D is the active form.

• Hormonal Interactions:   - Parathyroid Hormone (PTH): Activates enzymes to convert Vitamin D into its active form; hypersecretion raises blood calcium, while hyposecretion leads to deficiency.   - Calcitonin: Produced by parafollicular cells of the thyroid; deposits bone and lowers blood calcium levels.   - Vitamin D Function: Remodels bone to raise blood calcium levels in the short term, though it is essential for long-term bone health.

Synthesis and Physiological Action of Vitamin D

• Cutaneous Synthesis: UVB radiation ($285$ to $305\,\text{nm}$) strikes $7$-dehydrocholesterol in the skin to create pre-vitamin D.

• Metabolic Activation:   - Liver: Adds an enzyme to create $25$-hydroxy vitamin D.   - Kidney: Converts it to the active $1,25$-dihydroxy vitamin D form, stimulated by PTH.

• Target Organs:   - Kidney: Reabsorbs more calcium via tubules.   - Intestines: Activates genes for calcium uptake in the duodenum and ileocecal valve.   - Bone: Facilitates the release of calcium into the blood.

Skeletal Disorders and Calcium Homeostasis

• Osteomalacia: Inadequate deposition of bone affecting the osteoid and hydroxyapatites.

• Osteoporosis:   - Primary: Often caused by menopause due to dropping estrogen levels from the ovaries.   - Secondary: Caused by external factors like radiation therapy or muscle disorders.

• Rickets: Occurs in children; characterized by soft, bendy bones due to inadequate calcification from low Vitamin D or calcium.

• Paget's Disease: Characterized by irregular bone deposition, related to hormone imbalances.

Risk Factors for Vitamin D Deficiency

• Dietary Limitations: Naturally occurring Vitamin D is rare in food; fatty fish like salmon are primary sources. Milk usually requires massive quantities ($gallons$) to meet recommended allowances.

• Environmental Factors:   - Sun Exposure: Limited by clothing, indoor work, and glass (which filters UVB).   - Geography: High latitudes (> 30 degrees) and low altitudes reduce UVB penetration.

• Biological Factors:   - Obesity (High BMI): Vitamin D is lipid-soluble and gets sequestered in fat tissue, reducing blood availability.   - Melanin: Darker skin acts as a natural sunscreen, requiring longer sun exposure to synthesize Vitamin D.

Case Study: Vitamin D Synthesis in Los Angeles

• Los Angeles Vulnerability: Despite being sunny, the zenith angle of the sun prevents Vitamin D synthesis for most of the year.

• Synthesis Window: Effective synthesis only occurs from mid-May to early September.

• Clinical Observations: High prevalence of deficiency found in students and children (rickets) unless supplementing.

• Serum Levels:   - Deficient: < 20.   - Insufficient: $30$.   - Ideal: Over $60$ to $80$.

• Cancer Prevention: Vitamin D arrests cell division in the $G_0$ phase, providing protective benefits against cancer cell cycles.

Questions & Discussion

• Q: What is the risk of developing rickets or osteoporosis in Los Angeles?

• A: The risk is surprisingly high (enormous). Factors include high BMI, high latitude (> 30 degrees), sea-level altitude, and the fact that synthesis is only possible for a few months per year. Darker skin tones and lack of dietary supplementation further increase this risk.

• Guideline for Sun Exposure: Exposing $15\%$ of skin for $20$ minutes daily during ideal months without burning is recommended.