Phosphorus: Characteristics and Clinical Implications
Phosphorus is primarily an intracellular ion, not a major extracellular anion, and its deficiency is common in hospitalized patients, can contribute to insulin resistance, and is actively (not passively) absorbed from the gastrointestinal tract. 1
Phosphorus Distribution and Absorption
- Approximately 85% of phosphorus is found in bone, with the remainder distributed in soft tissues and only a very small fraction in extracellular fluid 1
- Phosphorus absorption from the gastrointestinal tract is an actively regulated process (not passive):
- Regulated by vitamin D, parathyroid hormone, and other factors
- Approximately 40-60% of animal-based phosphate is absorbed
- Plant-based phosphate is less absorbable (20-50%) 1
Phosphorus Deficiency in Hospitalized Patients
- Severe hypophosphatemia is common in hospitalized patients, not rare
- In a study of hospitalized patients, severe hypophosphatemia was found in 120 patients over a 16-month period 2
- Common causes include:
- Postoperative status (42.5% of cases)
- Medication effects (82% of cases), particularly from intravenous glucose, antacids, diuretics, and steroids
- Gram-negative septicemia (second most common cause) 2
- Mortality rates associated with severe hypophosphatemia were significant:
- 20% mortality with phosphorus levels between 1.1-1.5 mg/dL
- 30% mortality with phosphorus levels ≤1.0 mg/dL 2
Phosphorus Deficiency and Metabolic Effects
- Hypophosphatemia can lead to various metabolic disturbances, including insulin resistance
- In critically ill patients, hypophosphatemia is associated with:
- Worsening respiratory failure
- Increased risk of prolonged mechanical ventilation
- Cardiac arrhythmias
- Prolonged hospitalization 1
- Phosphorus deficiency can disrupt normal cellular metabolism, affecting insulin sensitivity and glucose handling 3
Clinical Implications and Management
- Both hypophosphatemia (<3.5 mg/dL) and hyperphosphatemia (>5.5 mg/dL) are linked to adverse outcomes in critical care settings 1
- Regular monitoring of phosphorus levels is recommended:
- Every 3-6 months for CKD Stage 3-4
- Monthly for CKD Stage 5
- Multiple times daily for patients on continuous kidney replacement therapy 1
- Oral phosphate supplements in combination with calcitriol are the mainstay of treatment for chronic phosphate wasting 1
Common Pitfalls and Caveats
- Phosphorus intake is systematically underestimated in national surveys 4
- Dietary phosphorus intake has been increasing in the US population, with grains being the largest source, followed by meats and milk products 1, 5
- Excessive phosphorus intake relative to calcium can disrupt hormonal regulation of phosphate, calcium, and vitamin D, potentially contributing to:
- Disordered mineral metabolism
- Vascular calcification
- Impaired kidney function
- Bone loss 4
- Even mild elevations of serum phosphate within the normal range are associated with cardiovascular disease risk in healthy populations 4