How do I calculate a renal diet for a 70‑kg, 1.75‑m tall male with stage 4 chronic kidney disease, serum potassium 5.2 mmol/L, serum phosphate 1.5 mmol/L, and urine output 500 mL?

Calculating a Renal Diet for Stage 4 CKD

For this 70-kg male with stage 4 CKD, hyperkalemia (5.2 mmol/L), and elevated phosphate (1.5 mmol/L = 4.6 mg/dL), prescribe 0.8 g/kg/day protein (56 g/day), 30-35 kcal/kg/day energy (2100-2450 kcal/day), restrict sodium to <2 g/day (<90 mmol/day), limit potassium to <2-3 g/day (<50-75 mmol/day), and restrict phosphorus to 0.8-1 g/day (800-1000 mg/day). 1, 2

Protein Calculation

• Target: 0.8 g/kg body weight/day = 56 g protein/day for this 70-kg patient with stage 4 CKD (eGFR <30 mL/min/1.73m²) not on dialysis 1
• At least 50-60% should be high biological value protein (animal sources: eggs, fish, poultry, lean meat) to ensure adequate essential amino acid intake 3
• Do not exceed 1.3 g/kg/day (91 g/day) as high protein intake accelerates CKD progression 1
• This patient is metabolically stable (not hospitalized for acute illness), so the 0.8 g/kg target is appropriate; if he were acutely ill or catabolic, protein needs would increase to 1.2-1.5 g/kg/day 1

Energy Calculation

• Target: 30-35 kcal/kg/day = 2100-2450 kcal/day to maintain neutral nitrogen balance and prevent protein-energy wasting 3, 4
• Adequate energy intake is critical to prevent the body from catabolizing dietary and endogenous protein for energy 4
• Use actual body weight for calculation unless the patient has significant fluid overload or obesity 1

Sodium Restriction

• Target: <2 g sodium/day (<90 mmol/day or <5 g salt/day) to optimize blood pressure control and reduce fluid retention 1
• This translates to approximately 2-5 g sodium chloride (table salt) per day depending on blood pressure and volume status 3, 4
• Avoid processed foods, canned goods, deli meats, and restaurant meals which contain high sodium 2
• Critical pitfall: Do not use potassium-based salt substitutes in this patient with hyperkalemia (K+ 5.2 mmol/L) 1, 2

Potassium Restriction

• Target: <2-3 g/day (<50-75 mmol/day) given serum potassium of 5.2 mmol/L (hyperkalemia) 2, 1
• For a 70-kg adult, this approximates 30-40 mg/kg/day 1
• Eliminate high-potassium foods: bananas, oranges, potatoes, tomato products, legumes, yogurt, chocolate, dried fruits 1, 2
• Eliminate all salt substitutes immediately as they contain potassium chloride 1, 2
• Pre-soaking root vegetables (especially potatoes) for several hours effectively lowers potassium content by 50-75% 1
• Choose foods with <100 mg potassium per serving or <3% daily value on nutrition labels 1
• Recheck potassium within 72 hours to 1 week after initiating dietary restriction 2

Phosphorus Restriction

• Target: 0.8-1 g/day (800-1000 mg/day) given serum phosphate of 1.5 mmol/L (4.6 mg/dL), which is at the upper limit of the target range (2.7-4.6 mg/dL) for stage 4 CKD 1, 2, 5
• Practical implementation: 1 serving animal protein/day, 1 serving dairy/day, moderate bread/cereals/pasta, vegetables/fruits in moderation 2
• Limit high-phosphorus foods: dairy products, processed meats, cola beverages, nuts, beans 2
• Work with a renal dietitian for individualized phosphorus counseling 1, 2
• Consider calcium-based phosphate binders if dietary restriction alone is insufficient, not exceeding 1.5 g elemental calcium daily 2, 5
• Monitor serum phosphate every 3-6 months and PTH every 6-12 months 5, 2

Fluid Management

• With urine output of 500 mL/day: Restrict total fluid intake to approximately 1000-1300 mL/day (500 mL urine output + 500-800 mL for insensible losses) 6
• Adjust based on presence of edema, blood pressure, and weight changes 6
• This patient's oliguria (500 mL/day) indicates advanced kidney dysfunction requiring strict fluid monitoring 6

Micronutrients and Monitoring

• Check PTH levels as elevated PTH may warrant stricter dietary phosphorus restriction even when serum phosphorus is within normal range 5, 7
• Monitor serum bicarbonate as metabolic acidosis worsens hyperkalemia risk; consider treatment if <18 mmol/L 2
• Ensure total calcium intake (dietary + binders) does not exceed 2000 mg/day given calcium level approaching upper limit 5
• Monitor bone-specific alkaline phosphatase to assess bone metabolism 5

Critical Medication Review

• Discontinue NSAIDs and COX-2 inhibitors immediately as these worsen renal function and dramatically increase hyperkalemia risk 2
• Eliminate all potassium supplements 2
• Do not reflexively discontinue RAAS inhibitors (ACE inhibitors/ARBs) as they slow CKD progression, but check potassium within 1-2 weeks after any dose adjustment 2
• Consider patiromer as potassium binder if dietary restriction insufficient, as it may simultaneously lower both potassium and phosphorus 2

Common Pitfalls to Avoid

• Do not confuse this with primary hyperparathyroidism: Stage 4 CKD causes secondary hyperparathyroidism with low-normal calcium and high phosphate, whereas primary hyperparathyroidism presents with high calcium and low phosphate 7
• Do not use chronic sodium polystyrene sulfonate (Kayexalate) for hyperkalemia due to bowel necrosis risk 2
• Do not normalize PTH to "normal range" (<65 pg/mL) as this can cause adynamic bone disease 7
• Do not restrict protein to delay dialysis initiation if the patient becomes catabolic or acutely ill 1
• Phosphate retention begins early in CKD, but hyperphosphatemia only becomes evident at stage 4 when GFR falls below 20-30 mL/min/1.73m² 7