Chronic kidney disease (CKD) is the progressive and irreversible loss of kidney function. In Italy, it affects approximately 5 million people, with 50,000 on dialysis. Kidney function is measured by glomerular filtration rate (GFR, eGFR): five stages are classified from G1 (eGFR > 90 ml/min: normal) to G5 (eGFR < 15 ml/min: end-stage renal disease). Diet is one of the cornerstones of CKD treatment at all stages, with different objectives for each phase: slowing progression toward end-stage renal disease (by reducing kidney workload through protein restriction), preventing metabolic complications (hyperkalemia, hyperphosphatemia, metabolic acidosis, dyslipidemia), and maintaining adequate nutritional status (malnutrition risk increases as CKD progresses).

Protein restriction: the fundamental principle

Restricting protein intake is the cornerstone of renal nutrition in stages G3-G5 (not on dialysis). The rationale: proteins are metabolized producing urea and other nitrogen-containing products (creatinine, uric acid) that must be eliminated by the kidneys. Reducing protein load decreases production of these uremic solutes, relieves residual kidney workload, and may slow CKD progression. The recommended protein intake according to 2024 KDIGO guidelines: CKD G3-G4 without dialysis: 0.6–0.8 g/kg/day of protein (vs. 1.0–1.2 g/kg/day for the general population). CKD G5 (pre-dialysis/conservative): further reduction possible to 0.3–0.5 g/kg/day with essential amino acid supplementation (keto-analogs: Ketosteril). Dialysis (hemodialysis and peritoneal dialysis): protein intake must be INCREASED (1.2–1.5 g/kg/day) because dialysis removes amino acids from the blood. In dialysis, protein restriction is counterproductive. The importance of protein quality: with severe protein restriction (below 0.6 g/kg/day), it is essential that consumed proteins have high biological value (complete essential amino acids): eggs, fish, white meat, dairy products (in quantities compatible with phosphorus restriction). Keto-analogs (Ketosteril): substitutes for essential amino acids with keto-acids that don't produce urea during metabolism. They allow ultra-restricted protein diets (0.3–0.4 g/kg/day of actual protein) without malnutrition, significantly slowing CKD progression.

Potassium restriction: which foods to limit

Hyperkalemia (elevated plasma potassium: above 5.5 mEq/L) is a serious CKD complication because it can cause potentially fatal cardiac arrhythmias. Diseased kidneys cannot adequately eliminate potassium. High-potassium foods (to limit or avoid): fruits: banana (358 mg K/100 g), kiwi (312 mg), dried apricot (1160 mg!), orange (181 mg), avocado (485 mg). Vegetables: potatoes (379 mg), tomatoes (237 mg), spinach (558 mg), dried mushrooms (very high). Legumes: beans (452 mg), lentils (369 mg). Nuts (very high). Dark chocolate. Coffee and fruit juices. How to reduce potassium in vegetables (multiple boiling): cut vegetables into small pieces, soak in cold water for at least 1–2 hours (change water twice), cook in abundant water (1:10 vegetable-to-water ratio), drain and discard cooking water (don't reuse it for soups: it contains extracted potassium). This process reduces vegetable potassium by 30–50%. It's not sufficient for all patients with severe hyperkalemia but helps those with mild-to-moderate CKD. Low-potassium foods (preferable): rice, pasta, bread, apple, pear, canned pineapple (rinsed), boiled green beans.

Phosphorus restriction: the hidden danger of additives

Hyperphosphatemia is a frequent complication of advanced CKD that contributes to cardiovascular calcification (the leading cause of death in kidney patients). Renal diet limits phosphorus to 800–1,000 mg/day (vs. 1,200–1,500 mg/day in a typical Italian diet). Phosphorus-rich foods to limit: dairy products (cheese in particular: 400–700 mg P/100 g), meat and fish (200–250 mg P/100 g), legumes, whole grains, nuts, egg yolks. The major hidden danger: inorganic phosphate additives. Inorganic phosphates (E338–E343: phosphoric acid and phosphate salts) are added as additives to: carbonated beverages (especially cola: phosphoric acid), industrial baked goods (commercial bread, cookies, snacks), processed cheese (cheese slices, cheese spreads), processed meats (ham, sausages, deli meats), breakfast cereals, instant soups, snacks. Inorganic phosphates are absorbed almost completely (90–100%) by the intestine, compared to 40–60% of organic phosphates from natural proteins. Phosphate from additives is far more dangerous for kidney patients than phosphate from whole, unprocessed foods. The practical lesson: in renal diet, ultra-processed products (which often contain phosphate additives) must be avoided more than natural phosphorus-rich products.

Sodium restriction: hypertension and fluid retention

Sodium reduction is recommended at all CKD stages to control blood pressure and fluid retention. Target: less than 2 g of sodium per day (equivalent to less than 5 g of table salt). The main problem is hidden sodium: bread and baked goods (Italian bread contains an average of 1.5–2 g of salt per 100 g), processed meats and deli products (ham: 800–1,200 mg sodium/100 g), aged cheeses, canned and preserved foods, sauces and condiments (soy sauce, ketchup, bouillon cubes), ready-made and frozen meals. How to reduce sodium without sacrificing flavor: use herbs and spices (basil, rosemary, turmeric, ginger, pepper) to season without salt, quality extra-virgin olive oil to enhance flavor, balsamic vinegar and lemon juice as salt alternatives on salads, prepare condiments at home (avoid commercial sauces), choose salt-free bread (Tuscan bread, traditional Apulian bread: naturally salt-free). Salt substitutes (potassium chloride, "diet salt"): absolutely avoid in CKD because it's pure potassium. It can cause fatal hyperkalemia.

Calcium and vitamin D in CKD

Calcium and vitamin D metabolism is profoundly altered in CKD. The kidneys produce the active form of vitamin D (calcitriol, 1,25-dihydroxycholecalciferol): as CKD progresses, calcitriol production decreases, causing secondary hyperparathyroidism (parathyroid hormone rises to compensate for reduced calcium), bone mass loss (renal osteodystrophy), hypocalcemia (low serum calcium). Treatment: calcitriol supplementation or analogs (Alfa-D3, Rocaltrol: prescribed by the nephrologist), calcium-based phosphate binders (calcium carbonate: bind phosphorus in the intestine before absorption, reducing hyperphosphatemia). Caution: at certain CKD stages, excess calcium (from supplementation and calcium binders) can contribute to cardiovascular calcification. The nephrologist must carefully balance calcium and phosphorus. Monitoring: calcium, phosphorus, parathyroid hormone (PTH), 25-OH vitamin D: measured regularly every 3–6 months in CKD G3-G5. Mineral metabolism abnormalities must be corrected promptly to prevent renal bone disease and reduce cardiovascular risk.

Diet in dialysis: differences from non-dialytic CKD

With the start of dialysis, some dietary restrictions change or reverse. Protein: as already noted, in dialysis protein intake must be increased (no longer restriction but high target: 1.2–1.5 g/kg/day). Potassium: dialysis removes potassium, but between sessions potassium accumulates again. Potassium restriction remains (especially between sessions: the 48–72 hours between dialysis in three-times-weekly hemodialysis). In continuous peritoneal dialysis, dialysis is daily and restriction is less severe. Phosphorus: restriction remains, in fact it's more important in dialysis because dialysis doesn't remove phosphorus efficiently. Phosphate binders (sevelamer, lanthanum, calcium carbonate) must be taken with every meal. Fluids: in hemodialysis, between sessions fluids accumulate causing edema and hypertension. Fluid restriction (often 1–1.5 liters of total liquids per day) is essential. This includes all water contained in foods (fruits, vegetables, soups). Calories: dialysis patients often struggle to reach adequate caloric intake (at least 30–35 kcal/kg/day). Malnutrition is very common in dialysis and worsens prognosis. Oral nutritional supplements specific for dialysis (low phosphorus and low potassium) are often prescribed.