Table of Contents
- 1. The Chemistry of Crystallization
- 2. Pathophysiology: Supersaturation
- 3. Types of Stones and Their Causes
- 4. The Critical Role of Hydration
- 5. Sodium: The Great Hidden Villain
- 6. Oxalate and Calcium: The Milk Myth
- 7. Animal Protein and Urinary pH
- 8. Citrate: The Natural Inhibitor
- 9. Conclusion
- Selected References
The 2-Liter Rule
The most effective prevention against kidney stones isn't just drinking water, but ensuring a urinary volume of at least 2 to 2.5 liters per day. This dilutes lithogenic solutes (like calcium and oxalate) below their saturation point, physically preventing crystals from clumping together and forming stones.
1. Introduction: A Memorable Pain
Nephrolithiasis, popularly known as kidney stones, is one of the most painful conditions described in medicine, often compared to childbirth. It affects about 10-15% of the world's population at some point in their lives, with an alarming recurrence rate: without preventive intervention, 50% of patients will have a new episode within 5 to 10 years.
More than an isolated event, stone formation is a symptom of metabolic and nutritional imbalance. The modern epidemic of lithiasis is closely linked to the Western diet, rich in salt, animal protein, and poor in fruits and vegetables (sources of citrate and water).
2. Pathophysiology: The Theory of Supersaturation
Urine is a complex solution containing dissolved mineral salts. Stone formation follows the laws of physical chemistry:
- Supersaturation: When the concentration of salts (such as calcium and oxalate) exceeds the solubility capacity of urine, they precipitate and form microscopic crystals.
- Nucleation: Crystals clump together on a protein matrix or cellular debris (Randall's Plaque).
- Growth and Aggregation: If not excreted, these crystals grow until they become macroscopic stones.
The body has natural inhibitors of this process, such as Citrate and Magnesium, which bind to calcium and prevent it from crystallizing. Lithiasis occurs when there is an imbalance between promoters (excess solutes) and inhibitors (lack of citrate/water).
3. Types of Stones and Metabolic Profile
Not all stones are the same. Dietary treatment strictly depends on the chemical composition of the stone:
| Stone Type | Prevalence | Main Dietary/Metabolic Causes |
|---|---|---|
| Calcium Oxalate | 70-80% | Low fluid intake, excess sodium, excess oxalate, lack of calcium in the diet. |
| Calcium Phosphate | 10-15% | Alkaline urine, hyperparathyroidism, renal tubular acidosis. |
| Uric Acid | 5-10% | Acidic urine (pH < 5.5), excess purines (red meat, seafood), metabolic syndrome. |
| Struvite (Infectious) | < 5% | Urinary tract infections by urease-producing bacteria. Requires antibiotic and surgical treatment. |
4. The Critical Role of Hydration
Water is the most potent "medicine" against lithiasis. Urinary dilution reduces the concentration of all stone-forming salts.
- Goal: Urinate clear (color of diluted lemonade) throughout the day.
- Strategy: Drink water distributed evenly over 24 hours. Drinking a glass of water before bed and upon waking to urinate at night is crucial, as nocturnal urine is the most concentrated.
- Citrus Drinks: Lemonade and orange juice are excellent as they provide extra citrate. Avoid cola-based sodas (rich in phosphoric acid) and sugary industrial juices.
5. Sodium: The Great Hidden Villain
Salt (sodium chloride) is the biggest enemy of those with calcium stones. The mechanism is renal: sodium and calcium compete for reabsorption in the renal tubules.
The recommendation is to limit sodium to < 2,300 mg/day (approx. 1 teaspoon of salt), avoiding processed meats, canned goods, ready-made seasonings, and fast food.
6. The Calcium Myth: Don't Cut the Milk!
A common and dangerous mistake is restricting dietary calcium. Epidemiological studies (such as Curhan et al.) have proven that diets low in calcium INCREASE the risk of stones.
Explanation: Oxalate (present in spinach, nuts, chocolate, beets) is absorbed in the intestine. If there is calcium in the meal, they bind in the intestine and are eliminated in the feces. If there is no calcium, oxalate remains free, is absorbed, goes into the blood, and is excreted by the kidneys, where it meets urinary calcium and forms stones.
Recommendation: Consume the normal amount of calcium (1,000-1,200 mg/day) through food, preferably along with oxalate-rich meals.
7. Animal Protein and Acid Load
Excess animal protein (red meat, chicken, fish, eggs) generates a metabolic acid load (sulfates). To buffer this acidity, the body:
- Withdraws calcium from bones (increasing calciuria).
- Reduces citrate excretion (removing natural protection).
- Acidifies urine (favoring uric acid stones).
Stone-forming patients should moderate animal protein intake to 0.8 - 1.0 g/kg of body weight and increase intake of fruits and vegetables, which are alkalizing.
8. Citrate and Potassium: The Protectors
Citrate is the most important inhibitor of calcium crystallization. It binds to calcium in the urine, forming a soluble complex that does not precipitate. Hypocitraturia (low citrate) is found in up to 60% of stone formers.
Citrus fruits (lemon, orange, melon) and vegetables are rich in citrate and potassium. Potassium also helps reduce urinary calcium excretion. In severe cases, pharmacological supplementation with Potassium Citrate is indicated.
9. Conclusion
Prevention of renal lithiasis is precise nutritional science. There is no single "kidney stone diet"; it must be personalized based on 24h urine metabolic analysis and stone type. However, the universal pillars of hyper-hydration, sodium restriction, normal calcium consumption, and a diet rich in vegetables (citrate) constitute the foundation for keeping kidneys stone-free.