The ketogenic diet, commonly known as keto, is an eating plan characterized by very low carbohydrate intake, moderate protein consumption, and high fat consumption. This dietary approach aims to shift the body’s metabolism into a state called ketosis, where it primarily burns fat for fuel instead of carbohydrates. While the ketogenic diet has gained popularity for various reasons, including weight management, some individuals express concern about a potential association between this diet and the formation of kidney stones. This article explores the physiological mechanisms and dietary habits that might explain this connection.
Metabolic Foundations of Stone Risk
Entering ketosis involves metabolic adjustments that can alter urine composition, increasing kidney stone risk. One primary change is a reduction in urinary pH, making urine more acidic. As the body breaks down fat, it produces acidic ketone bodies for kidney excretion. This acid load favors crystallization of stone-forming substances. Urine pH on a keto diet can decrease, with average values around 5.86 compared to 6.37 in control groups.
The ketogenic diet can decrease urinary citrate excretion, a natural stone inhibitor. Citrate binds with calcium in urine, preventing crystal formation. Lower citrate levels mean more free calcium is available to form stones. Chronic metabolic acidosis, often induced by ketosis, increases citrate reabsorption in kidney tubules, reducing its urinary presence.
The diet influences urinary calcium levels. Metabolic acidosis can decrease kidney calcium reabsorption, leading to increased urinary calcium excretion. This heightened urinary calcium (hypercalciuria) elevates stone formation risk. The body may also draw calcium from bones to buffer acid, further increasing urinary calcium.
Urine volume is another factor. Inadequate fluid intake, a challenge on keto, leads to more concentrated urine. Highly concentrated urine means stone-forming minerals are in higher saturation, making them more likely to precipitate. Frequent urination in early ketosis, as the body sheds water, can also contribute to dehydration if not counteracted by increased fluid intake.
Common Stone Types in Ketogenic Diets
The metabolic changes induced by a ketogenic diet predispose individuals to certain types of kidney stones. Uric acid stones are notably prevalent among those following this diet. The acidic urine environment created by ketosis reduces uric acid solubility, making it easier for crystals to form. Nearly half of kidney stones reported on ketogenic diets are uric acid stones.
Calcium-based stones, particularly calcium oxalate stones, are also commonly observed. While acidic urine directly promotes uric acid stone formation, the increased urinary calcium and reduced citrate levels associated with ketosis can facilitate the crystallization of calcium oxalate. Even though uric acid stones are more frequent on a ketogenic diet compared to the general population, calcium-based stones still represent a significant portion of reported cases. These stones can sometimes form as mixed uric acid and calcium-based compositions.
Dietary and Lifestyle Factors
Beyond metabolic effects, certain dietary and lifestyle choices on a ketogenic diet can increase kidney stone risk. Inadequate fluid intake is a concern. Maintaining optimal hydration by drinking sufficient water is a primary preventive measure.
High animal protein intake, often a characteristic of ketogenic diets, can contribute to stone risk. Excessive animal protein increases the body’s acid load, lowering urine pH and increasing urinary calcium excretion, favoring stone formation. While protein is necessary, an imbalanced intake heavily skewed towards animal sources can exacerbate these issues.
Reduced fruit and vegetable consumption is another factor. Many ketogenic diets severely restrict these food groups due to their carbohydrate content. Fruits and vegetables are natural sources of citrate and compounds that help alkalinize urine and inhibit stone formation. Their absence can lead to lower urinary citrate and a more acidic urine environment.
Sodium intake can play a role. Some ketogenic diets emphasize high-sodium foods like processed meats and cheeses. High dietary sodium can increase urinary calcium excretion, raising the risk for calcium-based stones. Monitoring and moderating sodium intake is a consideration for stone prevention.