From the Guidelines
The renal system plays a critical role in maintaining acid-base balance by regulating the excretion of excess acids or bases, reabsorbing bicarbonate, and generating new bicarbonate, with dietary patterns significantly influencing net endogenous acid production. The kidneys achieve acid-base homeostasis through several mechanisms, including the excretion of hydrogen ions into the urine and the reabsorption of bicarbonate into the bloodstream when blood pH decreases, as noted in a study on kidney disease and dietary patterns 1. Conversely, when blood pH increases, the kidneys excrete more bicarbonate and retain more hydrogen ions. This process involves specialized cells in the renal tubules, particularly in the proximal tubule where about 80% of filtered bicarbonate is reabsorbed.
The Western dietary pattern, characterized by high animal protein intake and low fruit and vegetable consumption, leads to an imbalance between nonvolatile acids and the alkali needed to buffer these acids, resulting in net endogenous acid production 1. This imbalance may lead to a chronic low-grade metabolic acidosis that worsens with age as kidney function declines. Key mechanisms by which reduced extracellular pH affects kidney function include altering NADPH oxidation, inhibiting nitric oxide production in mesangial cells, stimulating endothelial cell secretion of endothelin 1, and enhancing the activity of the sodium-hydrogen exchanger 3, all of which can accelerate kidney disease progression 1.
Some of the key points to consider in the renal system's role in acid-base balance include:
- The kidneys' ability to regulate acid-base homeostasis is influenced by dietary patterns, with high fruit and vegetable intake associated with lower net endogenous acid production 1.
- The production of ammonia by the kidneys acts as a buffer for excess hydrogen ions, forming ammonium that is excreted in urine.
- Dysfunction in renal mechanisms can lead to various acid-base disorders, including renal tubular acidosis.
- Treatment of high net endogenous acid production may involve sodium bicarbonate supplementation or increasing fruit and vegetable intake, with the latter yielding more benefits, as demonstrated by significant decreases in systolic blood pressure and total-body weight in addition to increased plasma bicarbonate levels and decreased net endogenous acid production 1.
Overall, the renal system's response to acid-base disturbances is relatively slow compared to respiratory compensation but provides a more powerful and sustained correction, highlighting the importance of considering dietary influences on acid-base balance in the management of kidney health 1.
From the Research
Role of the Renal System in Acid-Base Balance
The renal system plays a crucial role in maintaining acid-base balance in the body. This is achieved through several mechanisms, including:
- Reabsorption of bicarbonate filtered by the glomeruli 2, 3
- Excretion of titratable acids and ammonia into the urine 2, 3
- Regulation of acid transport processes in the surviving nephrons 3
Mechanisms of Acid-Base Regulation
In individuals with chronic kidney disease (CKD), the kidneys' ability to regulate acid-base balance is impaired, leading to acid retention and metabolic acidosis 2, 3. The extent of acid retention depends on the degree of kidney impairment and the dietary acid load 3.
- Urinary ammonia levels are used to gauge the renal response to acid loads 3
- Acid-base disorders lead to changes in mechanisms involved in acid-base balance maintenance, as well as other mechanisms tightly wired to it 4
Treatment and Management
Treatment of metabolic acidosis in CKD patients typically involves oral bicarbonate supplementation to maintain serum bicarbonate levels within the normal range 2. However, the target therapeutic range of serum bicarbonate and the drugs used for metabolic acidosis require further research 2.
- Other treatment options include increased fruit and vegetable intake and oral alkali supplementation 2
- Veverimer, a non-absorbed, counterion-free, polymeric drug, is emerging as a potential treatment for metabolic acidosis, but further research is needed 2
Clinical Implications
Abnormalities in blood bicarbonate concentration are common in patients with CKD, especially at the end-stage renal failure 5. Initiating hemodialysis does not completely solve this problem, and new guidelines are needed to correct acid-base disorders in hemodialysis patients 5.