What causes metabolic acidosis in patients with a history of diabetes or kidney disease?

Metabolic Acidosis: Causes in Patients with Diabetes and Kidney Disease

Primary Mechanisms in Chronic Kidney Disease

In patients with CKD, metabolic acidosis develops primarily because the kidneys lose their ability to excrete hydrogen ions and synthesize ammonia, leading to progressive acid accumulation in the body. 1, 2

The pathophysiology follows a clear progression:

• Impaired renal acid excretion becomes clinically significant when GFR decreases below 20-25% of normal, with the majority of CKD patients developing metabolic acidosis at this threshold 3
• Reduced ammonia synthesis by damaged nephrons limits the kidney's primary mechanism for buffering and eliminating daily acid production 1
• Decreased bicarbonate reabsorption and generation occurs as functional nephron mass declines 4

The severity typically correlates with renal function—plasma bicarbonate concentrations usually range from 12-22 mmol/L in advanced CKD, though values below 12 mmol/L are rare without an additional acid load 3

Dietary Contributions

Western dietary patterns significantly contribute to metabolic acidosis in CKD patients through an imbalance between acid-producing and alkali-providing foods. 1

Specific dietary mechanisms include:

• Animal proteins contain sulfur-containing amino acids (methionine, cysteine) that generate nonvolatile acids during metabolism, increasing the daily acid burden 1
• Low fruit and vegetable intake reduces available potassium citrate salts that normally generate alkali to buffer acids 1
• High cereal and grain consumption combined with low plant-based foods creates net endogenous acid production that overwhelms impaired renal compensatory mechanisms 1

Diabetes-Specific Causes

Diabetic Ketoacidosis

In diabetic patients, DKA represents an acute, severe form of metabolic acidosis caused by absolute or relative insulin deficiency coupled with elevated counterregulatory hormones. 5

The pathophysiologic cascade includes:

• Insulin deficiency combined with elevated glucagon, catecholamines, cortisol, and growth hormone leads to unrestrained lipolysis 5
• Hepatic fatty acid oxidation produces excessive ketone bodies (β-hydroxybutyrate and acetoacetate), causing ketonemia and metabolic acidosis 5
• Osmotic diuresis from hyperglycemia causes loss of water, sodium, potassium, and other electrolytes, worsening the metabolic derangement 5

DKA severity is classified by bicarbonate levels: mild (≥15 mmol/L), moderate (<10 mmol/L), and severe (variable definitions) 5, 6

Diabetes with Concurrent CKD

Interestingly, diabetic patients with advanced renal failure demonstrate less severe metabolic acidosis compared to non-diabetic CKD patients at equivalent levels of renal function. 7

• Diabetic nephropathy patients show mean bicarbonate levels of 20.7 mmol/L versus 18.2 mmol/L in non-diabetic CKD patients with similar GFR 7
• This difference may reflect more efficient extrarenal bicarbonate generation in diabetic patients, though the exact mechanism remains unclear 7
• The anion gap is also significantly lower in diabetic versus non-diabetic CKD patients (19.70 vs 22.35 mmol/L) 7

Drug-Induced Metabolic Acidosis

Metformin-Associated Lactic Acidosis

Metformin accumulation in CKD creates risk for lactic acidosis, though this complication remains rare even with moderate renal impairment. 5, 8

Key mechanistic points:

• Metformin is excreted unchanged by the kidneys, and impaired renal function prolongs its half-life and reduces clearance 5, 8
• Metformin decreases hepatic lactate uptake, increasing blood lactate levels and potentially triggering lactic acidosis when toxic levels accumulate 8
• Most cases occur with concurrent acute illness, particularly when acute kidney injury further reduces metformin clearance 5

The FDA revised safety guidelines now permit metformin use down to eGFR ≥30 mL/min/1.73 m², with dose reduction to 1000 mg daily for eGFR 30-44 mL/min/1.73 m² 5

Other Medications

Multiple pharmaceutical agents can induce metabolic acidosis through various mechanisms including impaired renal acid excretion, increased acid production, or bicarbonate loss. 9

Common culprits include:

• Carbonic anhydrase inhibitors (topiramate, acetazolamide) promote bicarbonate loss and can cause metabolic acidosis, with topiramate increasing metformin AUC by 25% 5, 9
• Antiretroviral therapy may cause life-threatening lactic acidosis 9
• Certain antibiotics like fosfomycin may contribute to 5-oxoproline accumulation, particularly in patients with sepsis and renal impairment 10

Additional Precipitating Factors

Acute Illness and Hypoperfusion States

Infection, sepsis, and conditions causing tissue hypoperfusion represent common precipitating factors for metabolic acidosis in both diabetic and CKD patients. 5

• Infection is the most common precipitating factor for both DKA and metabolic decompensation in CKD 5
• Hypoxic states including acute heart failure, cardiovascular collapse, myocardial infarction, and sepsis cause lactic acidosis and may precipitate prerenal azotemia 5, 8
• Dehydration and volume depletion from vomiting, diarrhea, or inadequate intake worsen renal perfusion and acid excretion 5

Complex Metabolic Scenarios

Multiple simultaneous causes frequently coexist, particularly in critically ill patients with diabetes and CKD. 10

A case report illustrates this complexity: a diabetic patient with prior gastric bypass surgery, urosepsis, and chronic paracetamol use developed severe HAGMA from combined elevations of L-lactate, D-lactate, ketones, 5-oxoproline, and acute renal failure 10

Clinical Pitfalls to Avoid

Several common errors can delay diagnosis or worsen outcomes:

• Assuming single etiology: Always calculate the anion gap and consider multiple simultaneous causes, particularly in diabetic CKD patients who may have ketoacidosis, lactic acidosis, and uremic acidosis concurrently 4, 10
• Overlooking medication contributions: Review all medications including over-the-counter agents (paracetamol) and supplements that may contribute to acidosis 9, 10
• Misinterpreting compensatory responses: Elevated bicarbonate in a diabetic patient may represent compensation for chronic respiratory acidosis rather than metabolic alkalosis—obtain arterial blood gas if uncertain 1
• Continuing metformin inappropriately: Metformin should be discontinued during acute illness with hypoperfusion risk, before contrast procedures in at-risk patients, and when eGFR falls below 30 mL/min/1.73 m² 5, 8