Which supplements can cause a low serum bicarbonate (total CO₂) level?

Supplements That Can Lower Serum Bicarbonate (Total CO₂)

Medications and supplements formulated with acid salts—particularly metformin hydrochloride and gabapentin hydrochloride—can significantly lower serum bicarbonate levels by imposing an exogenous acid load on the body.

Primary Culprits: Acid Salt-Containing Supplements and Medications

The most clinically significant supplements and medications that lower serum CO₂ are those containing hydrochloride (acid) salts 1:

• Metformin hydrochloride is the most potent offender, delivering approximately 15.4 mEq/day of acid load at a dose of 2550 mg/day 1
• Gabapentin hydrochloride contributes approximately 13.0 mEq/day of acid load at 2700 mg/day 1
• When used together, metformin and gabapentin were associated with a 1.8 mEq/L lower total CO₂ and 2.4 mEq/L higher anion gap 1
• Metformin alone was associated with 0.7 mEq/L lower total CO₂ and 1.0 mEq/L higher anion gap 1

Mechanism and Clinical Impact

How Acid Salts Lower Bicarbonate

Approximately 25% of commonly prescribed medications contain acid salts 1. These agents work by:

• Delivering an exogenous acid load that must be buffered by serum bicarbonate 1
• The mean daily acid load from medications in patients taking multiple acid salt-containing agents can reach 14.2 mEq/day 1
• Patients with high medication acid loads (≥7.7 mEq/day) had 1.7 mEq/L lower total CO₂ compared to those with low acid loads 1

Population at Highest Risk

Patients with chronic kidney disease (CKD) are particularly vulnerable 1:

• Impaired renal acid excretion limits the body's ability to compensate for exogenous acid loads 2
• The KDOQI guidelines recommend maintaining serum bicarbonate levels at 24-26 mmol/L in CKD 3-5D patients 2
• Bicarbonate supplementation is recommended in CKD 3-5D to reduce net acid production and slow kidney function decline 2

Common Pitfalls and Clinical Considerations

Overlooked Sources of Acid Load

Many clinicians fail to recognize that the hydrochloride salt form of common supplements and medications contributes to metabolic acidosis 1. When evaluating unexplained low bicarbonate:

• Review all medications and supplements for hydrochloride or other acid salt formulations 1
• Calculate the cumulative acid load from all sources, not just individual agents 1
• Consider that the effect is dose-dependent and cumulative across multiple agents 1

Distinguishing True vs. Spurious Low Bicarbonate

Pseudohypobicarbonatemia can occur with elevated serum proteins (such as paraproteins in multiple myeloma), causing falsely low bicarbonate measurements on basic metabolic panels 3. To differentiate:

• Compare total CO₂ from chemistry panel with calculated bicarbonate from arterial blood gas 3
• Look for discrepancies between these two measurement methods 3
• Check for elevated serum proteins or lipids that can interfere with assays 3

Management Strategy

When acid salt-containing supplements cause low bicarbonate 2, 1:

• First-line approach: Consider switching to non-acid salt formulations when available (e.g., metformin base instead of metformin hydrochloride) 1
• Bicarbonate supplementation: In CKD 3-5D patients, supplement with sodium bicarbonate or citric acid/sodium citrate solution to maintain bicarbonate 24-26 mmol/L 2
• Dietary modification: Increase fruits and vegetables to reduce net acid production in CKD 1-4 patients 2
• Monitor closely: Check bicarbonate levels regularly when initiating or adjusting doses of acid salt-containing agents 1

The anion gap elevation seen with these agents is not explained by lactate accumulation, indicating true metabolic acidosis from the acid salt load rather than metformin-associated lactic acidosis 1.