Substrate oxidation errors during combined indirect calorimetry-hyperinsulinemic glucose clamp studies.

Area of Science:

• Metabolic research
• Human physiology

Background:

• Indirect calorimetry combined with hyperinsulinemic euglycemic clamps measures glucose oxidation (Gox), fat oxidation (Fox), and energy expenditure (EE).
• Protein oxidation (Pox) is typically estimated from urinary nitrogen (N) excretion, assuming accuracy and stability during clamps.

Purpose of the Study:

• To assess the variability of basal urinary N excretion.
• To determine the impact of hyperinsulinemic clamps on urinary N excretion.
• To evaluate potential errors in Gox, Fox, and EE calculations due to urinary N measurement inaccuracies.

Main Methods:

• Eight healthy subjects underwent basal urinary N measurements on four separate days.
• Subjects also underwent a hyperinsulinemic euglycemic clamp with simultaneous urinary N measurements.
• Calculations of Gox, Fox, and EE were performed using standard indirect calorimetry equations.

Main Results:

• Basal urinary N excretion showed high day-to-day variability (36% CV).
• Urinary N excretion increased by 47% during the clamp due to increased urea clearance.
• Variability in basal urinary N led to potential errors of 11-23% for Gox and 16-24% for Fox.

Conclusions:

• Basal urinary N excretion is highly variable and not a reliable sole indicator of protein oxidation.
• Hyperinsulinemic clamps significantly alter urinary N excretion, introducing errors in metabolic calculations.
• Correcting for urea clearance changes may improve the accuracy of urinary N estimates during clamps.