What is BHB (Beta-Hydroxybutyrate)?
Beta-hydroxybutyrate (BHB or βOH) is a ketone body produced by the liver during fasting or carbohydrate restriction, and it serves as a critical diagnostic marker to distinguish between hyperinsulinemic hypoglycemia (such as insulinoma) and appropriate physiological responses to fasting.
Biochemistry and Physiology
BHB is the most abundant and stable ketone body produced through hepatic ketogenesis when glucose availability is limited. During fasting, the liver converts fatty acids into acetyl-CoA, which then undergoes enzymatic conversion through acetoacetate to BHB via β-hydroxybutyrate dehydrogenase 1. Unlike acetoacetate and acetone (the other ketone bodies), BHB is the predominant circulating form and provides the most reliable measurement for assessing metabolic state 2, 3.
Clinical Application During Fasting Tests
Diagnostic Thresholds
During a 72-hour fast, plasma BHB >2.7 mmol/L rules out organic hypoglycemia caused by inappropriate insulin secretion with approximately 100% sensitivity and specificity 1. This threshold is particularly valuable when glucose levels fall into the 0.50-0.60 g/L (3.0-3.3 mmol/L) range, where neither insulin nor C-peptide measurements can reliably distinguish patients from controls 1.
Two Primary Objectives
BHB measurement during fasting serves to:
- Verify patient compliance with the fasting protocol (rising BHB confirms true fasting state)
- Rule out hyperinsulinemic hypoglycemia without waiting for glucose to drop below 0.45 g/L (2.5 mmol/L) 1
Expected BHB Behavior
In patients without insulinoma during a 72-hour fast 4:
- Hours 12-18: Minimal change (0% increase)
- Hours 18-36: Rapid rise (333% increase)
- Hours 36-54: Continued elevation (210% increase)
- Hours 54-72: Further increase (167% increase)
In patients with insulinoma:
- BHB remains suppressed, typically ≤1.2 mmol/L even at 43 hours 5
- Excess insulin blocks lipolysis and ketogenesis, preventing BHB elevation
Practical Implementation
Sampling Protocol
Measure plasma BHB systematically every 6 hours during fasting when venous glucose exceeds 0.60 g/L, then every 1-2 hours once glucose falls below this threshold 1. This should be done alongside insulin, C-peptide, and proinsulin measurements 6.
Early Fast Termination
The 2009 Endocrine Society guidelines suggest BHB >2.7 mmol/L as a criterion to end the fast early 6. However, the 2013 French guidelines advise against routinely aborting the fast based solely on BHB levels, as rare insulinoma cases have shown BHB >2.7 mmol/L 1. The exception: in highly symptomatic patients with severe ketosis (nausea, vomiting), stopping at BHB >2.7 mmol/L is reasonable 1.
Recent quality improvement data demonstrates that using BHB >2.7 mmol/L as a stopping criterion reduced average fast duration from 57.6 to 49.7 hours without missing any insulinoma cases (all 7 cases developed hypoglycemia within 43 hours with BHB ≤1.2 mmol/L) 5.
Critical Pitfall
BHB does not exceed 2.7 mmol/L during fasting in cases of IGF (insulin-like growth factor) secretion from non-islet cell tumors 1. This extends BHB's diagnostic utility beyond insulinoma detection, providing information that insulin and C-peptide measurements alone cannot offer.
Advantages Over Other Markers
When glucose is in the intermediate range (0.45-0.60 g/L), insulin and C-peptide show only 85% sensitivity for distinguishing patients from controls 1. BHB >2.7 mmol/L in this glucose range definitively excludes hyperinsulinemic hypoglycemia with near-perfect accuracy 1, making it superior to traditional markers in this diagnostic gray zone.