A 52-year-old man with poorly controlled type 2 diabetes mellitus presents with altered mental status, rapid shallow breathing, and fruity-smelling breath. Serum glucose is 520 mg/dL, arterial pH is 7.18, and serum β-hydroxybutyrate is markedly elevated. Which is the most common ketone body in diabetic ketoacidosis?

Explanation

## Ketone Bodies in Diabetic Ketoacidosis **Key Point:** **β-Hydroxybutyrate** is the most abundant and predominant ketone body in diabetic ketoacidosis (DKA), comprising 70–80% of total ketones, followed by acetoacetate (20–30%) and acetone (<5%). ### The Three Ketone Bodies | Ketone Body | Structure | Proportion in DKA | NADH Dependency | Clinical Significance | | --- | --- | --- | --- | --- | | **β-Hydroxybutyrate** | Reduced form (alcohol) | 70–80% | Requires NADH | Most abundant; used by tissues for energy | | **Acetoacetate** | Keto form (ketone) | 20–30% | Can be reduced | Intermediate; unstable | | **Acetone** | Decarboxylation product | <5% | N/A | Volatile; causes fruity breath odor | ### Why β-Hydroxybutyrate Predominates in DKA 1. **Hepatic Redox State:** Severe lipolysis in DKA generates massive amounts of acetyl-CoA and NADH 2. **NADH-Driven Reduction:** The high NADH/NAD^+^ ratio favors the reduction of acetoacetate → β-hydroxybutyrate via β-hydroxybutyrate dehydrogenase 3. **Metabolic Efficiency:** β-Hydroxybutyrate is the preferred substrate for ATP generation in peripheral tissues (brain, heart, muscle) 4. **Accumulation:** The liver cannot consume ketones (lacks thiophorase), so they accumulate in blood ### Ketone Body Synthesis Pathway ```mermaid flowchart TD A[Acetyl-CoA<br/>from β-oxidation]:::action --> B[HMG-CoA synthase<br/>rate-limiting]:::action B --> C[HMG-CoA]:::outcome C --> D[HMG-CoA lyase]:::action D --> E[Acetoacetate]:::outcome E -->|β-hydroxybutyrate<br/>dehydrogenase<br/>+ NADH| F[β-Hydroxybutyrate<br/>70-80% of total]:::action E -->|Spontaneous<br/>decarboxylation| G[Acetone<br/>fruity breath]:::outcome F --> H[Released to blood<br/>for peripheral use]:::action style A fill:#e8f4f8 style F fill:#c8e6c9 ``` **High-Yield:** In DKA: - **β-Hydroxybutyrate** is the predominant ketone (70–80%) - **Acetone** causes the characteristic fruity odor (Kussmaul breathing) - Standard urine ketone dipsticks detect **acetoacetate** preferentially, not β-hydroxybutyrate, which may falsely underestimate severity **Clinical Pearl:** The **nitroprusside test** (used in older labs) detects acetoacetate and acetone but NOT β-hydroxybutyrate. Modern labs use β-hydroxybutyrate assay (more specific) or serum ketone measurement for accurate DKA diagnosis. **Warning:** Do not confuse the predominant ketone body with the ketone detected by dipstick — they are different! β-Hydroxybutyrate is most abundant but least detected by traditional urine dipsticks.