What is the expected respiratory quotient (RQ) in prolonged starvation?

Respiratory Quotient in Prolonged Starvation

The respiratory quotient (RQ) in prolonged starvation is 0.7. 1

Understanding Respiratory Quotient

Respiratory quotient (RQ) is the ratio of carbon dioxide production (VCO2) to oxygen consumption (VO2) and serves as an indicator of which fuel substrate is being predominantly metabolized:

• RQ = 1.0: Pure carbohydrate oxidation
• RQ = 0.8: Mixed substrate utilization
• RQ = 0.7: Predominant fat oxidation
• RQ > 1.0: Lipogenesis (fat synthesis from carbohydrates)

Metabolic Changes During Starvation

During prolonged starvation, the body undergoes significant metabolic adaptations:

1. Substrate Shift: The body shifts from using primarily glucose to using fatty acids as the main fuel source 2

2. Accelerated Starvation: Cirrhosis and other conditions can lead to accelerated starvation characterized by a reduction in respiratory quotient, which reflects the metabolic switch from glucose to fatty acids as primary fuel 2

3. Protein Conservation: After initial protein breakdown, the body adapts to conserve protein and primarily relies on fat stores, resulting in the characteristic RQ of 0.7 1

4. Ketone Production: Increased fat metabolism leads to ketone body production, which can provide alternative fuel for the brain

Evidence for RQ of 0.7 in Starvation

The respiratory quotient decreases in all subjects during starvation from approximately 0.85 to 0.70, as demonstrated in research measuring energy metabolism during exercise in normal subjects undergoing total starvation 1. This finding is consistent across multiple studies examining metabolic changes during prolonged fasting.

In a study examining the effects of a very-low-calorie diet, subjects with a respiratory quotient in the lower range (<0.72) were better able to maintain weight loss, indicating their bodies had adapted to primarily using fat as fuel 3.

Clinical Implications

Understanding the RQ in starvation has important clinical implications:

• Nutritional Assessment: RQ can be used to assess metabolic state and substrate utilization, though it has limitations for fine-tuning nutrition support 4

• Refeeding Risk: When reintroducing nutrition after prolonged starvation, the RQ will rise as metabolism shifts back to carbohydrate utilization 1

• Monitoring Overfeeding: An RQ >1.0 may indicate overfeeding, which can lead to respiratory compromise in patients with limited pulmonary reserve 4

Measurement Considerations

Indirect calorimetry is the gold standard for measuring RQ and energy expenditure 2. Hand-held calorimeters measuring oxygen consumption and calculating energy expenditure (assuming an RQ of 0.85) are more accurate than predictive equations for determining resting energy expenditure 2.

In summary, the respiratory quotient in prolonged starvation is 0.7, reflecting the body's adaptation to primarily utilize fat as its main energy source during periods of nutritional deprivation.