Respiratory Quotient (RQ) of 1 Indicates Carbohydrate as Main Fuel Source
A respiratory quotient (RQ) of 1.0 definitively indicates that carbohydrate is the main source of fuel being metabolized by the body. 1
Understanding Respiratory Quotient
The respiratory quotient (RQ) is the ratio of carbon dioxide production (VCO₂) to oxygen consumption (VO₂) at the tissue level. This value provides important information about which macronutrient is predominantly being metabolized:
- RQ = 1.0: Indicates primarily carbohydrate metabolism 1
- RQ = 0.7: Indicates primarily fat (fatty acid) metabolism 1
- RQ = 0.8: Indicates primarily protein metabolism 1
- RQ > 1.0: Indicates lipogenesis (conversion of carbohydrate to fat) 2
Physiological Basis
When carbohydrates are metabolized, the number of CO₂ molecules produced equals the number of O₂ molecules consumed, resulting in an RQ of exactly 1.0. This occurs because:
- The chemical equation for glucose oxidation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
- The ratio of CO₂ produced (6) to O₂ consumed (6) equals 1.0
In contrast, when fats are metabolized, fewer CO₂ molecules are produced relative to O₂ consumed (RQ ≈ 0.7), and protein metabolism yields an intermediate value (RQ ≈ 0.8) 1.
Clinical Applications
The RQ measurement has several important clinical applications:
Nutritional Assessment: RQ helps evaluate the efficacy of nutrition support regimens 3
Metabolic Status: It indicates whether a patient is primarily burning carbohydrates, fats, or proteins 1
Overfeeding Detection: An RQ >1.0 suggests overfeeding with carbohydrates, leading to lipogenesis 2
Respiratory Tolerance: Elevated RQ values (≥1.0) may be associated with reduced respiratory tolerance and mild respiratory compromise in critically ill patients 2
Important Distinctions
It's important to distinguish between RQ and the respiratory exchange ratio (RER):
RQ: Reflects tissue-level metabolic gas exchange and is determined by substrate utilization 1
RER: Measured at the mouth during gas exchange testing and can be affected by factors beyond metabolism, such as hyperventilation or hypoventilation 1
During steady-state conditions, RER equals RQ, but during non-steady states (e.g., hyperventilation, exercise above anaerobic threshold), RER may differ from RQ 1.
Clinical Pearls
An RQ of 1.0 in patients receiving total parenteral nutrition suggests efficient carbohydrate utilization 2
In hypermetabolic states (injury/sepsis), even with high glucose administration, RQ often remains below 1.0, indicating continued fat utilization 4
Certain medications can affect RQ values, potentially influencing weight gain patterns in diabetic patients 5
During exercise beyond the anaerobic threshold, RER often exceeds 1.0 due to additional CO₂ produced from bicarbonate buffering of lactic acid 1, 6
Therefore, the answer to the question is (c) carbohydrate.