In the fasted state the respiratory exchange ratio is close to 0.7. Taking into account which macronutrients can be stored, which macronutrient is providing most of the energy for cellular metabolism in the fasted state?

Question

BlackTom AI · Accepted Answer

RER (respiratory exchange ratio) = VCO2 / VO2 and specific macronutrients have characteristic RERs: carbohydrate ≈ 1.0, fat ≈ 0.70, protein ≈ 0.8. This key link between substrate oxidation and RER immediately points to which fuel predominates when RER ≈ 0.7.

1. Relevant knowledge points and concepts
- RER/RQ values for pure substrate oxidation (glucose ≈1.0, palmitate ≈0.7, average amino acids ≈0.8).
- Metabolic state in fasting: low insulin/high glucagon → glycogen depletion, increased lipolysis, fatty acid mobilization, ketogenesis, and fatty acid β-oxidation for ATP.
- Storage capacity and energetic density: fat ≈9 kcal/g (long-term, large stores), carbohydrate ≈4 kcal/g (limited glycogen stores), protein ≈4 kcal/g (functional, not stored primarily for fuel).

2. Step-by-step reasoning
- Observed RER ≈0.7 indicates predominant oxidation of a substrate with RER ≈0.7 (i.e., fat).
- During fasting glycogen becomes depleted within ~24 hours; therefore carbohydrate oxidation falls and fat oxidation rises.
- Hormonal milieu of fasting promotes adipose triglyceride breakdown → free fatty acids used by most tissues (except some obligate glucose users) for ATP.

3. Calculations / intermediate steps (palmitate example)
- Palmitate (C16H32O2) oxidation equation: C16H32O2 + 23 O2 → 16 CO2 + 16 H2O.
- RQ = CO2 produced / O2 consumed = 16 / 23 ≈ 0.695 ≈ 0.7. This matches the observed RER.

4. Why the correct answer is correct
- The RER value directly corresponds to fat oxidation, and fasting physiology mobilizes fat as the primary long-term energy store. Fat yields far more stored energy and is preferentially oxidized when carbohydrates are scarce.

5. Why other options are incorrect
- Carbohydrate (RQ ≈1.0): would produce an RER near 1.0, not 0.7; glycogen is limited in fasting.
- Protein (RQ ≈0.8): proteins contribute to gluconeogenesis but are not the major fuel source and their RQ is higher than 0.7.

Conclusion: Therefore, fat is providing most cellular energy in the fasted state; the correct answer is a. Fat.

In the fasted state the respiratory exchange ratio is close to 0.7. Taking into account which macronutrients can be stored, which macronutrient is providing most of the energy for cellular metabolism in the fasted state?单项选择题

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