From the Guidelines
The respiratory quotient in prolonged starvation is 0.7. During prolonged starvation, the body shifts to primarily using fat as its main energy source after glycogen stores are depleted. When fat is metabolized, it produces less carbon dioxide relative to the oxygen consumed compared to carbohydrate metabolism. This results in a respiratory quotient (RQ) of approximately 0.7, which is characteristic of fat metabolism [ 1 ]. In contrast, carbohydrate metabolism yields an RQ of 1.0, and protein metabolism typically gives an RQ of about 0.8. The low RQ of 0.7 during starvation reflects the body's adaptation to conserve glucose and protein by relying heavily on stored fat for energy production through beta-oxidation and ketogenesis.
Some key points to consider:
- The respiratory exchange ratio (RER) is often used as a rough index of metabolic events, and it can be measured by gas exchange at the mouth [ 1 ].
- A reduction in the respiratory quotient is a manifestation of a metabolic switch from glucose to fatty acids, which occurs in states of accelerated starvation, such as cirrhosis [ 1 ].
- The use of actual body weight, corrected for ascites, is considered safe when estimating energy needs in patients with cirrhosis [ 1 ].
- Nutritional intervention, led by a multidisciplinary team, can improve survival rates and quality of life in patients with cirrhosis [ 1 ].
Overall, the respiratory quotient of 0.7 in prolonged starvation is a result of the body's adaptation to conserve glucose and protein by relying on stored fat for energy production.
From the Research
Respiratory Quotient in Prolonged Starvation
The respiratory quotient (RQ) is a measure of the ratio of carbon dioxide production to oxygen consumption. In the context of prolonged starvation, the RQ can indicate the body's reliance on different energy sources.
- The study 2 found that after 22-hour fasting, the respiratory quotient decreased to 0.72 +/- 0.01, indicating a shift towards fat metabolism.
- Another study 3 reported that during a 5-day fasting period, the respiratory quotient was overwhelmingly in the fat-burning range, suggesting that the body was utilizing fat as a primary energy source.
- A study on the use of respiratory quotient to determine the efficacy of nutrition support regimens 4 found that RQs were generally lower than 1.0, indicating fat oxidation.
- The study 5 on the relationship between RQ and energy intake found that the 24-hour RQ strongly predicted 24-hour energy intake, but did not specifically address prolonged starvation.
- A recent study 6 on the effect of exogenous and endogenous ketones on respiratory exchange ratio and glucose metabolism found that fasting resulted in a decreased respiratory exchange ratio, indicating increased fat oxidation.
Possible Answer
Based on the studies, the correct answer is: (d) 0.7
This value is consistent with the decreased respiratory quotient observed in studies 2 and 6 during prolonged fasting, indicating a shift towards fat metabolism.