For fat metabolism, what is the respiratory quotient (RQ) value?

The respiratory quotient (RQ) is a measure used to determine which macronutrient is being metabolized by the body for energy. The RQ is calculated by dividing the volume of carbon dioxide produced by the volume of oxygen consumed during metabolism.

When fat is the primary source of energy, the RQ value is typically around 0.7. This low value indicates that for every unit of oxygen consumed, a smaller amount of carbon dioxide is produced, characteristic of fat metabolism. This is because fats require more oxygen to metabolize compared to carbohydrates, leading to a lower ratio of carbon dioxide output to oxygen intake.

In contrast, when carbohydrate metabolism predominates, the RQ is closer to 1.0, and for proteins, it generally ranges between 0.8 and 0.9. Therefore, the choice of 0.7 is consistent with the understanding of fat metabolism and its efficiency in energy production compared to other substrates.