What does an RER (Respiratory Exchange Ratio) of 1.0 indicate?

An RER (Respiratory Exchange Ratio) of 1.0 indicates that carbohydrates are the primary fuel source being utilized by the body during exercise. The RER is calculated by measuring the volume of carbon dioxide produced relative to the volume of oxygen consumed. An RER of 1.0 suggests that the body is predominantly oxidizing carbohydrates, which yield a carbon dioxide to oxygen ratio of about 1:1 due to the complete oxidation of glucose.

When carbohydrates are metabolized, carbon dioxide production matches oxygen consumption closely, hence the RER approximates to 1.0. This can occur during high-intensity exercise, where the demand for energy is high, and the body shifts to more rapid glucose metabolism to meet that energy requirement.

In contrast, an RER below 0.7 would typically indicate fat utilization as the primary fuel source, while values above 1.0 are often seen when anaerobic metabolism increases during very high-intensity exercise. Option D, mentioning both fat and carbohydrates being utilized equally, would not yield an RER of 1.0; instead, it would produce an intermediate value reflecting the mixed utilization of both substrates.