Fractional utilization refers to the fraction of VO2max that an athlete sustains during an event. It varies between events and between athletes. An athlete can sustain a greater percentage of VO2max for a shorter duration (5000m vs. 10,000m). Fractional utilization is sometimes referred to as lactate threshold. While the concepts are related, they are not the same.
Consider an athlete with a VO2max of 70 ml/kg/min: If we find that he can average 90% of VO2max over the course of a 5000m run, he averages 63.0 ml/kg/min for the race. If he averages 86% of VO2max during a 10,000m run, he averages 60.2 ml/kg/min.
Athletes may train for years, increasing fractional utilization. By increasing fractional utilization, an athlete will be able to complete a distance at a greater percentage of VO2max. For example, if that same athlete increases his fractional utilization during the 5000m to 93%, he can now average 65.1 ml/kg/min. That 2.1 ml/kg/min increase in utilization means more O2 is consumed. Assuming running economy does not change, more ATP is generated from aerobic metabolism for muscle contraction and the athlete completes the 5000m faster.
But what happens if economy decreases?
Economy has traditionally been measured by recording O2 uptake at certain running velocities. If an athlete consumes less O2, that means he's become more economical. We can also compare economy between athletes. The athlete that consumes the least amount of O2 for any given running velocity is the most economical at that pace. Economy gives us an indication of how much substrate, or which substrate, is being used for locomotion. One issue with a low carbohydrate diets is that more O2 is required to completely oxidize fatty acids than is needed for glucose. This means, economy suffers. And when economy suffers, pace declines for any given percentage of VO2max.
To my knowledge, low carbohydrate diets have no beneficial effects on VO2max or fractional utilization. And if VO2max and fractional utilization are unchanged (while economy is decreased) performance suffers; no matter the distance. The athlete may still only be able to complete the 5000m at 90% of VO2max, but more O2 is being used for substrate oxidation (decrease economy), and the ATP yield per unit of O2 consumed is decreased.
Dr. Ronald J. Maughan does a great job summarizing this issue with low carbohydrate diets here, beginning just after 23:00 and going to 34:00.
"The challenge is not to spare carbohydrate by promoting fat use. The challenge is rather to increase carbohydrate storage; and to increase carbohydrate utilization."