Respiratory Quotient.
https://en.wikipedia.org/wiki/Respiratory_quotientThe ratio of CO2 exhaled to oxygen inhaled varies according to the fat/carbohydrate mix.
Carbohydrate molecules have a higher oxygen content, and thus require less oxygen to metabolise, hence proportionally more CO2 is exhaled. Functional threshold was also estimated from this quotient (once RQ raised to 1.0, that was threshold). RQ can go above 1.0 when above threshold, AIUI.
What the testing I've done showed was that absolute rates of fat burning are substantially higher now than a year ago, right across the range of intensities below threshold. In absolute terms, in Feb 2015 at 140W, I was under 3kcal/min of fat, with around 9kcal/min total energy, so I had to find 6kcal/min of carbohydrate. That's 90g of carbohydrate per hour, which is more than can typically be ingested. Hence, it's unsustainable.
Now, according to the latest test, the ratios are reversed; I've not yet received the detailed numbers, but from what I saw, I'm burning over 6kcal/min of fat, and 3kcal/min of carboydrate, at that intensity. That is very sustainable, only requiring 45g/hour of carbohydrate, which is well within typical absorption limits.