[lazerpants]

I wonder if many people have tested VO2 max like this. Anecdotally, I know that I have a very difficult time getting my heart rate over 150 if I don't consume carbs, while I can easily get it 160+ if I've consumed carbs. I don't notice a qualitative difference, except during HIIT exercise, but I believe there is a subtle one based on my heart rate.

[bitexploder]

I have. I found a cool place that did lactate threshold (via constant blood sampling) Dexa fat scans and VO2 max).

The thing is you can use RPE. We basically mean sustained 4/5 effort (rating of perceived on a scale of 1-5). It's a good enough proxy. VO2 max is also tricky because some people have higher or lower VO2 max but go faster or slower. Higher max != Faster times.

I think the sweet spot for very low carb diets is ultra marathon where you keep your HR around 130-140 anyway, wherein you would use fat anyway. Simple fact is you can't max your power output in an aerobic capacity without carbs. This is not disputed. What we want to know is where the cut off point is in terms of percentage of VO2 max.

Edit: just adding, Noakes talks about ultra performance in his book. It covers everything I saw in that book. He dives deep into the body systems and energy production of the body to suss out the science involved.