Originally Posted by Jughed

I'm no scientist - so I won't offer an opinion.

Watch some of the videos from Dr San Milan on this. Plenty of content from him on youtube (and he isn't selling any products or plans). He is a research Doc that focuses in on energy use at the cellular level. Much of his research is geared towards health and disease - but he also uses it in his cycling coaching career. He coaches team UAE and Tadej - and they employ mass amounts of Z2 work for these guys.

According to him, Z2 is the best method for repairing and improving mitochondrial function, energy use, fat burning/fat as fuel - which is key for endurance. And it has benefits for anerobic work as well, better lactate clearing capabilities, better energy transfer between muscle fiber types... he explains it much better than I can.

I found him and others touting Z2 in my quest to crush my T2 diabetes - and so far, its worked fairly well.

His interviews are worth watching IMHO.

For health reasons, the majority of my riding is in Z2+/-. 7-8 hours per week, all year round. My volume is consistent even in what I call the "offseason". Offseason work is constant effort Z2 on the indoor trainer. Most of the time on the road I am in Z2 but will do a more spirited ride from time to time - the Z2 work is not as "clean". In the late winter/early spring I do dedicated blocks of interval training to sharpen the pencil.

My results from interval training are mixed. My high end goes up, but my endurance & FTP power ever so slowly creeps up.

My results from Z2 training are not mixed. My endurance and FTP power keep going up, and my VO2 max power is increasing as well.

I saw about a 8 point gain in FTP after (3) 3 week blocks of threshold and VO2 max intervals. I followed the plan, 2x per week intensity, recovery day, Z2 days... very limited results.
I saw a 35-40 point gain in FTP after a spring/summer of Z2 and free riding. And I can maintain a higher % of my FTP power for longer. But my top end sprint power - which I never really use for any real purpose - has declined.

Longevity and sustainability - interval training for me creates immense fatigue, slow recovery, makes getting on the bike a chore after the 2nd week... not something I look forward to. Z2, free riding with some efforts every so often - no or limited fatigue, I want to ride the bike, I can add volume at will - and I can go out and crush a Z3/Z4 ride whenever I want.