I've been farting around with this for about a month. I feel like I'm fully 'keto-adapted'. It's terrific for anything endurance related while I'm breathing, aerobic exercise of any kind is incredibly comfy and I feel like I can go for ever, but I am going to start cycling off it since I feel like going further will be detrimental to apnea.
It might not be bad for U/W hockey where I imagine CO2 is a limiting factor but in my case it limits the amount of on-demand explosive power I have at my disposal, and changed the way deeper dives feel. The explosive power might be something missing for U/W hockey though. For general shallow spearfishing I've found it to be very comfortable. For deeper stuff and DYN sets I had to slow my pace down. For pool swimming, I feel like it definitely affected my lactic threshold in the speed sense more than in the distance sense. Normally, I can easily do UW 50's all day at a 1:1 pace going 42 sec swim + 42 sec rest. When in ketosis it feels better to do something more like 50 sec swim + 50 sec rest.
In regards to dry holds/statics, they are noticeably more comfortable when I am in a state of ketosis but based on my O2 sat meter I was burning much more O2, and experiencing very little dive response. I do still experience healthy dive response on dry dynamics and anything aquatic while in ketosis. For divers where CO2 is a limiting factor ketosis might be useful but it would seem to be safer from a limit to perspective to increase CO2 tolerance, even more-so given that CO2 is usually a limiting factor for new divers.
re: catabolic. Medically, ketosis is very much a catabolic state since anabolic hormones (those that promote fuel storage or muscle growth) such as insulin and testosterone are reduced, with insulin being the major one. HGH is probably increased a little bit since I think insulin blocks HGH. Cortisol is very much increased; cortisol + insulin is at the very least cosmetically bad although at the end of the day anabolic since it promotes fat storage but cortisol + HGH/super low insulin (ketosis) promotes fat burning, again a catabolic state.
Again, very little study on freedivers but there does seem to be positive correlation between insulin, glucose, and EPO production. True ketosis results in staggeringly low insulin and blood glucose. I don't necessarily think it has wrecked my freedive training since at least I was experiencing plenty of hypoxia and I don't feel like I have lost muscle when it comes to any resistance training but strict keto for apnea seems to limit the benefits (ie greater RBC production) and would also be poor for max attempts. Since fat cannot be used for energy without O2 ketoadaptation doesn't help competition freediving. Ketoadaptation which leads to greater metabolic efficiency is certainly not a bad thing in the long run, but extended training while in true ketosis doesn't seem smart. I also played around with cyclic and targeted keto but my performance when I was carb loaded doing some benchmark DYN sets wasn't anything stellar. In fasted rats (ie starved), EPO production was completely blocked. While it can be 'gamed' to some degree to spare muscle and such, and isn't necessarily unhealthly, the endocrine state of ketosis looks almost identical to starvation.
I saw my biggest apnea gains in the past when I ate what felt good to me, which was a diet high in various meats (seafood, red meat, whatever), no conscious fat restriction (although none of this adding huge dollops of lard to every meal like in a keto diet), very little cheese/milk although a fair bit of whey protein, limited much grain/bread, and with most carbohydrates coming from moderate to high GI fruits as opposed to veggies. I love my coffee although it appears to interfere with iron absorption, as does tea. Hence the whole 'cutting caffeine' for freediving is probably more rooted in the iron absorption aspect (which is likely the limiting factor for many serious divers when it comes to RBC generation) than in the actual caffeine aspect.
