I don't really have a source, but anecdotal evidence... Some of my commercial diver friends got really fucked up after a while of doing it.
One of my friends in particular had a saturation dive go really bad and he came back a completely different person. Like going from someone living the life to a complete wreck in one month.
Famously the Navy SEALs' entry exam-slash-basic training is named "Basic Underwater Demolition/SEAL" as a carry-over from its WWII Underwater Demolition Team legacy, maybe GP is referring to that?
technically they didn't die _from_ doing underwater breath holding training _directly_. they fainted (the question at hand is - does the training up to this point cause brain damage) and then drowned.
"Collectively, these observations suggest that increased cerebral oxidative stress following prolonged apnea in trained divers may reflect a functional physiologic response, rather than a purely maladaptive phenomenon."
so, this paper neither is relevant for seal training nor does it claim any harm caused by breath holding, based on the Abstract section.
yeah, maybe this paper isn't the most relevant source. hopefully you can find a better one if learning about this is important to you, i suggest reading the papers it references, or related papers in google scholar. i'm super not motivated to help you right now because you're acting like if you're wrong about something it's my responsibility to convince you
this kind of bullshit is really frustrating. my comment already explained that the kind of brain damage caused by apnea isn't the kind the pathology reports found, so it absolutely doesn't matter whether seal training involves apnea or not, or exactly what conditions are needed for apnea-induced brain damage, unless you have some strong reason for believing that the news article's reporting of the pathology is wrong
so actually it's fine with me for you to continue being wrong if you want. i don't have anything to sell you
well, i read plenty about breathing techniques and training from various angles and also practice breath exercises where hyperventilation and breath holding play key roles. i never came across anybody claiming that holding your breath especially pre-fainting (which is essential when practicing apnoe diving - and btw you seem to confuse apnoe with sleep apnea?).
to add something of substance - here is a study that does not even find any "Link between Repeated Transient Chokes and Chronic Traumatic Encephalopathy Related Effects" and that kind of asphyxiation is obviously much more extreme than holding your breath.
never heard about that. can you back this up with a source or explanation?