: Frome here : Proliferation of once-secret human augmentation techniques.
: Surreptitious recovery of Spartan tissue and organs, industrial espionage,
: independent research, and the release of some augmentation protocols for
: medical usage has substantially eroded the UNSC’s monopoly on
: super-soldiers. Though the knowledge and tools are now available for any
: sufficiently motivated (and wealthy) private party to replicate a
: Spartan-equivalent enhancement package, few see the need to match the
: recruiting standards and scope of enhancement afforded to UNSC Spartans –
: and are willing to settle for cheaper and more invasive augmentations with
: unknown medical drawbacks and psychological costs. Many of the gene
: therapies, artificial organ implants,

I believe Initiation says they have their pancreas replaced entirely by an artificial organ that was "so new they don't even have a name for it yet." It also says that the heart has a material woven into it that lets them outrun a horse, the lungs can now breathe in methane for up to an hour, and cybernetic implants to monitor these tweaks for Spartan medical support. New Blood apparently (going off article references, haven't had the pleasure of reading it yet) claims that this goes for virtually every other organ in the body.