I talked to Eli Lilly a couple of
years ago about why the mix. It
seems that eons ago, when no one had
a clue as to differences in activity
curves, nor even differences in
immuunogenicty between beef and
pork, these two insulins were
thought to be absolutely equivalent.
So they saw no harm in throwing them
together in the same vial. However,
even as the error in this thinking
slowly became apparent, Lilly
continued making the blend simply
because there were so many people
using it successfully who didn’t
want to change to pure beef or pure
pork.
Finally, the reason for the pure
beef and pure pork (Iletin II
series) was that there were people
who were either (1) totally allergic
(breaking out all over their bodies)
to beef insulin due to it having
three amino acids different from
human insulin, which is what the
immune system expects to find in the
bloodstream, so pure pork was needed
here, or (2) they found the activity
curves of pure pork insulins so much
shorter than beef that they had
difficulty getting through the night
on pork crystalline insulin without
losing control of their blood sugars
(me for example), so pure beef
insulin was needed.