Stephan Guyenet wrote a post about this a few years back. Relevant section:
Elevated fasting insulin is a hallmark
of the metabolic syndrome, the
quintessential modern metabolic
disorder that affects 24% of Americans
(NHANES III). Dr. Lamarche and
colleagues found that having an
insulin level of 13 uIU/mL in Canada
correlated with an 8-fold higher heart
attack risk than a level of 9.3 uIU/mL
(1; thanks to NephroPal for the
reference). So right away, we can put
our upper limit at 9.3 uIU/mL. The
average insulin level in the U.S.,
according to the NHANES III survey, is
8.8 uIU/mL for men and 8.4 for women (2). Given the degree of metabolic
dysfunction in this country, I think
it's safe to say that the ideal level
of fasting insulin is probably below
8.4 uIU/mL as well.
...
So what's the ideal fasting insulin
level? My current feeling is that we
can consider anything between 2 and 6
uIU/mL within our evolutionary
template, although the lower half of
that range may be preferable.
