I agree that IGF-1 has positive effects on insulin sensitivity generally. However, the question is whether it is useful in the specific context of
chronic GH use causing insulin resistance.
The following paper gives some perspective on how IGF-1 improves insulin sensitivity:
We have investigated the metabolic actions of recombinant human IGF-1 in mice genetically deficient of insulin receptors (IR-/-). After intraperitoneal administration, IGF-1 caused a prompt and sustained decrease of plasma glucose levels in IR-/- mice. Plasma free fatty acid concentrations were unaffected. Interestingly, the effects of IGF-1 were identical in normal mice (IR+/+) and in IR-/- mice. Despite decreased glucose levels, IR-/- mice treated with IGF-1 died within 2-3 d of birth, like sham-treated IR-/- controls. In skeletal muscle, IGF-1 treatment caused phosphorylation of IGF-1 receptors and increased the levels of the phosphatidylinositol-3-kinase p85 subunit detected in antiphosphotyrosine immunoprecipitates, consistent with the possibility that IGF-1 stimulates glucose uptake in a phosphatidylinositol-3-kinase-dependent manner. IGF-1 receptor phosphorylation and coimmunoprecipitation of phosphatidylinositol3-kinase by antiphosphotyrosine antibodies was also observed in liver, and was associated with a decrease in mRNA levels of the key gluconeogenetic enzyme phosphoenolpyruvate carboxykinase. Thus, the effect of IGF-1 on plasma glucose levels may be accounted for by increased peripheral glucose use and by inhibition of hepatic gluconeogenesis. These data indicate that IGF-1 can mimic insulin's effects on glucose metabolism by acting through its own receptor. The failure of IGF-1 to rescue the lethal phenotype due to lack of insulin receptors suggests that IGF-1 receptors cannot effectively mediate all the metabolic actions of insulin receptors.
https://www.ncbi.nlm.nih.gov/pubmed/9153298/
So IGF1 seems to mainly exert its insulin sensitizing effects via the IGF1 receptor rather than the Insulin receptor. Both reduced gluconeogenesis and improved peripheral glucose uptake are very desirable and these are also the channels through which metformin works.
So it is tempting to conclude that IGF-1 gives all the posotive effects of Meformin w.r.t. insulin sensitivity, without metformin's downsides and some additional anabolic goodness.
There are 2 potential problems though:
1) receptor saturation and
2) receptor downregulation/desensitization
1) receptor saturation
Let's say someone takes 5Iu of pharma GH and their health is excellent. Then their IGF-1 levels will already be highly elevated. In other words, most of the IGF1 receptors will already be activated. Flooding the body with even more IGF-1 will thus only have a small marginal effect on insulin sensitivity. Much like going from 2.5g of Testosterone to 3g of Testosterone makes less of a difference than when going from 500mg to 1g. So when looking at the studies that show a significant effect of exogenous IGF-1 on insulin sensitivity in GH-deficient subjects, we cannot infer much about the effect size in GH-abusing bodybuilders
2) receptor downregulation/desensitization
IGF1 receptors are structurally very similar to insulin receptors. We know that for the latter, there is significant receptor downregulation after chronic exposure to supraphysiological insulin levels. Due to the structural similarities, it is very likely that the same is true for IGF1 receptors. Indeed, overwhelming anecdotal evidence by users of IGF1 products suggests significantly diminished effectiveness after weeks of use. So even if studies show that,
acutely, IGF-1 increases insulin sensitivity, it is unlikely that this effect will persist after chronic use, at least if supraphysiological levels are used (as would be the case in GH abusing bodybuilders).
So until there are studies showing that long-term use of exogenous IGF-1 with exogenous GH can alleviate the latter's detrimental effects in insulin sensitivity, I am very skeptical. Such studies
do exist for metformin and GH (
https://www.ncbi.nlm.nih.gov/pubmed/14983408) so the recommendation should be for people to use Metformin if their GH use leads to hyperglycemia (which should always be verified with a glucometer).