And when I say that, I really mean; you're wrong.
Metformin does not increase endogenous insulin secretion. In fact, the total amount of insulin secreted by the pancreas will be decreased. You have this medication mistaken with another class of anti-diabetic agents known as the sulfonylureas. Metformin is a very complicated drug and it seems each time I read up on it, I learn something new.
To my knowledge on metformin (which is quite elementary), there are a few mechanisms in which it can assist with maintaining a lower blood sugar. In simple terms, its main action is to increase insulin sensitivity, i.e you will need less insulin to lower any given unit of blood sugar.
It does this in a few different ways. Some of which I can recall are:
1. Turns off hepatic gluconeogenesis (i.e, it stops your liver from synthesizing glucose)
2. It increases the number of insulin receptors - I am unsure how correct this is. Its likely to be an oversimplification of what really happens, but it'll do.
3. It reduces glucose absorption from the gut
One of the less well known side effects is that it inhibits some of the anabolic pathways, meaning it will make muscle growth more difficult.
As a separate, compounding side effect to the above, there was a study which also showed it to lower testosterone in males.
to the OP:
-Reduced hunger on metformin is common, it comes about from having more stable blood sugar.
-Having the shits on metformin is common, particularly at first and on higher dosages. The side effect will likely decrease in severity over time
-If it is helping you drop weight, fantastic. Keep at it. Ignore the 'lower testosterone bullshit', losing fat is a health priority if you are greatly overweight, which I am assuming you are given it was prescribed for weight loss. Anti-aging people love metformin for its health benefits.
-It is not some wonder drug, which when stopped will have permanently improved your metabolic profile to mean you will no longer gain fat as easy ever again. Apparently, Biotest's latest supplement is capable of that, but who knows.
I'm done rambling, this went on longer than I expected.
Metformin is excellent for diabetics and the severely overweight. In everyone else, trying to gain muscle, its probably not ideal to be used regularly, although perhaps could be a nice addition to a cheat day, or those on AAS. Those are my thoughts, give them with the credibility they deserve; the credibility of some random on the internet.