the scientific explanation is SURVIVAL
if you didn't gain from from using insulin humans and every other animal with our body structures would have went extinct long ago
when not using exogenous insulin certain foods will spike insulin from your pancreas more than others. for example eating chocolate cake will spike insulin way more than eating a watermelon. even though they both have the exact same calories. crazy huh? but a lot of guys don't get this concept. when you continously spike insulin over and over then you become insulin resistant and your blood sugar will not come down after a meal, that is what we refer to as type 2 diabetes where now you need a drug or insulin to bring the blood sugar back down. ironically the more you do this the more you will need insulin over time vs. fixing the root issue of poor diet. but that is a discussion for another thread
now when you have excess insulin that you inject into the body you will store. does not matter what you eat, that is how insulin works to put on mass in the first place (which is what i try and explain to people on here all the time but they love to argue)