Hi
@lcarter I hope that a member who is Type2 but on insulin will come along to answer your questions.
Meanwhile I (a diet controlled Type 2) will give you my (limited) understanding of it all.
Firstly a few words of your post seem to be missing.
I think you are asking is why injected insulin brings your blood sugar levels down, but your own insulin doesn't?
This is almost certainly a misunderstanding, unless your pancreatic Beta cells have given up and so you no longer make any insulin yourself.
As a Type 2 you almost certainly suffer from insulin resistance which mean s it takes much more insulin than normal in order for it to force the blood sugar (glucose) out or the bloodstream and into the cells which either use or store it. If making all this extra insulin gets to be too much for your body, then one remedy is to use prescribed external insulin to make up the difference between the amount you make yourself and the amount that you actually need.
We are all different, so my diabetes isn't the same as that of each of my 5 moderator colleagues even though we are all T2 and mainly diet controlled.
Similarly, there are different kinds of insulin (human, porcine, synthetic etc) and withing the synthetics there are faster and slower acting insulins. But different people may react somewhat differently to a particular type of insulin from the way somebody else does. But, if you are still producing it, there is no reason to think there is anything wrong with your own human insulin - just that because of insulin resistance you are not making enough of it right now.
Now to address carbs: Carbs convert into glucose very easily and often very fast, sometimes while they are still in your mouth!
Some people, perhaps all people, produce some glucose from fats and proteins if they eat zero or very low carbs. There is a part of the brain which absolutely require glucose in order to work- so in cases of fasting or starvation our body needs to be make at least that amount of glucose even when no carbs are eaten. But the vast majority of the energy requirements of the body can be supplied by Ketones - which is why Carbohydrates are NOT an essential macro nutrient.
It's possible that in some people, they are not accustomed to 'burning fats' for ketones for energy and so their body produces a lot more glucose from proteins and fats than is actually necessary - thus they have to inject insulin to cover for the protein they eat if they are consuming nearly no carbs, but since making glucose from proteins and fats is 'expensive' and takes more time compared to doing it from carbs, the body always chooses carbs first!
So you have the choice - consume more carbs and inject more insulin, or consume few carbs and inject less insulin. The amount of external insulin needed is related to:
A). How bad your insulin resistance is.
B). How much insulin your body can still make.
C). How many carbs you eat.
D). How much protein and fat your body converts into glucose.