When giving insulin, it's important to know that the cat is actually eating normally and will have food 'on board' for when the insulin starts working. So, you can feed prior to the shot, or (if you're confident your cat is eating normally) you can give the shot while the cat is eating.
What you don't want to happen is that you give the shot and then find out that the kitty doesn't want to eat.
There are instances in which folks might wait a while after feeding before giving insulin. For example:
Some folks whose kitties have sensitive GI issues may wait a while after feeding to ensure their kitty isn't going to regurgitate the food.
And I've known some folks using faster acting insulins to feed first and then wait a short while to ensure that the food is getting into the system before they give the insulin shot.
In a nutshell though, the sequence is:
1. Test - to check the blood glucose is high enough to warrant that insulin dose.
2. Feed - to ensure the kitty will have food on board when the insulin kicks in.
3. Shoot - give insulin.
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