So when I do the gym is it wise to take my insulin then eat? About an hour before the gym?
In these early days, I think it might be safer to schedule exercise for before meals instead of after. Once you've got more experience of insulin, you'll have way more flexibility.
I don't know which fast acting insulin you're on, but, just as an example, I've posted the action graph for a commonly prescribed one, Novorapid, below.
You can see how it gets to work after about 10 to 20 mins (because it takes a while for it to be distributed around the body), peaks in 1 to 3 hrs, and lasts overall for 3 to 5 hrs.
Those timing margins are broad, because it varies between individuals - experimenting with bg test strips, and cgm - continuous glucose monitoring will help you find your margins.
It is good that it operates over an extended time period, because food digestion operates over an extended time period too.
When you eat carbs, they get broken down into glucose and then absorbed into your bloodstream. That doesn't happen all at once. An average human will absorb glucose at about 30g per hour, so if you're eating, say, a 70g meal, that's a couple of hours, so you want the insulin to still be active to deal with the ongoing feed in of carbs during that period.
But, because exercise also lowers bg, it means you need to think about how exercise fits in with the insulin "shape" over time.
If you were going to the gym after eating, the exercise and the still active insulin will both be influencing bg levels, so you need to factor that in. It's not like some conditions where you just "fire and forget" a pill.
For example, let's say I'm injecting for a 70g meal. These are my numbers, don't use them, it's just an example.
I'd typically take about 10u for that, and I'd probably inject about 20 mins before the meal to give time for the insulin to get to work.
10u would be fine if I knew I was just going to be sitting at my desk for the next 5 hrs.
But if I knew I was going to be walking around doing some shopping for a few hours after the meal, that's mild exercise, so I'd likely reduce the dose to maybe 7u.
If I was planning on some harder exercise like a long run or hillwalking, I might tail it back to only 3 to 5.
The message is when you're deciding on an insulin amount, you need to think about what you will be doing in the way of exercise during the activity phase of the insulin shot, typically 5 hrs.
People do this in different ways. Some get the spreadsheets out, some just wing it a bit.
CGM helps a lot with this - you get a continuous graph from readings every 5 mins so you can see if you are trending up or down, and them intervene before it gets too high or low, and it also shows how much active insulin you've got on board - IOB.
Don't be afraid of making mistakes. The vast majority of mistakes can be sorted out with a few dextrotabs, or, my favourite, a Tunnocks Caramel Wafer, and you can learn from them too - look back at the dose and carb amounts and decide whether it would have played out differently with lower or higher amounts.
Also, be prepared psychologically for the unpredictability of it. Food and insulin get exposed to a whole lot of biological processes after you eat and inject them, so 5u for 50g one day won't necessarily pan out the same way the next. That is not your fault, it's just how biology works.
Many of us are now using cgm and Stephen Ponder's Sugar Surfing techniques to adjust things on the fly to get around the vagaries of it, but keep that till later.
There's lots of really advanced ways of living with T1, but the NHS simply doesn't have the time or resources to teach you them, so I'd recommend books like Think Like a Pancreas by Gary Scheiner, Sugar Surfing by Stephen Ponder and Using Insulin by John Walsh. Oh, and ask us on the forum too - there's centuries worth of collective experience here.
It's scary at the moment, but in a few months time, you'll be surprised by how few limitations there are. I can go into any restaurant and eat pretty much anything on the menu, provided I think ahead a bit.
Good luck!