From a lactation standpoint, i'm not sure. From a health standpoint, instant is not good. It's highly processed and lacking in nutrients. They process out all the nutrition and then throw a few vitamins back in to say it has some. Your best bet is to get slow-cooking, steel-cut oats. It takes some getting used to, because the cooking processed takes longer and the oats need to sit in the fridge overnight. What I usually do is make a batch, do the overnight part, then the next day cook it up. This gives me 3-4 servings that I reheat via microwave over the next few days. Once I got into the routine, it really didn't take all that long.
I'm not sure about for lactation either, but I have a recipe that I love for the steel cut oats. It could be worth a try to see if it helps(and it's easy). https://www.thekitchn.com/recipe-baked-pu-159872
Steel cut has the best concentration of nutrients, but its texture is much different than 'regular' oatmeal -- more chewy. A good second choice would be old fashioned oats. Those are the ones that you cook on the stovetop (or in the microwave). We microwave old fashioned style oats all the time. 2min 30 sec and it's done, plus time for them to set up and cool enough to eat. I would avoid instant. Too processed and too much sugar in them. With old fashioned, you can sweeten them with a bit of honey or brown sugar, OR I like to just add dried fruit (cranberries, raisins, cherries) during the cook time. Yum!
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Re: Which oatmeal?
I'm not sure about for lactation either, but I have a recipe that I love for the steel cut oats. It could be worth a try to see if it helps(and it's easy). https://www.thekitchn.com/recipe-baked-pu-159872