We've also done full-fat cow's milk since 1 yr, and we still do even though Tyler's over 2 because he's pretty skinny. When we transitioned him to whole milk, we gradually starting giving him bottles colder and colder until he took them straight from the fridge so we wouldn't have to warm his sippy cups of cow's milk. You can heat it if you prefer, but I believe the reason the microwave isn't recommended is because it can create "hot spots" in the liquid that you can't detect, even after you've stirred, shaken, or tested it. That puts your LO in danger of being burned by hot liquid when he goes to swallow it. You might be thinking of breastmilk...the microwave does destroy the nutrients in that. HTH!