My elderly aunt taught me a lesson on making low fat mashed potatoes. After draining them, and using just a tiny dab of lowfat margarine, I start whipping them and adding, just a little at a time, fat free non dairy creamer.
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I discovered a great way for making mashed potatoes, and I don't use any fat in it. While making the potatoes for mashing, I put a can of vegetable broth into a pan, and just simmered it while the potatoes were cooking.