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It is a naturally occurring fiber that is found in grapefruit, so it is very easy to get into your diet. It is also cheap compared to a lot of other citrus fruits out there and even cheap compared to fiber supplements (not to mention that it is much more fun to eat than fiber supplements, even if you do not usually care for the taste of grapefruit). If your nutritionist recommends to get your grapefruit pectin through food, then you should consume grapefruit or grapefruit products. But if you do not want or have access to a lot of grapefruit, then they could ask you to take grapefruit pectin pills or grapefruit pectin powder.