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It is a polysaccharide which, in the presence of acid and sugar, gels. It is used as a thickener in jams, and occurs naturally in some fruit, like apples. In apples, it's mostly in the peel, and for someone beginning the diet, you normally wouldn't get too much from eating an apple, because you'd peel it. Pectin as an added ingredient is not legal, as it is a complex sugar.