Isomalt is a mixture of medium-sized sugars (a.k.a. oligosaccharides) that is prepared from sucrose by treatment with enzymes.
In skincare products, it acts as a water-binding humectant ingredient that also serves as a bulking and anticaking agent. It has a mildly sweet taste.
Science
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Fiume, M. et al. (2019). Safety Assessment of Monosaccharides, Disaccharides, and Related Ingredients as Used in Cosmetics. International Journal of Toxicology, Vol. 38(Supplement 1) 5S-38S