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You would think that breakfast cereal is free from animal products, but this might not in fact be the case. Over in the U.K., The Independent and several British outlets are reporting today that some cereals may contain a derivative from a popular animal by-products: sheep’s wool. While animal products in everyday foods aren’t exactly uncommon (for example, gelatin is made out of collagen, which is derived from animal proteins), The Telegraph reports that this sheep-related element can be found in “popular breakfast cereals including Rice Krispies, Corn Flakes and Cheerios.”

While many cereals carry a vegan brand, “The reason why breakfast cereals such as Cheerios and Rice Krispies do not carry the vegan logo might be because they are both fortified with either vitamin D3 or vitamin D2.” And the former can be derived from animal sources such as sheep’s wool, making such cereals non-vegan. HuffPost explains that “Sheep secrete a product called lanolin from their glands to make their woolly coats water-resistant. This grease also happens to contain vitamin D.” It goes on to list cereals with the non-vegan-friendly substance: “Coco Pops, Rice Krispies, Frosties, Corn Flakes, Special K, Crunchy Nut, Bran Flakes, Sultana Bran, Rice Krispies Multi-Grain Shape, and Fruit N Fibre.”

For those who want their cereal sheep-wool-grease free, Kellogg’s has released a brand called W.K. Kellogg, “a delicious range of plant powered cereals that champion healthy lifestyles,” like no-sugar granola. As this news is all over the U.K. today, it’s uncertain how much U.S. cereals are affected. But those who feel strongly about adhering to their vegan lifestyles should look out for cereals that list vitamin D3 on the package.