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B12 is generally the biggest concern. Vegans need to take vitamin supplements to get it. The B12 in those supplements is produced in factories doing bacterial fermentation

True herbivores have stomachs that can ferment B12 from the plants they eat. We do not

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Another one can be iron, depending on your genes. There’s plenty of iron-rich vegetarian options, but heme iron in animal sources typically is absorbed at rates of ~40% while non-heme iron in plant sources is absorbed at 1-10% depending on the specific source. Non-heme iron absorption is also blocked by calcium and magnesium that compete for the same receptors. It’s not impossible to get enough iron in a vegan diet without supplementation, but it requires some careful planning of what sources you eat and how you cook it, and if you’re genetically predisposed to anemia it can be risky without monitoring your bloodwork.

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