Vitamins B12, K2, DHA based Omega-3 (the kind your brain runs on) are all animal-based nutrients not found in plants.
Furthermore, some plants like Spinach for example, have oxalates that bind to nutrients and prevent your body from absorbing them. It binds to calcium and prevents your body from absorbing it. Animal foods have a lot of calcium. Plants not so much, and you couple that with the antinutrient effect of some plants and you end up with deficiencies. That's not even getting into lectins and phytic acid.
Farm animals are also supplemented with b12. There is b12 in some algae but it is not reliable source and it is the only thing smart vegans should supplement. There is no RDA for EPA and DHA, just for ALA but you can buy algae derived EPA/DHA if you want. Just FYI, yes, brain is mostly DHA but DHA from supplements cant cross blood-brain barrier. For k2 the best source bay far is Natto which is fermented soy and it is in the form of mk7 which stays much longer in the blood thank mk4 form of k2 from animal sources. Other fermented veggies also have k2 but much less. D3 over the winter can be obtained buy eating sun dried mushrooms (i dried mine over the summer), yes plants are amazing and mushrooms make d2 and d2 vitamin in large quantities by drying them in the sun or under UV light. Etc, etc..
But in the end just look at vegan populations or plant based eating population and on average they live longer and healthier so they are not missing anything in the diet, not anything essential anyhow
> Animal foods have a lot of calcium. Plants not so much
Broccoli has plenty of calcium.[1] B12 is easily supplemented and also present in dairy (which vegetarians consume) and eggs (which also many vegetarians consume). I'd never heard of vitamin K2 but it also is apparently present in eggs, butter, cheese and natto (fermented soy).[2] I love meat but let's not state with complete confidence that eating animals is mandatory for good health. India has more lacto-ovo vegetarians than the rest of the world put together and they're doing fine.
Furthermore, some plants like Spinach for example, have oxalates that bind to nutrients and prevent your body from absorbing them. It binds to calcium and prevents your body from absorbing it. Animal foods have a lot of calcium. Plants not so much, and you couple that with the antinutrient effect of some plants and you end up with deficiencies. That's not even getting into lectins and phytic acid.