1. Protein. It's possible to get enough protein on an animal-sourced-food-free diet and if you're a lacto-ovo vegetarian (deathless animal products) it is easy. However, bioavailability is a concern for vegan protein, given the prote-ase inhibitors in lentils, soya & grains. Protein powders have this removed and are usually enriched with leucine, which is a sine qua non of muscle-building and is naturally present in meat. 2. Vitamin A Not everyone can cleave plant Vit A, which is b-carotene (in carrots), into retinol, which is animal Vit A. Everyone with a working gut can absorb the Vit A in a cow's liver, which is retinol. Both forms are fat-soluble, which means they hitch a ride in the body's lipid transport system to get into your body (the infamous LDL cholesterol - having highish levels is good in my view, but being in like the 10s and 20s is a marker of LDL receptor failure). They can't as easily do this if you don't drench your mashed carrots in fat, though, and I don't suspect a vegan would want to do this - in Scotland or even England, without coconuts shipped in from Panama (which was a Scots colony once!), where would one get practical quantities of non-toxic fat? I mean, I guess linseed but nobody in nature has a blender to get the fat out of flax, whereas anyone who can chase an elk for days with a knife can get the (typically meagre) fat out of the elk. 2a. Vitamin A involvement in iron absorption Retinol is not involved in iron absorption, but in deficiency, red blood cells will not form correctly. 3. Vitamin B12 Not present in a whole food plant based diet unless including animal products (vegetarian, pescetarian or bivalve-vegan, not vegan) or supplementation. If supplementing, bioavailability is questionable. 4. Iron Present in the nonhaeme form in vegetables. This is not as easily absorbed as haeme iron, even if you discount oxalic and phytic acid. If all someone eats is vegetation, I have every reason to believe they aren't getting enough iron, even if they are, due to oxalate and phytate, among other factors. This is why I admonish folks to eat liver if they can tolerate it and think they're anaemic. 5. Vitamin C This is the only essential nutrient vegans and vegetarians should not get a problem with no matter what permutation they follow (an all-cooked diet can of course cause scurvy). Most vegetarian diets have large quantities of raw foods that are high in vitamin C. Ascorbate is actually an issue for me because I eat mostly cooked and need to eat something raw to get enough, so I eat strawberries sliced in cream thrice a week to cover that base 6. Vitamin K The plant form, K1, goes through a weird process to become K3, a synthetic form, and then be metabolised into K2. K3 is hepatotoxic, and not everyone makes it from K1. 7. EPA & DHA ALA (plant omega-3) competes with linoleic acid (plant omega-6) for the enzyme that elongates them to EPA and arachidonic acid respectively. A WFPB diet will generally contain an exorbitant amount of linoleic acid, swamping any ALA out. Not everyone can make the EPA to DHA conversion - for them, DHA, found, like EPA, only in animal products, is an essential nutrient. 8. Cholesterol and plant sterols It isn't clear that a human can be cholesterol-deficient, due to the fact that the body makes it, but statin therapy side effects (from inhibiting cholesterol and CoQ10 synthesis) present a clear symptom profile on which someone could be diagnosed as having a cholesterol deficiency. If the body's own cholesterol synthesis is only sufficient in the context of a diet containing animal-sourced foods, one would expect to see statin side effects in vegans and some vegetarians not taking statins. I'm not sure the real life experience of vegans and some vegetarians bears this out. In people with sitosterolaemia https://en.wikipedia.org/wiki/Sitosterolemia , plant sterols are absorbed at an extremely high rate and replace the body's own cholesterol with a nonfunctional plant version. An effective treatment, though not the standard of care, is a plant-free diet somewhat akin to mine - all cholesterol in such a diet is functional animal cholesterol that can be used if it gets absorbed. A sitosterolaemic being told by his Doc to go vegan would be analogous to guns being sold to depressives at the pharmacy - it'd make the issue worse. 9. Carbohydrates Most plant-based diets are extraordinarily high in carbohydrates - which in the gut, break down into simple sugars (glucose, galactose if you tolerate lactose, and fructose) relatively quickly. They are not an essential macronutrient for most individuals. People at risk for diabetes, or who have (either type) or previously had (successfully-managed type 2) diabetes, should not eat high or even normal quantities of carbohydrates if they want their blood glucose levels to become or remain normal. Correcting hypos is an obvious exception - if your bg is 2.0 mmol and you're still conscious but shaking like a leaf, eat a few tablespoons of rice or drink 2-4g of carbs from a dextrose solution.