Possibly iron. There's heme and non-heme forms of iron and some people are just physically incapable of using non-heme (plant based) iron. It's uncommon but definitely a thing.

[–] BonfireOvDreams@lemmy.ml 6 points 1 year ago* (last edited 1 year ago)

If for some reason he medically required heme-iron, I'd rather publicly subsidize the price difference for them to eat impossible meat as that does contain heme-iron. No more requirement to rely on animal products for that. As far as I'm aware though, it's just a concern of absorption rate. If the absorption rate is the concern they should just focus on taking a higher dosage supplement - which would not require heme-iron.