Edgar Cayce led me to iodine, but that was more faith based. Brownstein helped convince me that Iodine was necessary but it was Dr.D.C.Jarvis's 1958 book Folk Medicine: A Vermont Doctor's Guide to Good Health that made me think about nutrients in a way I had never thought before.
Dr.Jarvis (who was not a vet) studied farmed animal groups. Farmed animals eat what they are fed, they can't follow nature to access their needed foods. Dr.Jarvis would try to learn the difference between the healthy herd & sickly herd & change the sickly herd's diet to make them a healthy herd. He studied cows, goats, mink, dogs, etc. The goats were dying due to a lacking micronutrient. Apparently only a speck of this nutrient was needed at least once every 5 years, otherwise the herd died. He claims this is vital info.
He was a proponent of kelp, raw apple cider vinegar & raw honey for all animals (including humans) He explained that even though honey appeared to have no vital nutrients, like the goats, vital nutrients didn't need to be needed in quantity or on a daily or even annual basis to be essential.
His Science was more observation based but he could take a sickly herd & restore it to health with a few simple additives (mainly kelp & ACV). It's a fascinating read & I highly recommend it.