Vitamin B12 is made by neither animals nor plants, but by microbes. Thankfully, in our sanitized world there are safe, cheap, convenient sources. Vitamin B12 deficiency is common in vegans who don’t eat B12 fortified foods or take B12 supplements and can result in paralysis, myelopathy, psychosis, or atherosclerosis. A plant-based diet may improve Alzheimer’s symptoms, but if without B12 supplementation, a plant-based diet may make things worse. It is imperative that those eating plant-based diets include B12-fortified foods in their diet or take B12 supplements, especially pregnant or nursing women (see here and here). Testing is not necessary, but supplementing is—it may even increase one’s lifespan. The recommended intake is 4-7 micrograms a day (but to get that much in daily or weekly supplements you have to take much more). Raw food diets are no exception to this rule. Omnivores under age 50 are rarely deficient in vitamin B12 (though may be deficient in other nutrients).
Topic summary contributed by Wyatt.