Certain bacteria that live in the guts of animals are the only organisms that make B12. As such, these animals accumulate the nutrient in their tissues, and you’ll find high concentrations in cows, goats, and other ruminant animals and in seafood such as fish and shellfish. B12 regulates the expression of genes, meaning it prevents things like cancer from happening.
Most processed foods are fortified with folic acid, which masks B12 deficiency, signs of which are aggression, obsessive-compulsive behavior, sleeplessness, and tingling in the arms and legs. Don’t settle for low levels even if they are “low normal.” Two hundred to 1,100 picograms per milliliter is normal, but levels of under 400 increase your risk of mental health problems like dementia and depression.