Being deficient in B12 can bring on serious memory loss, even dementia, as we age, explains Dr. David Smith, a researcher at the University of Oxford's Oxford Project to Investigate Memory and Aging (OPTIMA). An important source of vitamin B12, yogurt helps hold off many of the cognitive problems that can come with age, like memory loss and cognitive decline, he says. Smith and his colleagues have discovered that B12 "stops the brain from shrinking," a known marker for cognitive decline. It’s also important to note that you don't need to be old to need B12; a deficiency can lead to fatigue and problems concentrating right now. Suspect you’re deficient? Ask your doctor to do a simple blood test.