Shrinkage doesn’t just happen at the pool. It can also take place inside your skull, especially when your brain lacks sufficient vitamin B12. The RDA for men is 900 mcg. A deficiency of this essential nutrient can cause significant drops in gray matter volume, which has been linked to memory loss. And even your doctor may not be able to tell that it’s happening. According to a new Rush University Medical Center study, low B12 levels often go undetected in blood tests. In fact, even when blood levels of the vitamin appeared normal, between 15 percent and 17 percent of people they studied weren’t absorbing enough of the vitamin and had serious deficiencies. But don’t worry your possibly-shrinking head too much—just include any of our top-five B12-packed foods listed here and never get accused of being small-minded again.