Tired? Achy? Can’t remember where you put your keys? You may be low in vitamin B12, essential for proper blood-cell and nerve function. Men over age 40 are more likely to become deficient in the vitamin, even if they eat foods high in B12 like salmon, steak, and eggs, due to age-related loss of the stomach acid needed to absorb nutrients. Talk to your doc about taking a daily 1,000-mcg B12 supp.