No one wants a vitamin deficiency, especially when that vitamin is one that takes care of your nerves and blood cells. And that goes double if this same nutrient is one of several that keep you from experiencing anemia, a condition that makes you incredibly tired and weak and gives you a lovely ghostly pallor.
The vitamin is B12, and you can only get it from animal foods -- lamb, liver, sardines, clams and beef all have plenty. Vegetarians can stick with eggs and some cheeses, but vegans are at risk for vitamin B12 deficiency.
Another group at risk is people older than 60, for reasons medicine doesn't yet understand. In older men, this deficiency has been linked to a low sperm count and lazier sperm. In the hopes that the cure for abnormal sperm production is as simple as a B12 supplement, researchers have performed multiple studies on men with fertility issues, but unfortunately, the results have been inconclusive. There simply isn't enough scientific evidence yet to determine whether B12 supplements can fight male infertility.