Vitamin B-12 might not be commonly known and understood, but that doesn't make it any less important. In fact, it's essential for normal activity in nerves. Deficiency can lead to anemia -- a lack of healthy, oxygen-providing red blood cells. It's also part of the B-complex group of vitamins, a few more of which are on this list and all of which are essential for metabolism and energy [source: Sease].
B-12 is abundant in meat, especially salmon and tuna, as well as in eggs and cheese. However, there are not many sources in vegetables, so vegetarians and vegans are at a high risk for deficiency [source: PCRM]. A lack of B-12 can lead to fatigue and shortness of breath. Men who work out regularly need to take extra care to insure they're getting enough in their diet.
Most men aren't going to find it challenging to get enough B-12 unless they're vegan or vegetarian, however it's still an important vitamin and a key part of a well balanced diet.
If you avoid certain foods that are deemed to be healthy, you may be avoiding this next vitamin.