The National Institute of Health recommends that the average adult needs 2.4 micrograms of vitamin B-12 per day, or more if you're pregnant or breastfeeding, this vitamin is essential for nerve function and the creation of healthy blood cells. It is also found in seafood. One of the easiest ways to increase this vitamin in your diet is to eat more fresh bluefin tuna. While canned tuna is a good source of B12, fresh fish has twice as much at 5 microgram of B-11 per 3-ounce. This can cause dizziness and fatigue, but also lead to mobility problems and memory loss. The ultimate guide to buying fresh tuna is to select cuts with more marbling.