Some studies say omega-3 fatty acids and vitamin B12 may play a role with brain chemicals that affect mood and other brain functions. Low levels may be linked to depression. Fatty fish like salmon, tuna, and mackerel have omega-3s. So do flaxseed, nuts, soybeans, and dark green veggies. Seafood is a good B12 source, but vegetarians can get it in fortified cereals, dairy products, and supplements.