Tuna contains some mercury, which is toxic to developing brains and nervous systems. Women who are pregnant or planning to become pregnant must limit the amount they eat. Canned light tuna has less mercury than albacore tuna. Women can safely eat up to six servings of light tuna per month, but should limit albacore to three servings or less monthly. For the healthiest sandwich, choose tuna canned in water. You’ll get 73 calories and 0.8 grams of total fat from 3 ounces of light tuna packed in water. When it’s packed in oil, the same portion has double the calories and seven times more fat.