Fish oil contains Omega 3 fatty acids, particularly EPA AND DHA. There is only one plant source for EPA and DHA and that is algae. Omega 3’s cannot be obtained by diet alone; you would need to eat fish every day, every meal to equal 1300 mg, the necessary daily amount to keep you out of deficiency.
EPA is good for heart, joint, mood and immunity. DHA is good for eyes, brain, fetal development and memory.
If 1300 mg is necessary per day to keep you out of deficiency, what is too much? No studies have been done to show what is too much, however, for pain and inflammation (joint health) you might want to have upwards of 2000 mg per day.
FIsh in a supplement should come from wild caught fish in the Atlantic. Sardines and Anchovies are the best source.
You CAN take too much cod liver oil – it contains Vitamin A, which can be toxic if taken in excess.
And, the FDA removed the previous warning of taking too much fish oil if you are on blood thinners. Those on blood thinners are safe at 3000 mg or less.