The sniff test

It is well established that omega-3 fatty acids from fish oil are free of mercury, PCBs, furans, and other pesticide residues. Several independent analyses have all agreed: little to none are contained in fish oil. In the Consumer Lab series of assessments, for example, no fish oil supplement failed because of any heavy metal or pesticide residue.

However, oxidative byproducts are a problem. Just as fish that sits on the store shelf or your refrigerator too long starts to smell "fishy," so will fish oil. When fish or fish oil becomes rancid, smelling like rotten fish at its worst, it means that