DHA is an omega-3 essential fatty acid critical for brain, retina, and nervous system development, particularly in puppies and kittens. The best dietary sources are fish oils (salmon, menhaden, sardine). Plant-based DHA precursors (ALA from flaxseed) are converted to DHA at very low efficiency in dogs and even less in cats — for cats, dietary DHA must come from animal sources because their conversion is essentially zero.