What critical components of a balanced diet might be missed if one exclusively consumes lean fish like cod or haddock?

EPA and DHA omega-3 fatty acids.

While lean fish such as cod or haddock are valuable components of a healthy diet, providing excellent sources of high-quality protein alongside essential minerals like iodine and selenium, they inherently lack the defining characteristic of oily fish. The primary nutritional benefit derived from oily varieties is the substantial presence of long-chain omega-3 fatty acids, specifically EPA and DHA. Therefore, an eating plan relying solely on lean fish will miss out on the unique, well-documented benefits associated with regular intake of these essential fatty acids, necessitating the inclusion of fatty fish for comprehensive nutritional coverage.