This nutrient serves as a building block of brain tissue and is the primary structural fatty acid in the gray matter of the brain, as well as in the retina of the eye. Thus, this nutrient is essential for healthy brain development and mental function and is important for nerve signal transmission in the brain, eyes, and nervous system. Research has also revealed a strong inverse correlation between the levels of this nutrient in red blood cells and cardiovascular disease, confirming its cardio-protective (heart-protecting) effects. This nutrient must be obtained either from the diet or from supplements. What is this nutrient?