Fish oil has no direct benefit to the eyes after eating, while cod liver oil supplementation helps to replenish vitamin A. If vitamin A is deficient, it often results in insufficient replenishment of the key substance of the visual cycle, 11-cis-retinal, reduced synthesis of retinol, reduced sensitivity to low light, the time required to see substances from light to dark, that is, the dark adaptation time is significantly prolonged, and in severe cases, night blindness can occur. Vitamin A deficiency can cause severe epithelial keratinization, which can cause loss of conjunctival mucus-secreting cells, keratinization, and reduced glycoprotein secretion, thus causing corneal dryness, also called dry eye disease, and vitamin A is also known as the anti-dry eye vitamin. Cod liver oil is rich in vitamin A, which helps prevent damage to the eyes brought about by vitamin A. Fish oil preparations contain mainly unsaturated fatty acids, which have no significant benefits for the eyes, but are helpful for the heart and brain vessels, so it has some preventive effect against atherosclerosis.