The best time to supplement cod liver oil for babies varies slightly depending on the specific situation, usually up to 2 weeks of age, or up to 3 weeks of age if the usual sun exposure is low. The daily physiological requirement of vitamin D for babies is at least 200-400 units, while newborns are more deficient in the first few months of life and are prone to night blindness and rickets. To prevent vitamin D deficiency, cod liver oil can be added to premature babies, twins and newborns with digestive tract diseases after birth, and cod liver oil can be added to full-term babies from the second week of life; at the same time, appropriate amounts of calcium can be taken, and more sunlight can help synthesize vitamin D on the skin surface to promote bone growth and development. If the baby is not calcium deficient, cod liver oil alone can be taken. In addition, in spring and summer, when there is enough sunshine, 20-30 minutes of sunshine every day can reduce the amount of cod liver oil. In autumn and winter, when sunshine is short, cod liver oil should be taken daily.