What key nutrient does whole milk lack for infants?

Whole milk is known to lack sufficient levels of iron, which is crucial for infants' growth and development. Infants go through rapid growth stages, and iron is essential for the formation of hemoglobin in red blood cells, which carries oxygen throughout the body. When infants consume whole milk as a primary food source, they may miss out on crucial sources of iron found in breast milk or iron-fortified formula.

Although whole milk does provide important nutrients like proteins, fatty acids, and vitamin D, it does not meet infants' iron requirements. This can lead to iron deficiency anemia if complementary iron-rich foods are not introduced at the proper age. Therefore, while whole milk provides many nutrients, its lack of iron makes it unsuitable as a sole source of nutrition for infants.