Which of the following is not a main component of milk?

The main components of milk are water, fat, protein, and various sugars. Among these components, lactose is the primary sugar found in milk, and while it is significant, it is not always classified in the same way as the major macronutrients like fat, protein, and water.

Water, which constitutes about 87% of milk, is essential for transport and maintaining the fluid balance. Fat contributes to the energy content and creaminess of milk, protein provides essential amino acids critical for growth and repair in the body, and these are all considered key macronutrients. Sugar, while important, acts primarily as an energy source rather than a structural or functional component of milk, making it less of a foundational element in the context of what defines milk's main components.

In this context, identifying sugar as not being a main component reflects a focus on those elements that dominate its nutritional profile, primarily fat, protein, and water.