Unlike minerals such as iron and calcium, protein is a macronutrient in the human diet. Why is protein a macronutrient?

Let's discuss first about difference between micro - and macro-nutrients. Micronutrients are those which are needed in small quantities and their function is not that vast. But macronutrients are the ones which control all the metabolic cycles of the cell, so they are needed in large quantities. This is the basic difference between the two terms.

Proteins are macro-nutrients because they are needed in large amount, as they are involved in some of the most important functions of our body. For example, proteins act as enzymes, involved in immunity, maintaining homeostasis of the body, etc. That is why they are considered as macronutrients.