Final answer:
To identify proteins in food, nitrogen should be measured because it is a specific component of amino acids, unlike other macronutrients such as carbohydrates.
Step-by-step explanation:
To determine whether a food sample contains protein, you would measure the element nitrogen. Proteins are composed of amino acids, which in turn are made up of hydrogen, carbon, oxygen, and nitrogen. Specifically, nitrogen is a distinctive component of amino acids that is not present in the same form in other macronutrients like carbohydrates and fats. Carbohydrates, for example, consist only of carbon, hydrogen, and oxygen. Hence, nitrogen's presence is a key indicator of proteins in a food sample.