Because it's unlikely that a specific deficiency is your only single problem.
Based on what? You seem to be assuming that the only way to be deficient in something is to have a junk diet. It is quite easy to be deficient in just one nutrient.
Based on what? You seem to be assuming that the only way to be deficient in something is to have a junk diet. It is quite easy to be deficient in just one nutrient.