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.
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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.