Final answer:
Products with aspartame must have a warning about phenylalanine due to the risk it poses to individuals with phenylketonuria (PKU).
Step-by-step explanation:
Products containing aspartame must carry a warning label because aspartame is broken down by the body into several components, including phenylalanine. This is particularly important for individuals with the genetic condition phenylketonuria (PKU) who are unable to metabolize phenylalanine. The warning is required to inform consumers about this potential health risk.