When you eat something with aspartame (a sugar substitute), your body uses the phenylalanine in it to build proteins, either as-is or after turning it into another substance called tyrosine.

When your body breaks down aspartame (an artificial sweetener), it turns into three things you already get from foods like meat, beans, and fruit—and your body doesn’t store them up over time.

When people eat or drink things with aspartame (like diet soda), their blood phenylalanine goes up — and this happens more if they haven’t eaten recently, even if they don’t have the genetic condition PKU.

When you eat or drink something with aspartame, your body breaks it down into smaller pieces, including methanol (which turns into carbon dioxide), and two amino acids—aspatic acid and phenylalanine—that your body absorbs.