Diet Coke cans feature a warning label about phenylalanine exclusively to protect infants born with a rare missing liver enzyme that turns this common artificial sweetener into a…

A closer look

Next time you grab a diet soda to feel healthy, take a glance at the fine print. Phenylketonuria (PKU) is a rare pediatric metabolic disorder where an infant is born entirely missing the enzyme needed to break down the amino acid phenylalanine. For 99.9% of the population, it's totally harmless. But for babies with PKU, a single drop of aspartame or high-protein food causes massive brain toxicity. This is why hospitals prick newborn heels within hours of birth: catching it early means a strictly controlled, terrifyingly precise diet can literally save a child from profound intellectual disability. Biology really is a game of molecular roulette.