What it is
Fructose isolated and sold on its own — sweeter than sucrose, cheap, and a way to spike sweetness without raising the "sugar" line's glycemic number.
Why your biology objects
Pure fructose with no glucose to carry it across the gut overloads the GLUT5 transporter and spills to the liver, where unregulated fructokinase converts it to fat and uric acid. This is the exact molecule the 1:1 rule is built around.