Swedish people as whole have some of the best teeth in the world, but it wasn't always so. In the 1930s, even three-year-olds had cavities in most of their teeth. There weren't enough dentists to repair teeth, and no one knew how to prevent tooth decay, because they didn't know what caused it. They needed to do some controlled experiments. What they did was highly unethical, and would never be permitted today.
During the second World War, at a mental hospital outside of Lund, Sweden, researchers forced a group of patients to ingest 24 pieces of a sticky, light brown substance in a single day. These severely disabled patients were involuntary participants in a long-term study commissioned by the state medical board in cooperation with big industry, and this coerced feeding would continue for three years. The four to six doses that they consumed four times a day over that time were in some ways sweeter than their typical medicines—but also more troubling. No benefit to the patient was ever expected. Rather, the goal was to measure the damage inflicted by the substance over time and determine a dosage safe for public consumption.
That substance was caramel, and it inflicted so much damage to the patient's teeth that we learned it was sugar that caused tooth decay. Read about the experiments and the legacy they left behind, at Atlas Obscura.
(Image credit: Harris & Ewing/Library of Congress)