From the Wikipedia page on vitamins:
In east Asia, where polished white rice was the common staple food of the middle class, beriberi resulting from lack of vitamin B1 was endemic. In 1884, Takaki Kanehiro, a British trained medical doctor of the Imperial Japanese Navy, observed that beriberi was endemic among low-ranking crew who often ate nothing but rice, but not among officers who consumed a Western-style diet. With the support of the Japanese navy, he experimented using crews of two battleships; one crew was fed only white rice, while the other was fed a diet of meat, fish, barley, rice, and beans. The group that ate only white rice documented 161 crew members with beriberi and 25 deaths, while the latter group had only 14 cases of beriberi and no deaths. This convinced Takaki and the Japanese Navy that diet was the cause of beriberi, but mistakenly believed that sufficient amounts of protein prevented it.
It’s amazing how much we take modern knowledge for granted. I can’t read that without thinking about how obvious it is that only eating white rice is going to make you sick, since it’s nothing but simple carbohydrates. But even the idea of a carbohydrate only dates back to around the same time. Nobody actually had any solid idea of how these things worked. A great deal of practical heuristics developed over the course of history through trial and error and tradition (a power formula over a long enough span of time! One wonders how many poor saps died before anyone figured out which mushrooms were actually safe to eat).
Now we take vitamin supplements in the hope of living healthier, even though evidence is racking up that it’s a useless practice. And as science marches on, in 50 years we’re going to look like complete idiots. As usual.