Because the Japanese army only ate white rice in the 1800+ , they lacked those nutrients which made the 1/3 of the army sick and many deaths.
While the poor japs ate only the brown rice, they didn’t have this problem because it was better, white rice was considered a rich thing rich but rich people they had other food to eat, so they didn’t suffer it at all
A good reminder on how nutrition is so important. (Take note that white rice can be stored for longer)