Why did people from southern Asia commonly suffer from beriberi sickness in late 1800s?

eating toxic sea algae
853
inbreeding
1600
drinking dirty water
4130
eating mostly white rice
5098
Beriberi is a sickness caused by extreme deficiency of vitamin B1, thiamine. White rice is generally a poor source of vitamins, and completely lacks thiamine. The sickness became common in Asia in 1800s with the increased processing of rice.
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