How were vitamins and daily requirements determined by usda?

By a team : Of "experts". Each was asked how much of this and that vitamin is essential and the average of their answers was used to compile the rda, e.g. Thiamine (vitamin b1) rda is 1 to 1.5 mg/day. But prolonged deficiency damages essential enzymes and the traditional deficiency disease is beriberi. It required 100 mg of thiamine 3 times a day for months to treat. Thiamine def is common today.