During the Second World War, Yudkin served in the Royal Army Medical Corps and was posted to Sierra Leone. While there he studied a skin disease that was prevalent among local African soldiers and discovered that it was due not to an infection, as had been believed, but to riboflavin deficiency.[16] He found that the Army had devised a uniform diet for its soldiers in the four British West African colonies (Gambia, Sierra Leone, Gold Coast and Nigeria).