One of the things we've come to know is that in parts of China where the intensity of agriculture has gone up substantially even though the diet hasn't changed much—it's either wheat- or rice-based—in both cases the intensity of agriculture depletes the soil of zinc. The consequence of depleted soil is that the grains themselves are depleted in zinc. Let's say the aboriginals on the B.C. coast whose diet was fish are not zinc-deficient because they get enough from their foodstuffs. People in the world whose primary caloric intake comes from grain have had an increased problem with zinc deficiency due to intense agriculture. There may be many other explanations, but that's one we've come to know.
