Primarily constipation.  It can also lead to gallstones, elevated cholesterol, increased heavy metals, decreased intestinal flora.  Decreased intestinal flora can lead to lowered immune function, decreased metabolism of metabolites such as estrogen metabolites, decreased vitamin formation, and decreased bone density.  Our primary silica source also comes from fiber.  A lack of silica can lead to bone loss, tendonitis, osteoarthritis, osteoporosis, diverticulitis, emphysema, arteriosclerosis, etc.