Firstly, we cannot digest fiber. So, if we consume too much fiber too quickly, the bacteria in our large intestine will break it down, producing excess amounts of gas, thus causing bloating and cramps. Secondly, excess consumption of fiber can lead to deficiencies of calcium, magnesium, etc.
Now, fiber is either insoluble or soluble. If we consume too much of insoluble fiber, then it can accelerate the bowel movements, reducing the absorption of excess water, thus causing diarrhea. However, if we consume too much soluble fiber and don’t drink enough fluids, it can slow down our digestive process, thus leading to hard stools or constipation. In rare cases, it can even cause intestinal blockages.