I've been trying a high-fiber diet for three weeks and have experienced stomach irritation, cramps, and the sudden need to use the bathroom at inconvenient moments. Can you have too much fiber? Is there proof fiber really prevents cancer? Is long-term consumption of substances the body can't properly digest itself a health hazard?
You can have too much of anything, fiber included, as several gravelbrains have already shown. Surgeon James McClurken reported a while back on a 50-year-old man who complained of abdominal pain and constipation. McClurken sliced him open and found “a large amount of bran-like material with a dry, thick, toothpaste-like consistency” in the lower small intestine. Seems the guy had eaten two large bowls of bran with minimal milk the day before. His intestines absorbed what liquid there was and his guts basically turned to concrete. After the blockage was removed the patient was put on a liquid diet for a few days and was soon OK.
Intestinal blockage is rare, but too much fiber and not enough liquid can cause abdominal distention, cramps, and flatulence. There was fear at one time that a very high fiber diet (50 to 60 grams per day, twice what many dieticians recommend and four times what the average American gets) could prevent the body from absorbing certain minerals, but this has since been discounted. On the other hand, the link between low fiber and colon cancer is firmly established. Fiber is also thought to be useful in treating and/or preventing heart disease, diabetes, and various gastrointestinal disorders, such as constipation. The lesson? Eat fiber in moderation, wash it down with lots of liquids, and in general don’t be an idiot — useful advice in any context.
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