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It is poor digestion, partly, but instead of taking more supplements to fix it, we should be eating human foods, which means mostly fruit and few (raw) veggies.
The book "Dying to Get Well" nicely explains what is a human food and what is not, with many nice references if you wish to read further. I haven't read it myself, but "Mad Cowboy: Plain Truth from the Cattle Rancher Who Won't Eat Meat" also explains it well apparently.