Anyway, I tried (and have thus far failed) to find it. I may have to get the book in order to get enough information to track it down. Along the way, I stumbled across some research based on a women's cohort study done in the UK (not dissimilar to our own Nurses Health studies). Never mind what they were trying to show. They had a calculation on the healthiness of their diet, based on WHO guidelines, one of which was fiber intake. They would mark you healthy if you had above a certain amount of fiber (20 some odd grams, IIRC) _and below 40_. That kind of stuck out, because if you eat a plant based diet involving a lot of beans and those beans aren't processed soy product, and you eat at least 2000 calories a day, you are probably going to blast (heh) right past 40 g.
Why was this considered unhealthy (especially since other elements of the study indicated that near-vegetarian women were healthier, etc.)? You can find a variety of people who say too much fiber in the diet will interfere with mineral absorption, however, it's hard to imagine that eating whole foods is going to generate that problem, given how much additional mineral is coming in with the whole foods. Google books version of Becoming Vegetarian confirms that, but then adds that supplementing fiber, whether with bran or something like psyllium or whatever, can really mess with your mineral absorption and no compensating benefit to replace those minerals. In any event, the WHO guidelines don't top out at 40 anyway -- they top out at 54.
Update: ADA sez for children: take their age, add 5 and that's how many grams of fiber they should get per day. For T., that's 7. IOM fiber guidelines 2005 has a couple of ways of calculating fiber for kids, but it'd be around 14-19 or so. As a minimum. Still failing to find the WHO guidelines. Oh, and that guy who was feeding men 1 1/2 Cup beans to men and watching their cholesterol drop 19%? He was taking those victims, er, volunteers and moving them from whatever low-fiber/high-fat American default diet they at and maxing out their fiber for the day in one, foul, er, fell swoop.