Fiber and nutrient absorption
Ok, so this came up last week sometime and I knew I had numbers for it and I came across this in the folder of the never to be finished mondo opus book. I'm going to assume that I had some decent source (probably the Stipanuk book) for the numbers. Anyhow, there you go. Ultimately, it's a fairly small effect.
I should mention that very high fiber intakes can increase the amount of nutrients being lost to excretion. For example, higher fiber intakes may reduce fat absorption by about 3 percent, protein by maybe 5%. Itís been estimated that increasing fiber intake from 18 to 36 g/day would increase caloric losses by about 100 cal/day or so. Essentially, the fiber prevents the enzymes from being able to reach the nutrients, and they simply pass through unabsorbed. This is another reason that high-fiber intakes can be useful while dieting: decreased caloric absorption. Of course, no athlete or bodybuilder wants decreased protein absorption so its sort of a mixed blessing.