Over the years nutrition and diet have undergone a lot of research. I remember the low fat craze from a while back and it can work. I also remember the low carb craze, which isn't over yet, and it can work also.
As the title of this thread might imply, I think that fiber might have a significant impact on our nutritional status. However, don't go and overdose on fibrous foods all of a sudden. Not only will you regret it, getting too much can be unhealthy, just like anything else.
However, the modern western diet is commonly regarded as deficient in fiber. In fact, the high availability of processed carbohydrates is synonymous with the removal of fiber from our carbohydrate sources. I think most of us are aware of the consequences with respect to insulin, but there is also an issue with respect to our digestive tract.
Certain types of fiber are able to bind with bile acids. When this happens, cholesterol and lipids in the bile that is bound in this way are not reabsorbed into the body. This forces the liver to synthesize more bile, which makes use of lipoproteins in the bloodstream.
For example, I'm sure just about everyone has heard that fiber has been proven to help lower cholesterol? Well, to put it simply, this removal of cholesterol in the digestive tract, and hence it's consumption while the liver synthesizes more bile, is how this occurs.
Here is an interesting study (see Appendix table A-2C) which attempts to determine the rate at which bile is secreted from the liver:
Mathematical model of biliary lipid secretion: a quantitative analysis of physiological and biochemical data from man and other species
To make a long story short, they conclude that the amount of bile produced is generally related to body weight, such that we produce approximately 0.22ml/kg/hr. So, at a bodyweight of near 100kg, I should produce about 22ml/hr or 528ml/day. I'll go with 500ml/day for simplicity.
So, if I don't eat any fiber, the body will very efficiently reabsorb almost all the cholesterol and lipids contained in that half litre of bile.
Here is a different interesting study (see table 2) which allows us to get an idea of the amount of lipids contained in the bile of an individual.
High level of deoxycholic acid in human bile does not promote cholesterol gallstone formation
So, unless I'm getting the wrong data, it amounts to be approximately 13g/dl. People with gallstones apparently have lower lipid levels in their bile. Anyhow, if 500ml equates to 5dl, I'm circulating and recirculating a total of approximately 65g of lipids through my digestive tract during the course of a day.
Remember, this is 65g of mostly reused lipids, as it is generally going to be reabsorbed, resynthesized and recirculated again and again during any given day,
While I didn't find anything which tells me how much bile is released in response to a meal (as it may be produced at a fairly constant rate by the liver it is also stored in the gall bladder), the fact that fiber has a dramatic effect on cholesterol let's us know it isn't trivial.
Based on all of this, I'm going to suggest that body composition changes on a controlled diet are going to be easier with appropriate levels of fiber.
Remember, you can get unsafe levels of fiber, so don't simply go crazy with this concept. Also, remember that you need fats for various bodily functions, you can't just stop eating them.
That being said, if your insulin levels are relatively low due to not eating refined carbohyrates, and lipids are being bound to fiber in your digestive tract, you know that your liver is not going to stop producing bile, so you can guess that lipids will be taken out of the bloodstream. With low insulin levels, the fat cells should release more.
This also coincides with the fact that nutritionists are now telling us to consume whole oats, beans, and fibrous fruits and vegetables as our carbohydrate sources. I suspect there is more to this than simply insulin control.