I believe that, like sugar alcohols, soluble dietary fiber should be counted on a 0.5g carb/gram fiber basis. However since most dietary fiber is not listed as to whether it is soluble or insoluble, this can be difficult to do. Erring on the side of caution, a 1g for every 3g total dietary fiber would be a good compromise. But many of the LC "fibers" such as polydextrose, inulin and glucomannan (shiritaki noodles) are essentially all soluble so 1g counted for 2g is a better count.
Insoluble dietary fiber, IDF, passes through the human digestive system unchanged so it is non-nutritive in all aspects. Further, since it adds bulk to food and assists in stool formation and "moving things along" IDF can reduce the nutrient absorbtion from the food we do eat. This is essentially the rap of the "fiber is overrated" contingent.
SDF may not be metabolized by the body, but it is fermented by intestinal bacteria to short chain fatty acids (SCFA) that are absorbed and metabolized. Some of these SCFA's are used by the intestines themselves as fuel, but the rest go mainly to the liver to be metabolized.
The "joys" of SDF are many from a weight loss and general health POV. To list a few I've read we have:
- When SDF absorbs water it swells forming a gel. This gives a feeling of fullness in the stomach and slows the progression through the digestive tract so you may feel full longer.
- This slowing of the digestion has been shown to improve the stability of BG levels in diabetics.
- SDF's are fermented in the large intestine to produce short chain fatty acids (SCFA's) that can be used by the intestine's cells as well as absorbed by the body. These SCFA's are believed to help with irritable bowel syndrome (IBS) by boosting the mucosal cells and perhaps reduce colorectal cancer risks. They are also believed to contribute to lowering LDL levels.
- SDF's do have a smaller than normal caloric nutritive value to us humans as compared to other carbs (1.5 cal/g to 2.5 cal/g, so I'll use an average of 2 cal/g)
What becomes of the rest of the SCFA's? We hear "fatty acid" and presume these are metabolized like other dietary fats, so eating soluble fiber has all the bennies above plus they are like ingesting fats! But these are metabolized more like carbs -- the SCFA's are prime substrates for gluconeogenesis!!
According to this (which focuses on inulin type SDF's) the major byproducts of SDF fermentation are lactate (15%) and SCFA's (40% of which these are further classified as approx 15% butyrate, 28% propionate and 67% acetate respectively). The SCFA's are absorbed or otherwise utilized for energy at a rather high 90-95% rate, and the latter two are either entirely (propionate) or partially (acetate) metabolized by the liver to PRODUCE GLUCOSE via gluconeogenesis (and lactate is also listed as a substrate for gluconeogenesis). Scroll down at this citation.
I have read several reports around the LC webosphere of BG increases with low carb breads and pastas. And, of course, the numerous reported stalls many seem to experience when including these products in their diet. This may well be why. Those "fibers" are like eating half their grams in sugar.
Also consider that, like all bacteria, the population will increase when their food supply increases. So more SDF is feeding these. Sounds great for digestion, but these same bacteria also ferment something else -- "regular" carbs that have not been completely digested previously. So in addition to adding to your carbs, you are very likely increasing the digestion/absorbtion of the other carbs in your diet. Be consoled that you have a healthy and efficient digestive system, but you may be getting more "carbs" than you accounted for.
There may well be some mitigating factors to this in terms of the inefficiency and/or energy requirements for the gluconeogenesis pathways for these substrates, but to me it seems pretty clear now that soluble fibers should NOT be subtracted on a 1:1 ratio to find net carbs. At best they could be subtracted as 1:2. Here's a link to one source of fiber content for a pretty good sized list of foods: Fiber Contents