I've been doing a lot of research on fiber over the past week or so in an effort to sort out the "bottom line" of fiber, carbs and calories. I invite constructive criticism of the conclusion I have come to -- I actually hope to be wrong on this! -- but that conclusion isn't going to be good news to a good many ears here.
That is that 1 g soluble dietary fiber (SDF) = ~ 0.5g net glucose carbs!
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. Bonus for just about anyone wanting to lose weight regardless of the strategy employed. (I know this is gross, but think corn on the cob )
But as I've learned (thanks Jimmy!) recently, SDF may not be metabolized by the body, but it is fermented in the large intestine to short chain fatty acids (SCFA) that are absorbed in the large intestine 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)
So far so good, right?! Well the curious scientist in me was interested in what is to become of all 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!
Not so fast, unfortunately.
As it turns out, the SCFA's produced from SDF's are prime substrates for gluconeogenesis!!
According to this http://books.google.com/books?id=y2rvBwM...#PPA141,M1
(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 & 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 here (including KlediKlutz's ongoing experiments now including Julian bread) 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.
It gets worse It turns out 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. WHAMMO For those who may stall on soluble fiber, at least be consoled that you have a healthy and efficient digestive system
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: http://huhs.harvard.edu/assets/File/OurS..._Fiber.pdf
So my apologies for saying that those that go by total carbs were really just VLC, because that statement was based on the simplified view of fiber being totally non-nutritive. Now I know better, and total carbs may be the way to go with low carb products that contain high levels of soluble fiber -- my now beloved shirataki noodles are entirely SDF. I mostly stay away from the "frankenfoods" (any baked goods to this point are made with natural fibers), but everyone should also beware of things like polydextrose found in a lot of LC goodies that are classed as a soluble fiber and may therefore be contributing to your carb intake. Also, flaxseeds are about 1/3rd soluble fiber so are not the essentially carb-free food they are thought to be.
So if you're finding low net carbs isn't doing it for you, you may want to take a look at the *kind* of fiber being subtracted out to calculate that net result. Surely this is why Atkins suggested getting most of your carbs from non-starchy veggies because that fiber is mostly IDF.