Short chain fatty acids (SCFAs) are produced in the large intestine by the action (fermentation) of bacteria on undigested carbohydrates. SCFAs are absorbed through the colon wall. Those little fatty acids are in fact responsible for providing us with a few more calories from the food we eat. Just a few. Maybe accounting for about 10% of our total caloric intake 1, 2
What Type of Carbohydrates Produce SCFAs?
Carbohydrates that fail, for whatever reason, and there are a number of reasons, to get digested and absorbed before they reach the large intestine have the potential to produce SCFAs. Two types of carbohydrates that result in the production of SCFAs are oligosaccharides and resistant starch. Neither of these are technically considered dietary fiber, although they act in similar ways.
They're not monosaccharides, like glucose and fructose.
They're not disaccharides, like table sugar.
They're not long-chain polysaccharides like starch.
They're still a carbohydrate, and are included in the "Total Carbohydrates" line (not the Dietary Fiber line) of the Nutrition Facts label. Their chain length is slightly longer than that of a disaccharide. Because of that structural difference most of this carbohydrate escapes digestion and ends up in the colon, contributing little to post-meal glucose levels.
Sources: legumes (beans, peas) and any member of the onion/garlic family.
Resistant starch is simply starch, a polysaccharide, that resists digestion. As with oligosaccharides above, resistant starch is included in the "Total Carbohydrates" line (not the Dietary Fiber line) of the Nutrition Facts label. Almost 100% of this starch escapes digestion and ends up in the colon. It results in little or no blood glucose rise.
Sources: legumes, raw potato (but who could eat one!), green banana, cooled cooked potatoes, cooled cooked grains, cooled cooked pasta (cooling allows the heated starch to rearrange itself making it difficult for our bodies to digest), and high-amylose starch of the type found in some corn and rice. (See my post True or False: A Carb is a Carb is a Carb, for a picture of amylose starch, and for an explanation of how it resists digestion.)
I'm discovering many good things about these fiber-like non-fibers, so a few more posts are coming that will go into detail. Here, I'll just concentrate on the one benefit I mentioned at the start: SCFAs' ability to moderate blood glucose.
The two studies below demonstrate the phenomenon of the "second meal effect" (SME), where certain carbohydrates in a preceding meal improve glucose tolerance in subsequent meals.
1. Colonic Fermentation Of Indigestible Carbohydrates Contributes To The Second-Meal Effect (2006)
"Our results show that fermentable carbohydrates, independent of their effect on food GI, have the potential to improve postprandial responses to a second meal by decreasing NEFA competition for glucose disposal and, to a minor extent, by affecting intestinal motility."2. Effects Of GI And Content Of Indigestible Carbohydrates Of Cereal-Based Evening Meals On Glucose Tolerance At A Subsequent Standardised Breakfast (2006)
"The reduction in blood glucose IAUC (0–120 min) at the breakfast was approximately 46%, following the evening meal with boiled barley kernels as compared with the evening meal with WWB."Eating barley for dinner almost halved total blood glucose (area under the curve) during the 2 hours following breakfast the next day when compared to eating white bread for dinner. (There was an almost 10 mg/dl lower blood sugar at 30 and 45 minutes after breakfast.) Barley also resulted in lower blood sugars the morning after dinner meals of spaghetti or spaghetti+wheat bran.
"The improved glucose tolerance at breakfast, following an evening meal with barley kernels appeared to emanate from suppression of FFA levels, mediated by colonic fermentation of the specific indigestible carbohydrates present in this product."
The mediator of the second meal effect is thought to be SCFAs produced by bacteria in the colon.
The mechanism for the second meal effect is thought to be, primarily, a reduction in competition between glucose and fat uptake after a meal - resulting in improved glucose clearance and lower blood sugar.
A lesser, although demonstratable, mechanism for the SME is thought to be decreased gastric emptying.
With these two mechanisms in operation - the foods we eat hours and up to a day after a meal that produces SCFAs, even if those subsequent meals contain a lot of easily digestible (high GI) carbohydrate, will be digested more slowly, and the glucose that enters the bloodstream afterwards will be cleared faster.
The examples above are why I say that reducing carbohydrates is not the only way to reduce blood glucose. It's not as simple as carbs in, carbs out (to the bloodstream), at least as I'm coming to understand it. In the same vein as Taubes' claim that a calorie is not a calorie is not a calorie, it seems that a carb is not a carb is not a carb.
It's not that low-carb diets can't be used to manage blood sugar. Reducing carbohydrate-rich foods (especially highly-refined ones) and eating more fat and protein did help my BG levels. But, at least in my case, the more of certain types of carbohydrates I'm eating, the lower my blood sugars. Increasing the amount of whole grains (whole oats, whole hulled barley, whole quinoa) and beans in my diet has pushed my sugars lower than they were when I was eating mostly meat and vegetables. The real surprise for me is that I've been eating some cooled, cooked sweet potato almost daily for a few months now and my blood sugars are in the 80s afterwards.
2 Stomach Bug Makes Food Yield More Calories
Photo of boiled hulled barley: Homegrown.