So What Is Resistant Starch Anyway?
The common run of the mill starch that we are all familiar with is a long chain ( a polymer in chemistry terminology ) of glucose molecules occurring as either amylopectin or amylose in most plants. Proportionally speaking, starch is comprised of 2/3 amylopectin and 1/3 amylose. It turns out that structurally, amylopectin and amylose very closely resemble glycogen, so much so that one of the first informal names for glycogen was animal starch. This being the case, it should be of no surprise that human beings have no trouble in breaking off individual glucose molecules from the polymer via the amylase family of enzymes to use as an energy substrate.
When it comes down to how to make a glucose chain, you tend to be spoilt for choice since glucose is a 6 carbon molecule and each carbon has the capacity to form 4 bonds, giving us roughly 24 binding sites. In practice, however, the vast majority of bonds in amylopectin and amylose chains are formed between the first and fourth glucose carbon atoms ( 1-4 bonds ) while a very small portion of the bonds in amylose chains form between the first and last carbon atoms of the glucose molecule ( 1-6 bonds ).
The Many Flavours of Amylase
You may recall that I mentioned earlier that there is a family of amylase enzymes. There are in fact three variants: alpha, beta, and gamma amylase. All three variants are capable of breaking down glucose 1-4 bonds, but only gamma amylase can process 1-6 bonds. The reality for you, if you are a mammal, is that you only possess alpha amylase, so any starchy food that you eat which has 1-6 glucosidic bonds will pass through your stomach and small intestine and reach your large intestine intact. Or, in other words, 1-6 glycosidic bonds are "resistant" to your amylase.
What's So Great About Carbohydrates That Pass Intact Into the Large Intestine?
Well, in and of itself, nothing, really. However, you do have fairly substantial bacterial colonies in your colon, and it turns out that some of these can ferment various carbohydrates that make it through the digestive tract to reach them. The end products of this fermentation can include short chain fatty acids (SCFA) like acetate, propionate and butyrate, and those in turn serve to nourish the cells lining your colon. Any excess SCFAs are absorbed from the colon and oxidized by the host ( you ).
Some studies have associated colonic butyrate with decreased rates of colon cancer, so there may be something to be said for the occasional butyrate high colonic. But, does this mean that we need to go out of our way to consume indigestible carbohydrates in order to feed our intestinal mucosa?
All that Glitters is not Gold
Any fermentation that goes on in the large intestine occurs principally for the benefit of the bacterial fermentor, it is only incidental that the host ( you ) derives some benefit. Of the end products of fermentation there are two general categories: energetic compounds and wastes. The energetic compounds are extracted and retained by the fermenting bacteria, while the waste products are eliminated to the greater environment. Ultimately then, the SFAs being generated by fermentation are actually a waste product from the perspective of the fermentor. Given this, you might find yourself wondering about what other waste products might result? Typically, these include hydrogen, methane, carbon dioxide, and hydrogen sulfide gases.
Depending on your sense of humour, you might find this amusing, particularly if you find yourself at a posh dinner party where some other guest, hopefully someone you don't particularly like, is undergoing a significant amount of colonic fermentation. But if you reflect for a moment about the many individuals who are lactose intolerant and the degree of intestinal problems they experience from drinking, say, one glass of milk, you might find the prospect of colonic fermentation a bit more sobering.
Consider that in a lactose intolerant individual, the operative mechanism is a deficiency of the enzyme necessary to hydrolyze the lactose disaccharide into its constituent monosaccharides, glucose and galactose. As a result of this, lactose passes through the stomach and small intestine intact … at which point we pick up the earlier thread about the dubious benefits of carbohydrates passing intact into the large intestine.
Whole milk contains approximately 5% lactose, so a 200 ml glass would provide you about 10g of lactose. Which is to say that the difference between the lactose tolerant and someone who is forced to make a mad dash to the bathroom to undergo a bout of explosive diarrhea is a mere 10g of a resistant carbohydrate.
What if I'm Hell Bent on Colonic Fermentation?
You may be fortunate in that you are not lactose intolerant, but it is a safe bet that you are raffinose and stachyose intolerant because humans lack the galactosidase enzyme necessary to metabolize these oligosaccharides. Since you cannot enzymatically process either raffinose or stachyose, these carbohydrates are "resistant" to your digestive efforts.
Beans are relatively high in both of these sugars, so in order to simulate the effects of a 10g lactose load in a lactose intolerant individual you would need to eat 1kg of cooked beans. Of course, for the truly hardcore, don't cook your beans, and you triple the oligosaccharide dose per bean, so only 300g needed … be advised that you may come to hate your bowel movements, however.
Alternatively, since there does seem to be an infatuation with all things potato of late, you could attempt to do this by eating 2.4 kilograms of cold cooked potatoes. If you insist on eating your potatoes warm, however, you're then looking at approximately 5 kgs of potatoes, or 11 lbs. worth.
But Butyrate is Really Good, No?
Actually, yes, butyrate is a reasonably beneficial short chain fatty acid. But there is more than one way to skin the butyrate cat. If we are intent on getting some butyrate in the colon, then the trick is to find some way to bypass digestion and have carbohydrates made available for the intestinal microflora to ferment, but I'll save that for a later post.