While you sit around, your gut microbes ferment carbohydrates into short-chain fatty acids (SCFA) such as acetate, propionate, and butyrate. They also convert dietary and endogenous nitrogenous compounds into ammonia and microbial protein, while synthesizing and activating B vitamins and vitamin K.
The amount and variety of SCFAs produced are determined by the types of food ingested, how long the food stays in the gut, and which microbial species are present. The benefits of all these reactions result in increased energy for the gut epithelial cells, balanced absorption of salt and water, nitrogen recycling, breakdown of complex lipids and cholesterol, and detoxification of waste compounds.
The biggest impact a person can have on levels of SCFAs is what they eat. Research has consistently found that low carbohydrate diets reduce production of SCFAs, while high carbohydrate diets increase production. So in this case, consuming more carbohydrates is a good thing. This production of SCFAs is primarily due to bacterial fermentation of resistant starch (RS) or non-starch polysaccharides (NSP).
Resistant starch is considered a third type of dietary fiber. In fact, it’s so amazing that it even has its own Facebook page, but so does my dog. Though I didn’t see a Facebook page for soluble fibers, trans fats or vitamin C, so not all nutrients get them. Considering that very low-carbohydrate diets are often used to promote weight loss, it’s good to know how quickly these negative effects can happen.
In a cross-over design, Russell, et al., compared a maintenance diet with both a high-protein/moderate-carbohydrate diet (HPMC) and a high-protein/low-carbohydrate diet. After just 4 weeks, the HPLC diet resulted in decreased proportions of butyrate, which also greatly reduced concentrations of fiber-derived, antioxidant phenolic acids such as ferulate. In addition, the HPLC diet increased several hazardous metabolites.
In another study comparing a very low-carbohydrate/high-fat diet (LC) with a high-carbohydrate/high-fiber/low-fat diet (HC), significant reductions in butyrate, total SCFAs, and counts of bifidobacteria were seen in the LC group.
So, how long does it take to increase SCFAs or butyrate?
A randomized cross-over study by McOrist, et al., compared the effects of foods supplying 25g of NSP with 25g of NSP plus 22g of RS/d over 4 weeks. Butyrate levels increased most significantly in response to the RS. The study found that 27% of the butyrate level was due to BMI, 16% to protein intake, 6% to starch intake, 2% to carbohydrate intake, 4% to fiber intake, and 2% to the intake of fat. Butyrate levels also decreased exponentially with slower fecal transport. After 50 hours, the level was almost zero. Total SCFAs (except butyrate) were lower in females than in males especially during the luteal phase of the ovarian cycle.
In summary, higher intake of carbohydrates and fiber (specifically RS) can increase SCFAs - butyrate levels, in particular. And that’s a good thing! However, you probably already knew that.
~ Dr. Elizabeth Redmond
- Russell WR, et al. Am J Clin Nutr. 2011 May;93(5):1062-72.High-protein, reduced-carbohydrate weight-loss diets promote metabolite profiles likely to be detrimental to colonic health.
- McOrist AL, et al. J Nutr. 2011 May;141(5):883-9. Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch.