When it comes to studying the effects of complex dietary carbohydrates on the gut microbiota, resistant starch (RS) is a type of dietary fibre that is receiving increasing attention as a dietary intervention that can benefit the host through mechanisms that include altering the gut microbiota. Although starch is a major energy source in human and animal diets, little is known about the biochemistry, mechanism(s), pathway(s) and cell signalling by which RS affects host nutrition and health.
A recent review, led by Dr. Yulong Yin from the College of Life Sciences at Hunan Normal University and the Chinese Academy of Sciences at the Institute of Subtropical Agriculture from the Ministry of Agriculture in Changsha (China), explores current knowledge about resistant starch’s impact on nutrition through its effects on the gut microbiota.
Three kinds of starch are classified according to the rate and extent of digestion and absorption of starch in the small intestine: rapidly digestible starch (RDS) -the starch fraction that is digested to glucose within 20 min after ingestion and causes a rapid increase in blood glucose levels; slowly digestible starch (SDS) -a starch fraction that is digested to glucose within 20-120 min; and resistant starch (RS) -the starch portion that is digested after 120 min, is not absorbed in the small intestine and is fermented by the gut microbiota within the colon. As has been explained in a previous post, there are 5 types of RS that can be found naturally in vegetable foods (mainly grains, legumes, seeds, tubers and green banana) or can be produced industrially and incorporated afterwards into food products. All these types of starch can act at several sites including the oesophagus, liver, stomach, small intestine and large intestine.
The amylose (starch’s lineal fraction) content varies depending on the different forms of starch, and when its content is high this will imply lower digestible activity of starch by host enzymes and, as a result, a major fraction of undigestible starch reaches the colon and is accessible to the commensal gut microbiota. RS has the highest content of amylose followed by SDS and finally RDS.
Food processing has significant effects on starch nutritional fractions. For instance, cooking increases RDS and RS content, whereas it decreases SDS content. Besides this, food storage increases both RDS and SDS, but has no effect on RS levels. For instance, cooking and cooling in the fridge (to 4ºC) tubers, grains and legumes increase their content of RS. Besides this, RS content varies also depending on the food variety within a specific category. For instance, RS content of rice varies with rice variety and cooking method. Among different rice varieties and cooking techniques, refrigerated long-grain rice cooked in a conventional rice cooker had the highest RS content and refrigerated short-grain rice cooked in a pressure cooker had the lowest RS content.
The review emphasizes that several experimental and clinical studies have found that RS has positive effects on inflammatory status and it may have a protective role against type 2 diabetes mellitus, cardiovascular health, gastric health, colonic colitis, and chronic kidney disease progression. Underlying mechanisms that could explain the beneficial impact of RS for treating those conditions include improvement of cardiovascular disease morbidity (via multiple mechanisms, such as decreasing serum triacylglycerols), attenuating inflammatory markers and reducing oxidative stress. Further human studies with bigger samples and longer follow-up periods are needed to explore the impact of RS in those conditions.
In addition, RS has specific beneficial effects on the gut environment including increased populations of Ruminococcus bromii -a dominant member of the phylum Firmicutes that plays a primary role in releasing energy from dietary starches that escape digestion by host enzymes through its exceptional activity against particulate resistant starches. Besides this, ingestion of food products rich in RS has been shown to increase luminal short-chain fatty acid levels, modulate microbial metabolism and improve markers of glucose homeostasis and insulin sensitivity. Interestingly, the increased butyric acid after administration of RS is a variable response that depends on each person’s individual gut microbiome. These effects of RS on the gut environment suggest that RS can have a positive impact on the physiological functions of the gut microbiota and on the host: metabolic activities, trophic effects on intestinal epithelia and on immune structure and function, and protection of the colonised host against invasion by pathogens.
Regarding cell signalling pathways that can explain physiological effects of RS, using animal models it has been well documented that RS has strong anti-inflammatory properties, possibly through the induction of a regulatory T cell response, and direct and indirect effects related to increased peptide YY (PYY) -a hormone made in the small intestine in response to a meal, that helps to reduce appetite and limit food intake- and glucagon-like peptide (GLP)-1 levels and reduced protein levels of free fatty acids and interleukin (IL)-6 via either the inflammatory IL-10 pathway or GLP-1 receptor hormone pathways. These effects suggest that RS may have a role in weight gain not only by targeting gut microbiota, but also by inducing alterations in appetite regulating hormones. Supplementation of RS has also represented an approach for treating acute diarrhoea in children via increasing production of short-chain fatty acids, which are absorbed by colonic epithelial cells and enhance Na-dependent fluid absorption, therefore resulting in conservation of fluid and electrolytes. Distal effects of RS beyond the gut include, among others, attenuation of disruption in vitamin D homeostasis in type I diabetic rats.
However, some undesirable effects have been reported with a RS-supplemented diet in mice, including lack of weight gain and increased anxiety-related behaviours. These data suggest further research is needed in order to elucidate the role of diets rich in RS in humans and animal models.
In conclusion, when studying the role of complex carbohydrates in nutrition RS should be included based on the increasing amount of evidence that supports its effects on host health via modulating the gut microbiota. Although current research in this area is at an early stage, further studies will depict a clearer picture on specific recommendations for including RS as a part of the daily diet.
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