Michael J. Keenan

Dietary resistant starch upregulates total GLP-1 and PYY in a sustained day-long manner through fermentation in rodents.

Glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) are anti-diabetes/obesity hormones secreted from the gut after meal ingestion. We have shown that dietary-resistant starch (RS) increased GLP-1 and PYY secretion, but the mechanism remains unknown. RS is a fermentable fiber that lowers the glycemic index of the diet and liberates short-chain fatty acids (SCFAs) through fermentation in the gut. This study investigates the two possible mechanisms by which RS stimulates GLP-1 and PYY secretion: the effect of a meal or glycemic index, and the effect of fermentation. Because GLP-1 and PYY secretions are stimulated by nutrient availability in the gut, the timing of blood sample collections could influence the outcome when two diets with different glycemic indexes are compared. Thus we examined GLP-1 and PYY plasma levels at various time points over a 24-h period in RS-fed rats. In addition, we tested proglucagon (a precursor to GLP-1) and PYY gene expression patterns in specific areas of the gut of RS-fed rats and in an enteroendocrine cell line following exposure to SCFAs in vitro. Our findings are as follows. 1) RS stimulates GLP-1 and PYY secretion in a substantial day-long manner, independent of meal effect or changes in dietary glycemia. 2) Fermentation and the liberation of SCFAs in the lower gut are associated with increased proglucagon and PYY gene expression. 3) Glucose tolerance, an indicator of increased active forms of GLP-1 and PYY, was improved in RS-fed diabetic mice. We conclude that fermentation of RS is most likely the primary mechanism for increased endogenous secretions of total GLP-1 and PYY in rodents. Thus any factor that affects fermentation should be considered when dietary fermentable fiber is used to stimulate GLP-1 and PYY secretion.

Effects of Resistant Starch, A Non-digestible Fermentable Fiber, on Reducing Body Fat

Objective: To assess the effects of energy dilution with non‐fermentable and fermentable fibers on abdominal fat and gut peptide YY (PYY) and glucagon‐like peptide (GLP)‐1 expressions, three rat studies were conducted to: determine the effects of energy dilution with a non‐fermentable fiber, compare similar fiber levels of fermentable and non‐fermentable fibers, and compare similar metabolizable energy dilutions with fermentable and non‐fermentable fibers.

Peptide YY and Proglucagon mRNA Expression Patterns and Regulation in the Gut

Objective: Peptide YY (PYY) and glucagon‐like peptide‐1 are important in the control of energy homeostasis and are both secreted from the gut in response to ingested nutrients. However, more studies are needed on nutrient regulation of their gene expression patterns in specific areas of the gut. This study detailed PYY and proglucagon (the gene that encodes glucagon‐like peptide‐1) gene expression patterns and regulation in the gut. We further examined the regulation of PYY and proglucagon mRNA by a diet containing fermentation‐resistant starch (in vivo) and butyrate (in vitro).

Uncoupling of Inflammation and Insulin Resistance by NF-κB in Transgenic Mice through Elevated Energy Expenditure

To study the metabolic activity of NF-κB, we investigated phenotypes of two different mouse models with elevated NF-κB activities. The transcriptional activity of NF-κB is enhanced either by overexpression of NF-κB p65 (RelA) in aP2-p65 mice or inactivation of NF-κB p50 (NF-κB1) through gene knock-out. In these models, energy expenditure was elevated in day and night time without a change in locomotion. The mice were resistant to adulthood obesity and diet-induced obesity without reduction in food intake. The adipose tissue growth and adipogenesis were inhibited by the elevated NF-κB activity. Peroxisome proliferator-activator receptor γ expression was reduced by NF-κB at the transcriptional level. The two models exhibited elevated inflammatory cytokines (tumor necrosis factor-α and interleukin-6) in adipose tissue and serum. However, insulin sensitivity was not reduced by the inflammation in the mice on a chow diet. On a high fat diet, the mice were protected from insulin resistance. The glucose infusion rate was increased more than 30% in the hyperinsulinemic-euglycemic clamp test. Our data suggest that the transcription factor NF-κB promotes energy expenditure and inhibits adipose tissue growth. The two effects lead to prevention of adulthood obesity and dietary obesity. The energy expenditure may lead to disassociation of inflammation with insulin resistance. The study indicates that inflammation may prevent insulin resistance by eliminating lipid accumulation.

Effect of boron on vitamin D deficient rats

The effects of different levels of dietary boron were determined in vitamin D deficient rats. Vitamin D deficient diets containing either 0.158 ppm or 2.72 ppm of boron were fed to rats for 11 w, and calcium, magnesium, and phosphorus apparent absorption and balance were measured in the twelfth week. Higher apparent absorption and balance values for calcium and phosphorus were observed in the rats with higher dietary boron, but very few differences were seen in body wt, organ wt, and bone parameters. Balance measurements represented the present status of the rats after 12 w on the diets, but other measurements represented an accumulation over the lifetime of the rat, including a suckling period with ample vitamin D and boron. The data demonstrated that when rats are vitamin D deficient, as indicated by hypocalcemia, the level of boron in the diet affects mineral balance.

The intestinal microbiota in aged mice is modulated by dietary resistant starch and correlated with improvements in host responses.

Dietary interventions might prevent or reverse age-related declines in health through modification of the activity and composition of the intestinal microbiota. As a first step toward more comprehensive evaluations of single dietary components on healthy aging, 16S rRNA gene amplicon sequencing was applied to determine the structure of the bacterial communities in the ceca of 20-month-old healthy mice fed energy-controlled diets containing 0, 18, or 36% type 2 resistant starch (RS) from high-amylose maize (HAM-RS2). The cecal microbiota of mice fed a diet depleted in RS and containing the readily digestible carbohydrate amylopectin were dominated by bacteria in the Firmicutes phylum and contained low levels of Bacteroidetes and Actinobacteria. In contrast, mice fed diets containing HAM-RS2 were colonized by higher levels of Bacteroidetes and Bifidobacterium, Akkermansia, and Allobaculum species in proportions that were dependent on the concentration of the dietary fiber. The proportions of Bifidobacterium and Akkermansia were positively correlated with mouse feeding responses, gut weight, and expression levels of proglucagon, the precursor of the gut anti-obesity/diabetic hormone GLP-1. This study showed that aging mice harbor a distinct microbiota, which can be modulated by RS and enriched for bacteria that are associated with improved health.

Dietary resistant starch increases hypothalamic POMC expression in rats.

Resistant starch (RS) is fermentable dietary fiber. Inclusion of RS in the diet causes decreased body fat accumulation and altered gut hormone profile. This study investigates the effect of feeding RS on the neuropeptide messenger RNA (mRNA) expressions in the arcuate nucleus (ARC) of the hypothalamus and whether vagal afferent nerves are involved. The rats were injected intraperitoneally with capsaicin to destroy unmyelinated small vagal afferent nerve fibers. The cholecystokinin (CCK) food suppression test was performed to validate the effectiveness of the capsaicin treatment. Then, capsaicin‐treated rats and vehicle‐treated rats were subdivided into a control diet or a RS diet group, and fed the corresponding diet for 65 days. At the end of study, body fat, food intake, plasma peptide YY (PYY) and glucagon‐like peptide 1 (GLP‐1), and hypothalamic pro‐opiomelanocortin (POMC), neuropeptide Y (NPY), agouti‐related peptide (AgRP) gene expressions were measured. RS‐fed rats had decreased body fat, increased POMC expression in the hypothalamic ARC, and elevated plasma PYY and GLP‐1 in both the capsaicin and vehicle‐treated rats. Hypothalamic NPY and AgRP gene expressions were not changed by RS or capsaicin. Therefore, destruction of the capsaicin‐sensitive afferent nerves did not alter the response to RS in rats. These findings suggest that dietary RS might reduce body fat through increasing the hypothalamic POMC expression and vagal afferent nerves are not involved in this process. This is the first study to show that dietary RS can alter hypothalamic POMC expression.

Role of Resistant Starch in Improving Gut Health, Adiposity, and Insulin Resistance

The realization that low-glycemic index diets were formulated using resistant starch led to more than a decade of research on the health effects of resistant starch. Determination of the metabolizable energy of the resistant starch product allowed for the performance of isocaloric studies. Fermentation of resistant starch in rodent studies results in what appears to be a healthier gut, demonstrated by increased amounts of short-chain fatty acids, an apparent positive change in the microbiota, and increased gene expression for gene products involved in normal healthy proliferation and apoptosis of potential cancer cells. Additionally, consumption of resistant starch was associated with reduced abdominal fat and improved insulin sensitivity. Increased serum glucagon-like peptide 1 (GLP-1) likely plays a role in promoting these health benefits. One rodent study that did not use isocaloric diets demonstrated that the use of resistant starch at 8% of the weight of the diet reduced body fat. This appears to be approximately equivalent to the human fiber requirement. In human subjects, insulin sensitivity is increased with the feeding of resistant starch. However, only 1 of several studies reports an increase in serum GLP-1 associated with resistant starch added to the diet. This means that other mechanisms, such as increased intestinal gluconeogenesis or increased adiponectin, may be involved in the promotion of improved insulin sensitivity. Future research may confirm that there will be improved health if human individuals consume the requirement for dietary fiber and a large amount of the fiber is fermentable.

Failure to ferment dietary resistant starch in specific mouse models of obesity results in no body fat loss

UNLABELLED Resistant starch (RS) is a fermentable fiber that decreases dietary energy density and results in fermentation in the lower gut. The current studies examined the effect of RS on body fat loss in mice. In a 12 week study (study 1), the effect of two different types of RS on body fat was compared with two control diets (0% RS) in C57Bl/6J mice: regular control diet or the control diet that had energy density equal to that of the RS diet (EC). All testing diets had 7% (w/w) dietary fat. In a 16 week study (study 2), the effect of RS on body fat was compared with EC in C57BL/6J mice and two obese mouse models (NONcNZO10/LtJ or Non/ShiLtJ). All mice were fed control (0% RS) or 30% RS diet for 6 weeks with 7% dietary fat. On the seventh week, the dietary fat was increased to 11% for half of the mice and remained the same for the rest. Body weight, body fat, energy intake, energy expenditure, and oral glucose tolerance were measured during the study. At the end of the studies, the pH of cecal contents was measured as an indicator of RS fermentation. Compared with EC, dietary RS decreased body fat and improved glucose tolerance in C57BL/6J mice but not in obese mice. For other metabolic characteristics measured, the alterations by RS diet were similar for all three types of mice. The difference in dietary fat did not interfere with these results. The pH of cecal contents in RS fed mice was decreased for C57BL/6J mice but not for obese mice, implying the impaired RS fermentation in obese mice. CONCLUSIONS (1) decreased body fat by RS is not simply due to dietary energy dilution in C57Bl/6J mice, and (2) along with their inability to ferment RS, RS fed obese mice did not lose body fat. Thus, colonic fermentation of RS might play an important role in the effect of RS on fat loss.

Dietary-resistant starch improves maternal glycemic control in Goto-Kakizaki rat

SCOPE Dietary prebiotics show potential in anti-diabetes. Dietary resistant starch (RS) has a favorable impact on gut hormone profiles, including glucagon-like peptide-1 (GLP-1) consistently released, a potent anti-diabetic incretin. Also RS reduced body fat and improved glucose tolerance in rats and mice. In the current project, we hypothesize that dietary-resistant starch can improve insulin sensitivity and pancreatic β cell mass in a type 2 diabetic rat model. Altered gut fermentation and microbiota are the initial mechanisms, and enhancement in serum GLP-1 is the secondary mechanism. METHODS AND RESULTS In this study, GK rats were fed an RS diet with 30% RS and an energy control diet. After 10 wk, these rats were mated and went through pregnancy and lactation. At the end of the study, pancreatic β cell mass, insulin sensitivity, pancreatic insulin content, total GLP-1 levels, cecal short-chain fatty acid concentrations and butyrate producing bacteria in cecal contents were greatly improved by RS feeding. The offspring of RS-fed dams showed improved fasting glucose levels and normal growth curves. CONCLUSION Dietary RS is potentially of great therapeutic importance in the treatment of diabetes and improvement in outcomes of pregnancy complicated by diabetes.