Foods for appetite control: can carbohydrates stimulate the ileal brake and enhance satiety?

Abstract

New therapeutic approaches targeting appetite suppression for the management of body weight and prevention of obesity are currently of widespread interest. The research within this thesis has focused on the ileal brake, which is an endogenous appetite suppression mechanism that is activated by the presence of macronutrients in the ileum. The aim of the thesis was to investigate whether this mechanism can be activated firstly by direct delivery of glucose, and if so, secondly through oral delivery of protected available carbohydrate (CHO) from foods into the ileum. If resulting in suppression of appetite and food intake, it was anticipated that the findings in this thesis may have important implications for the development of food-derived appetite suppressants and/or therapeutics. The first clinical trial (Chapter 4) in this thesis used a model in which 14 healthy male volunteers were intubated with a 3.8 m long naso-ileal (NI) tube, the aim of which was to deliver glucose directly to the ileum, the distal small intestine (SI). The trial was a 5-day, randomised, residential, cross-over design investigating delivery of glucose (15 g) and saline to both the ileum and the duodenum. In this carefully controlled NI tube feeding study, there was evidence that the direct delivery of glucose into the ileum enhanced aspects of subjectively-rated satiety,glucagon-like-peptide-1 (GLP-1) and peptide YY (PYY) release, and decreased short-term energy intake (EI) compared with the direct delivery of saline into the ileum (-10%, -481 kJ, trend) and glucose into the duodenum (-22%, -988 kJ, Tukey’s post-hoc, P < 0.05) at an ad libitum lunch meal. An in vitro study (Chapter 5) was conducted to investigate the mechanisms by which intestinal enteroendocrine L-cells in the ileum couple glucose sensing to GLP-1 secretion using NCI-H716 cells, a human L-cell line model. The findings that GLP-1 secretion was triggered by glucose and inhibited by a well-established sGLT-1inhibitor and the presence of sGLT-1 transcript in the NCI-H716 cells suggest that the action of sGLT-1 is involved in glucose sensing. The findings that GLP-1 secretion was triggered by fructose and the presence of TAS1R2 receptor and α-gustducin transcripts also suggest that the activation of sweet taste receptors may participate in the regulation of the secretion of GLP-1. In light of these results, it is possible to infer that glucose-induced GLP-1 secretion from L-cells in the ileum is a consequence of the combined effect of sGLT-1 and activation of the sweet taste receptors. The aim of the second clinical trial (Chapter 6) was to compare the effects of low-dose (500 mg) and high-dose (1500 mg) commercially available grape seed extract (GSE), hypothesised to induce post-meal CHO malabsorption in the proximal SI with a placebo control, consumed at the same time as a standardised high-starch breakfast meal on appetite over the subsequent 3-4 hours and EI at a subsequent ad libitum lunch meal in 20 healthy males. The trial was a randomised, double blind, placebo controlled, 3-treatment, cross-over design in which GSE and placebo were delivered using gastric resistant (GR) capsules, intended to protect the contents from degradation in the stomach and to maximise the delivery of active components through the SI, and maximise contact with the starch-rich meal. This study found that GSE at both doses were safe and well tolerated, but did not significantly modulate appetite and EI when compared to a matched placebo. Nevertheless, there were two key methodological issues highlighted for the conduct of a future trial. Firstly, GR capsules being gastric acid resistant have the advantage of protecting their contents from degradation in the stomach, but also delay their release. The absence of response to encapsulated GSE may possibly be attributed to the disintegration and release times of the capsule. Secondly, the use of plant extract with the potential to inhibit the uptake of glucose in the proximal SI, incorporating GSE, which may further promote the delivery of glucose to the ileum. The aim of the third clinical study (Chapter 7) was to investigate the effects of GSE and onion skin extract (OSE) individually at dose of 1500 mg and in a combination at dose of 750 mg each, with the potential to suppress starch digestion and/or the absorption of glucose in the proximal SI, versus acarbose (a pharmaceutical starch blocker) at a dose of 50 mg and placebo control in 20 healthy male volunteers. The trial was a randomised, double blind, placebo controlled, 5-treatment, cross-over design. Each treatment was consumed 60 minutes prior to a standardised high-starch breakfast meal, taking the estimated disintegration time of the GR capsule into consideration, on glycemic response and appetite over the subsequent 4-5 hours and EI at a subsequent ad libitum lunch meal in healthy males. The trial was a randomised, double blind, placebo controlled, 5-treatment, cross-over design. There was evidence in support of short-term suppression of glycemic response by GSE, suggesting that a degree of CHO malabsorption may have been induced by reducing starch digestion. However, there was little evidence of suppression of appetite and EI over the immediate 4-5 hr post prandial period following consumption of the breakfast meal. In light of the results of this study, it was of interest to determine the extent of the ability of plant extracts to induce CHO malabsorption in the proximal SI to deliver sufficient glucose to the ileum from a starch-based meal in a clinical setting. A non-randomised, open-labelled, and non-blinded pilot clinical trial (Chapter 8) in 4 healthy subjects with an ileostomy in whom it is possible to sample the contents of the ileum directly was conducted to determine whether glucose and any other starch-derived available CHOs can be detected within the ileum, when consumed with GSE and quercetin. It was of particular interest in this study firstly to assess efficacy of GSE at a higher dose, i.e. 2000 mg, and of quercetin and secondly to determine why there was a short duration of the effect of acarbose as a positive control on postprandial glycemic responses observed in the previous clinical study. A total of 14 different treatments were prepared and participants were able to choose the total number of treatment. GSE and quercetin individually at doses of 500 and 2000 mg and a combination at a dose of 2000 mg each were administered. They were delivered using either GR capsules 60 minutes prior to a standard high-starch breakfast or non-GR vegetable capsules at the same time as a standard high-starch breakfast. Positive control, acarbose at a dose of 50 mg was given in GR capsules 60 minutes prior to a standard high-starch breakfast or in non-GR vegetable capsules at the same time as a standard high-starch breakfast. Difficulty in recruiting and retaining healthy subjects with an ileostomy led to a small number of eligible participants (n=4). The results of this study did not support whether GSE and quercetin have the ability to induce CHO malabsorption in subjects with an ileostomy. However, the postprandial blood levels of glucose in response to acarbose administration observed in a single participant provided further support for the evidence indicating that the efficacy of acarbose varies with timing of administration, exerting its maximum effect at the start of a meal and possibly explains why there was a short duration of the effect of acarbose as a positive control on postprandial glycemic responses observed in the previous clinical study. In conclusion, this thesis has demonstrated for the first time in humans that the ileal brake mechanism of suppression of appetite and food intake can be induced by glucose, if protected from absorption in the proximal SI and allowed to travel distally into the ileum through a NI tube. However, additional studies with a larger sample size are required to confirm these effects. Moreover, the challenge to trigger the ileal brake mechanism through oral delivery of protected CHO to deliver glucose to the ileum remains.