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Growing evidence from animal studies supports the anti-diabetic properties of some dietary polyphenols, suggesting that dietary polyphenols could be one dietary therapy for the prevention and management of Type 2 diabetes. This review aims to address the potential mechanisms of action of dietary polyphenols in the regulation of glucose homeostasis and insulin sensitivity based on in vitro and in vivo studies, and to provide a comprehensive overview of the anti-diabetic effects of commonly consumed dietary polyphenols including polyphenol-rich mixed diets, tea and coffee, chocolate and cocoa, cinnamon, grape, pomegranate, red wine, berries and olive oil, with a focus on human clinical trials. Dietary polyphenols may inhibit α-amylase and α-glucosidase, inhibit glucose absorption in the intestine by sodium-dependent glucose transporter 1 (SGLT1), stimulate insulin secretion and reduce hepatic glucose output. Polyphenols may also enhance insulin-dependent glucose uptake, activate 5′ adenosine monophosphate-activated protein kinase (AMPK), modify the microbiome and have anti-inflammatory effects. However, human epidemiological and intervention studies have shown inconsistent results. Further intervention studies are essential to clarify the conflicting findings and confirm or refute the anti-diabetic effects of dietary polyphenols.

Background: Limited evidence suggests that a higher ratio of protein to carbohydrate during weight loss has metabolic advantages. Objective: The objective was to evaluate the effects of a diet with a high ratio of protein to carbohydrate during weight loss on body composition, cardiovascular disease risk, nutritional status, and markers of bone turnover and renal function in overweight women. Design: The subjects were randomly assigned to 1 of 2 isocaloric 5600-kJ dietary interventions for 12 wk according to a parallel design: a high-protein (HP) or a high-carbohydrate (HC) diet. Results: One hundred women with a mean (±SD) body mass index (in kg/m²) of 32 ± 6 and age of 49 ± 9 y completed the study. Weight loss was 7.3 ± 0.3 kg with both diets. Subjects with high serum triacylglycerol (>1.5 mmol/L) lost more fat mass with the HP than with the HC diet (x̄± SEM: 6.4 ± 0.7 and 3.4 ± 0.7 kg, respectively; P = 0.035) and had a greater decrease in triacylglycerol concentrations with the HP (−0.59 ± 0.19 mmol/L) than with the HC (−0.03 ± 0.04 mmol/L) diet (P = 0.023 for diet × triacylglycerol interaction). Triacylglycerol concentrations decreased more with the HP (0.30 ± 0.10 mmol/L) than with the HC (0.10 ± 0.06 mmol/L) diet (P = 0.007). Fasting LDL-cholesterol, HDL-cholesterol, glucose, insulin, free fatty acid, and C-reactive protein concentrations decreased with weight loss. Serum vitamin B-12 increased 9% with the HP diet and decreased 13% with the HC diet (P < 0.0001 between diets). Folate and vitamin B-6 increased with both diets; homocysteine did not change significantly. Bone turnover markers increased 8–12% and calcium excretion decreased by 0.8 mmol/d (P < 0.01). Creatinine clearance decreased from 82 ± 3.3 to 75 ± 3.0 mL/min (P = 0.002). Conclusion: An energy-restricted, high-protein, low-fat diet provides nutritional and metabolic benefits that are equal to and sometimes greater than those observed with a high-carbohydrate diet.

Animal studies indicate that the composition of gut microbiota may be involved in the progression of insulin resistance to type 2 diabetes. Probiotics and/or prebiotics could be a promising approach to improve insulin sensitivity by favourably modifying the composition of the gut microbial community, reducing intestinal endotoxin concentrations and decreasing energy harvest. The aim of the present review was to investigate the effects of probiotics, prebiotics and synbiotics (a combination of probiotics and prebiotics) on insulin resistance in human clinical trials and to discuss the potential mechanisms whereby probiotics and prebiotics improve glucose metabolism. The anti-diabetic effects of probiotics include reducing pro-inflammatory cytokines via a NF-κB pathway, reduced intestinal permeability, and lowered oxidative stress. SCFA play a key role in glucose homeostasis through multiple potential mechanisms of action. Activation of G-protein-coupled receptors on L-cells by SCFA promotes the release of glucagon-like peptide-1 and peptide YY resulting in increased insulin and decreased glucagon secretion, and suppressed appetite. SCFA can decrease intestinal permeability and decrease circulating endotoxins, lowering inflammation and oxidative stress. SCFA may also have anti-lipolytic activities in adipocytes and improve insulin sensitivity via GLUT4 through the up-regulation of 5'-AMP-activated protein kinase signalling in muscle and liver tissues. Resistant starch and synbiotics appear to have favourable anti-diabetic effects. However, there are few human interventions. Further well-designed human clinical studies are required to develop recommendations for the prevention of type 2 diabetes with pro- and prebiotics.

The health care costs of overweight, obesity, and type 2 diabetes are increasing,¹ and conventional weight-loss diets with daily energy restriction are difficult to adhere to in the long term.² Intermittent energy restriction, short periods of severe energy restriction followed by longer periods of habitual eating, has been suggested as a potential strategy because it offers a reduced burden of dietary restriction and may therefore be more acceptable.³ In animal studies, intermittent energy restriction appears to be equally effective as, if not more effective than, continuous energy restriction for the reduction of disease risk factors.⁴ Studies in the healthy overweight and obese population have determined that intermittent energy restriction is an effective method for achieving weight loss comparable to that achieved by continuous energy restriction, with weight loss of 3 to 5 kg after approximately 10 weeks.⁵ Other methods of intermittent energy restriction, such as alternate-day modified fasting, have also demonstrated similar weight loss compared with that achieved by continuous energy restriction.⁶ For individuals with type 2 diabetes, investigators have evaluated the effects of intermittent very-low-calorie diets used within a continuous energy restriction diet compared with continuous energy restriction alone,^7-9 but they have not compared intermittent dieting with continuous dieting. We previously published a pilot trial that demonstrated that a 2-day intermittent energy restriction diet provides an effective alternative method for both glycemic control and weight loss compared with a continuous energy restriction diet over a 3-month period.¹⁰ The objective of this randomized noninferiority trial was to ascertain the long-term effects of a 2-day intermittent energy restriction diet compared with continuous energy restriction during a 12-month period for patients with type 2 diabetes. We hypothesized that equal improvements to hemoglobin A_1c (HbA_1c) level and weight would occur, as seen in our 3-month pilot trial, thus offering a successful alternative treatment strategy for use in clinical practice.

Understanding and manipulating how emulsion structure impacts on fat digestion is an important step towards understanding the role of fat in our diet. This article reports on the nature of emulsion structuring within the digestive tract and how it affects the dynamics of fat digestion. Emulsions were designed a priori to have specific structuring behaviours (stable, coalesced, partially coalesced and fully broken) under gastrointestinal conditions, through careful emulsifier selection and control of solid fat composition. The impact these structures had on lipolysis was then assessed in vitro using a digestion model and in vivo by measuring the postprandial change in blood triglyceride concentration as a marker of fat absorption. The major factor controlling the rate of fat digestion in vitro was the droplet surface area available for lipase adsorption, which was governed by emulsion instability. The rate of fat absorption in vivo was only affected by large changes in the droplet surface area, and only if these changes remained until the droplets reached the small intestine. This was most evident in emulsions that had undergone extensive partial coalescence under gastric conditions. Partial coalescence resulted in a dramatic reduction in triglyceride absorption, in part because the network of fat crystals provided the agglomerates with an internal scaffold to resist re-dispersion as they passed through the pylorus. The differences in fat absorption profile achieved by controlling emulsion structural stability during digestion provide a basis for examining the physiological effects of food structure on lipid metabolism, which will be the subject of a follow-up clinical paper.