Recent studies have documented the importance of postprandial hyperglycaemia in the incidence of chronic diseases, including type 2 diabetes. Inhibition of digestive enzymes, including membrane-bound brush-border α-glucosidases, leads to slowed carbohydrate digestion and absorption, and reduced postprandial glycemia. Nuts are widely eaten around the world and have the potential to inhibit α-glucosidases through their content of polyphenols and other bioactive compounds. According to our recent systematic review(1), no study has investigated the inhibitory effects of nut extracts on human α-glucosidase activities. Almost all studies in this area have been conducted on yeast α-glucosidase, with only a few using rat α-glucosidase. While there is no sequence homology between yeast and human α-glucosidase, there is 74% to 78% sequence homology between rat and human α-glucosidases(1). The lack of studies on the effect of bioactive compounds from nuts on human α-glucosidases, along with the growing attention to nuts as an important component of a healthy diet with the potential to reduce the risk of chronic diseases(2), highlights the need for research to evaluate the inhibitory effect of nut extracts on human α-glucosidases. The aim of the current study is to explore the inhibitory effect of extracts from nuts on human carbohydrate digestive enzymes. Walnuts and almonds were ground and defatted with hexane, extracted in 80% (v/v) acetone, and further purified using solid-phase extraction to obtain phenolic-rich extracts. The Folin–Ciocalteu assay was used to approximate the polyphenol content of the samples. Following our recently published detailed protocol(3), cell-free extracts from human intestinal Caco-2/TC7 cells were used as a source of α-glucosidase in enzyme inhibition assays, with sucrose, maltose and isomaltose as substrates and appropriate controls. The assay products were quantified using high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Glucose production in the presence of various concentrations of phenol-rich nut extracts was compared using a one-way ANOVA and half-maximal inhibitory concentration (IC50) values were calculated. The Folin–Ciocalteu data demonstrate that walnut extracts comprise a relatively high polyphenol content, with 18.1 ± 0.23 mg (epigallocatechin gallate [EGCG] equivalent) per gram of fresh weight, while almond extracts contain 0.87 ± 0.03 mg EGCG equivalent/g of fresh weight. The walnut phenolic-rich extract dose-dependently inhibited human intestinal sucrase and maltase activities (both p<0.01), with IC50 values of 1.67 mg/mL and 2.84 mg/mL, respectively. We demonstrate that phenolic-rich walnut extracts can inhibit human α-glucosidases in vitro and therefore walnuts may contribute to slowing carbohydrate digestion in humans. As such, we plan to assess the effects of walnuts on postprandial glycaemia in vivo.

We previously reported that the addition of a specified mulberry fruit extract (MFE) to rice consistently reduces post-prandial glycaemic (PPG) and post-prandial insulinemic (PPI) responses. This research tested whether this effect generalises to a broad range of rice types, reflecting the wide variation in rice characteristics known to influence glycaemic responses. In a randomised, balanced, partial factorial crossover design, Sona Masoori (SM), Bora Saul (BS), Gobindobogh (Gb) and Banskati (Bn) rices were tested with and without 0·37 g MFE. Healthy, normal-weight Indian adults (N 120) each consumed four of the eight possible boiled rice meals, all containing about 50 g available carbohydrate. The primary outcome was the effect of MFE on PPG, expressed as the percentage change in the positive, incremental AUC over 2 h. The mean effect of MFE on PPG for all rice types combined was −11·4 % (P < 0·003). The reduction in PPG was in a qualitatively similar range for all rice types (–9·8 to −15·1 %), and this was statistically significant for Bn. MFE also reduced the corresponding PPI response to all rice types combined by a mean of 10·1 % (P < 0·001; range −6·1 to −13·4 %), and the reduction in PPI was statistically significant for SM, Gb and BS. In conclusion, addition of 0·37 g MFE modestly reduced PPG and PPI responses to rices in general, and the effects were statistically significant for specific rice types.

Type II diabetes is considered the most common metabolic disorder in the developed world and currently affects about one in ten globally. A therapeutic target for the management of type II diabetes is the inhibition of α- glucosidase, an essential enzyme located at the brush border of the small intestinal epithelium. The inhibition of α-glucosidase results in reduced digestion of carbohydrates and a decrease in postprandial blood glucose. Although pharmaceutical synthetic inhibitors are available, these are usually associated with significant gastrointestinal side effects. In the present study, the impact of inhibitors derived from edible brown algae is being investigated and compared for their effect on glycaemic control. Carbohydrate- and polyphenolic-enriched extracts derived from Ascophyllum nodosum, Fucus vesiculosus and Undaria pinnatifida were characterised and screened for their inhibitory effects on maltase and sucrase enzymes. Furthermore, enzyme kinetics and the mechanism of inhibition of maltase and sucrase were determined using linear and nonlinear regression methods. All tested extracts showed a dose-dependent inhibitory effect of α-glucosidase with IC50 values ranging from 0⋅26 to 0⋅47 mg/ml for maltase; however, the only extract that was able to inhibit sucrase activity was A. nodosum, with an IC50 value of 0⋅83 mg/ml. The present study demonstrates the mechanisms in which different brown seaweed extracts with varying composition and molecular weight distribution differentially inhibit α-glucosidase activities. The data highlight that all brown seaweed extracts are not equal in the inhibition of carbohydrate digestive enzymes involved in postprandial glycaemia.

Polyphenol- and fibre-rich foods (PFRF) have the potential to affect postprandial glycaemic responses by reducing glucose absorption, and thus decreasing the glycaemic response of foods when consumed together. A randomised, single-blind, cross-over study was conducted on sixteen healthy volunteers to test whether PFRF could attenuate postprandial blood glucose in healthy volunteers when added to a source of carbohydrate (starch in bread). This is the first study to examine the effects of a meal comprised of components to inhibit each stage of the biochemical pathway, leading up to the appearance of glucose in the blood. The volunteers were fasted and attended four visits: two control visits (bread, water, balancing sugars) and two test visits (single and double dose of PFRF) where they consumed bread, water and PFRF. Blood samples were collected at 0 (fasted), 15, 30, 45, 60, 90, 120, 150 and 180 min after consumption. The PFRF components were tested for α-amylase and α-glucosidase inhibitory potential in vitro. Plasma glucose was lower after consumption of both doses compared with controls: lower dose, change in mean incremental areas under the glucose curves (IAUC)=−27·4 (sd 7·5) %, P<0·001; higher dose, IAUC=−49·0 (sd 15·3) %, P<0·001; insulin IAUC was also attenuated by−46·9 (sd 13·4) %, P<0·01. Consistent with this, the polyphenol components of the PFRF inhibited α-amylase (green tea, strawberry, blackberry and blackcurrant) and α-glucosidase (green tea) activities in vitro. The PFRF have a pronounced and significant lowering effect on postprandial blood glucose and insulin response in humans, due in part to inhibition of α-amylase and α-glucosidase, as well as glucose transport.

The amitochondriate parasitic protist Trichomonas vaginalis can utilize either glucose or maltose as carbon and energy source. The mechanisms of maltose utilization were explored with uptake experiments using radio-isotope labelled maltose in combination with the silicone-oil centrifugation technique and enzymatic assays measuring maltose hydrolysis. The uptake of maltose label became saturated after 2–3 h. The uptake of maltose as a function of the external maltose concentration was linear at low concentrations with no further increase at higher levels, kinetics characteristic of reactions obeying Michaelis–Menten kinetics preceded by a diffusion-limited step. Increased viscosity of the medium resulted in decreased maltose uptake, indicating an extracellular location of the diffusion-limited step. Most of the cellular α-glucosidase activity of T. vaginalis was detected on the cell surface, suggesting that maltose is hydrolysed to glucose outside the cell. Glucose interfered more with maltose uptake, and maltose less with glucose uptake, than would be expected if 1 mol of maltose were the equivalent of 2 mol of glucose. This pattern of interaction indicated that the interference occurs before the common metabolic pathway and even before the transport step, supporting the idea of extracellular maltose hydrolysis. We conclude that maltose is hydrolysed to glucose in the boundary layer of the cell, a process akin to membrane digestion in vertebrate enterocytes and on the teguments of helminths. The glucose formed is then transported by the glucose carrier of the organism.