Polyphenols and Glucose Homeostasis in Humans

Influence of Harvesting Stage on Phytochemical Composition, Antioxidant, and Antidiabetic Activity of Immature Ceratonia siliqua L. Pulp from Béni Mellal-Khénifra Region, Morocco: In Silico, In Vitro, and In Vivo Approaches

Ceratonia siliqua L. is a medicinal plant that has long been used in traditional Moroccan medicine to treat many diseases. This study aimed to assess the impact of the stages of the immature phase of carob pulp (M1, M2, M3, M4, and M5) on phytochemical composition, antioxidant activity, and antidiabetic activity of Ceratonia siliqua L. The identification of the phenolic profile by HPLC-UV/MS-MS and the study of the antidiabetic effect by in silico, in vitro, and in vivo studies were carried out for extracts with high contents of phenolic compounds from immature wild carob pulp from the communes of Timoulit (TM), Bin Elouidane (AW), and Ouaouizerth (TG) in the province of Azilal in the Béni Mellal-Khénifra region. The results revealed a gradual increase in total sugar content over the pulp's ripening period, reaching a value of 2134 ± 56.23 mg GE/100 g fresh weight (FW) for TG. The three locations showed peak values for total polyphenol content (TPC), total flavonoid content (TFC), and total condensed tannin (TCT) at the M2 stage. AW had the highest concentrations of TPC (3819 ± 226.4 mg GAE/100 g FM), TFC (1034 ± 57.08 mg QE/100 g FM), and TCT (1472 ± 28.46 mg CE/100 g FM). The DPPH assay (7892 ± 296.1 mg TE/100 g FM) and the FRAP assay (278.2 ± 7.85 mg TE/100 g FM) both demonstrated that the TG zone is a highly potent antioxidant zone. In contrast, the AW site exhibited a markedly elevated value of 725.4 ± 103.6 mg TE/100 g FM in the ABTS assay. HPLC-UV-MS/MS analysis showed that the methanolic extracts of immature carob pulp (MEICP) from the three areas contained several different chemical compounds. The most prevalent were 3-O-p-coumaroyl-5-O-caffeoylquinic acid, quercetin 3-methyl ether, gallic acid, and galloylquinic acid. Immature carob pulp extract (ICPE) from AW showed the strongest in vitro inhibition of pancreatic α-amylase (IC50 = 0.405 µg/mL) and TG extracts were most potent against intestinal α-glucosidase (IC50 = 0.063 µg/mL). In vivo, AW, TG, and TM extracts significantly reduced postprandial glycemia in rats, with AW having the greatest effect. These results highlight the antidiabetic potential of ICPE. The 3-O-p-Coumaroyl-5-O-caffeoylquinic acid showed better affinity for α-amylase compared to acarbose and interacted significantly with several amino acid residues of the enzyme. Similarly, this molecule and 3,4-Dicaffeoylquinic acid demonstrated a strong affinity for α-glucosidase, suggesting their potential as natural inhibitors of enzymes involved in carbohydrate metabolism. Most of the compounds are not substrates of P-glycoprotein and exhibited high intestinal absorption. Furthermore, the majority of these compounds did not act as inhibitors or substrates of CYP450 enzymes, reinforcing their suitability for development as oral medications. These results underscore the potential of immature carob pulp as a promising antidiabetic agent.

An Overview of the Potential of Medicinal Plants Used in the Development of Nutraceuticals for the Management of Diabetes Mellitus: Proposed Biological Mechanisms

Diabetes mellitus (DM) is a chronic metabolic disorder in which the pancreas does not produce enough insulin or the body cannot effectively use it. The prevalence of diabetes is increasing steadily, making it a global public health problem. Several serious complications are associated with this disease. There are a number of different classes of antidiabetic medications. Interestingly, traditional medicine can also be used for the development of novel classes of hypoglycemic therapeutics. This article summarizes an update of the potential of various important medicinal plants used in the development of nutraceuticals for the management of diabetes mellitus, and a proposal of their biological mechanisms.

Secondary Metabolites in the Treatment of Diabetes Mellitus: A Paradigm Shift

Diabetes mellitus (DM) is a chronic, polygenic and non-infectious group of diseases that occurs due to insulin resistance or its low production by the pancreas and is also associated with lifelong damage, dysfunction and collapse of various organs. Management of diabetes is quite complex having many bodily and emotional complications and warrants efficient measures for prevention and control of the same. As per the estimates of the current and future diabetes prevalence, around 425 million people were diabetic in 2017 which is anticipated to rise up to 629 million by 2045. Various studies have vaguely proven the fact that several vitamins, minerals, botanicals and secondary metabolites demonstrate hypoglycemic activity in vivo as well as in vitro. Flavonoids, anthocyanin, catechin, lipoic acid, coumarin metabolites, etc. derived from herbs were found to elicit a significant influence on diabetes. However, the prescription of herbal compounds depend on various factors, including the degree of diabetes progression, comorbidities, feasibility, economics as well as their ADR profile. For instance, cinnamon could be a more favorable choice for diabetic hypertensive patients. Diabecon®, Glyoherb® and Diabeta Plus® are some of the herbal products that had been launched in the market for the favorable or adjuvant therapy of diabetes. Moreover, Aloe vera leaf gel extract demonstrates significant activity in diabetes. The goal of this review was to inscribe various classes of secondary metabolites, in particular those obtained from plants, and their role in the treatment of DM. Recent advancements in recognizing the markers which can be employed for identifying altered metabolic pathways, biomarker discovery, limitations, metabolic markers of drug potency and off-label effects are also reviewed.

A phase I single-blind clinical trial to evaluate the safety of oil palm phenolics (OPP) supplementation in healthy volunteers

Plant phenolics are being increasingly consumed globally with limited scientific and clinical evidence pertaining to safety and efficacy. The oil palm fruit contains a cocktail of phenolics, and palm oil production results in high volumes of aqueous by-products enriched in phenolics and bioactives. Several lines of evidence from in vitro and in vivo animal studies confirmed that the aqueous extract enriched in phenolics and other bioactives collectively known as oil palm phenolics (OPP) is safe and has potent bioactivity. A phase one clinical trial was conducted to evaluate the safety and effects of OPP in healthy volunteers. In this single-blind trial, 25 healthy human volunteers were supplemented with 450 mg gallic acid equivalent (GAE)/day of OPP or control treatments for a 60-day period. Fasting blood and urine samples were collected at days 1, 30 and 60. Medical examination was performed during these trial interventions. All clinical biochemistry profiles observed throughout the control and OPP treatment period were in the normal range with no major adverse effect (AE) or serious adverse effect (SAE) observed. Additionally, OPP supplementation resulted in improvement of total cholesterol and LDL-C levels, compared to the control treatment. The outcomes support our previous observations that OPP is safe and may have a protective role in reducing cholesterol levels.

Polyphenol-rich curry made with mixed spices and vegetables benefits glucose homeostasis in Chinese males (Polyspice Study): a dose-response randomized controlled crossover trial

PURPOSE

To investigate acute effects of two doses of a polyphenol-rich curry made with seven different spices and four base vegetables, eaten with white rice, on 24 h glucose response, postprandial insulinemia, triglyceridemia and 24 h urinary total polyphenol excretion (TPE).

METHODS

Randomized, controlled, dose-response crossover trial in healthy, Chinese men [n = 20, mean ± standard deviation (SD) age 23.7 ± 2.30 years, BMI 23.0 ± 2.31 kg/m²] who consumed test meals matched for calories, macronutrients and total vegetables content, consisting either Dose 0 Control (D0C) or Dose 1 Curry (D1C) or Dose 2 Curry (D2C) meal. 24 h glucose concentration was measured using continuous glucose monitoring (CGM), together with postprandial plasma insulin and triglyceride for up to 7 h. Total polyphenol content (TPC) of test meals and urinary TPE were measured using the Folin-Ciocalteu assay.

RESULTS

TPC for D0C, D1C and D2C were 130 ± 18, 556 ± 19.7 and 1113 ± 211.6 mg gallic acid equivalent (GAE) per portion served, respectively (p < 0.0001). Compared with D0C meal, we found significant linear dose-response reductions in the 3-h postprandial incremental AUC (iAUC) for CGM glucose of 19% and 32% during D1C and D2C meals respectively (p < 0.05) and non-significant linear dose response reductions in iAUC of insulin (p = 0.089). Notably, we found significant dose-dependent increases in postprandial triglyceride with increasing curry doses (p < 0.01). Significant increases in TPE with increasing curry doses were also observed (p < 0.01).

CONCLUSION

Polyphenol-rich curry intake can improve postprandial glucose homeostasis. The longer term effects remain to be established.

Ceratonia siliqua L. (immature carob bean) inhibits intestinal glucose absorption, improves glucose tolerance and protects against alloxan-induced diabetes in rat

BACKGROUND

This study was designed to investigate the effects of immature carob pod aqueous extract (ICPAE) on intestinal glucose absorption in vitro and in vivo using an oral glucose tolerance test (OGTT) as well as the potential antidiabetic effect in alloxan-induced diabetic rats. OGTT was carried by administration of glucose (2 g kg^-1 , p.o.) and after treatment with extract (50, 100 and 200 mg kg^-1 body weight). Diabetes was induced by single intraperitoneal injection of alloxan (150 mg kg^-1 ). However, the extracts at various doses or glibenclamide (GLB, 10 mg kg^-1 body weight) were given by oral administration for 2 weeks.

RESULTS

ICPAE (50-2000 µg mL^-1 ) exerted dose-dependent reduction of sodium-dependent glucose transport across isolated mice jejunum and the maximal inhibition exceeded 50%.The ICPAE treatment improved glucose tolerance. More importantly, ICPAE at various doses showed a significant reduction in blood glucose and biochemical profiles in diabetic rats.

CONCLUSION

Our findings confirm that the degree of maturity of carob characterized by a different phytochemical composition may be responsible for these actions. Therefore, these compounds may be used as a food supplement in hyperglycemia and diabetes treatments. © 2016 Society of Chemical Industry.

Addition of Orange Pomace to Orange Juice Attenuates the Increases in Peak Glucose and Insulin Concentrations after Sequential Meal Ingestion in Men with Elevated Cardiometabolic Risk

BACKGROUND

Prospective cohort studies show that higher dietary fiber intake is associated with reduced cardiovascular disease risk, yet the impact on postprandial glucose and insulin responses is unclear.

OBJECTIVE

This study aims to evaluate the effects of orange beverages with differing fiber concentrations on postprandial glycemic responses (secondary outcome measure) after a sequential breakfast and lunch challenge in men with increased cardiometabolic risk.

METHODS

Thirty-six men (aged 30-65 y; body mass index 25-30 kg/m(2): fasting triacylglycerol or total cholesterol concentrations: 0.8-2.2 or 6.0-8.0 mmol/L, respectively) were provided with a high-fat mixed breakfast and were randomly assigned to consume 240 mL Tropicana (PepsiCo, Inc.) pure premium orange juice without pulp (OJ), OJ with 5.5 g added orange pomace fiber (OPF), juice made from lightly blended whole orange, or an isocaloric sugar-matched control (Control) on 4 occasions separated by 2 wk. A medium-fat mixed lunch was provided at 330 min. Blood samples were collected before breakfast and on 11 subsequent occasions for 420 min (3 time points postlunch) to determine postprandial glucose, insulin, lipid, and inflammatory biomarker responses. Repeated-measures ANOVA was used for data analysis.

RESULTS

OPF significantly (P < 0.05) reduced the maximal change in glucose concentrations (1.9 ± 0.21 mmol/L) reached after breakfast compared with other treatments (2.3-2.4 mmol/L) and after lunch (3.0 ± 0.05 mmol/L) compared with OJ (3.6 ± 0.05 mmol/L). The maximal change in insulin concentration (313 ± 25 pmol/L) was also lower compared with Control (387 ± 30 pmol/L) and OJ (418 ± 39 pmol/L) after breakfast. OPF significantly delayed the time to reach the peak glucose concentration compared with Control and OJ, and of insulin compared with Control after breakfast.

CONCLUSION

OPF consumed with breakfast may lower postprandial glycemic and insulinemic responses to typical meal ingestion in men with increased cardiometabolic risk. This trial is registered at clinicaltrials.gov as NCT01963416.

New insights into seaweed polyphenols on glucose homeostasis

CONTEXT

Polyphenol-rich marine macroalgae are gaining dietary importance due to their influence over diabetes mellitus and the role as a vital source of high-value nutraceuticals. Their assorted beneficial effects on human health include competitive inhibition of digestive enzymes, varying the activity of hepatic glucose-metabolizing enzymes, lowering the plasma glucose levels, and lipid peroxidation, delaying the aging process.

OBJECTIVE

In this paper, we review the health beneficial effects of polyphenols and phlorotannins from brown seaweeds with special emphasis on their inhibitory effects on carbohydrate-metabolizing enzymes.

METHODS

A survey of literature from databases such as Sciencedirect, Scopus, Pubmed, Springerlink, and Google Scholar from the year 1973 to 2013 was done to bring together the information relating to drug discovery from brown seaweeds as a source for diabetes treatment.

RESULTS

Over the past two decades, 20 different bioactive polyphenols/phlorotannins have been isolated and studied from 10 different brown algae. Discussion of the positive effect on the inhibition of enzymes metabolizing carbohydrates in both in vitro and in vivo experiments are included.

CONCLUSION

Despite the recent advancements in isolating bioactive compounds from seaweeds with potential health benefit or pharmaceutical behavior, studies on the polyphenol effectiveness on glucose homeostasis in human beings are very few in response to their functional characterization. Added research in this area is required to confirm the close connection of polyphenol rich seaweed-based diet consumption with glucose homeostasis and the exciting possibility of prescribing polyphenols to treat the diabetes pandemic.

Modulation of glucose transporter protein by dietary flavonoids in type 2 diabetes mellitus

Diabetes mellitus (DM) is a metabolic diseases characterized by hyperglycemia due to insufficient or inefficient insulin secretory response. This chronic disease is a global problem and there is a need for greater emphasis on therapeutic strategies in the health system. Phytochemicals such as flavonoids have recently attracted attention as source materials for the development of new antidiabetic drugs or alternative therapy for the management of diabetes and its related complications. The antidiabetic potential of flavonoids are mainly through their modulatory effects on glucose transporter by enhancing GLUT-2 expression in pancreatic β cells and increasing expression and promoting translocation of GLUT-4 via PI3K/AKT, CAP/Cb1/TC10 and AMPK pathways. This review highlights the recent findings on beneficial effects of flavonoids in the management of diabetes with particular emphasis on the investigations that explore the role of these compounds in modulating glucose transporter proteins at cellular and molecular level.

Effects of a beetroot juice with high neobetanin content on the early-phase insulin response in healthy volunteers

Produce rich in phytochemicals may alter postprandial glucose and insulin responses by interacting with the pathways that regulate glucose uptake and insulin secretion in humans. The aims of the present study were to assess the phytochemical constituents of red beetroot juice and to measure the postprandial glucose and insulin responses elicited by either 225 ml beetroot juice (BEET), a control beverage matched for macronutrient content (MCON) or a glucose beverage in healthy adults. Beetroot juice was a particularly rich source of betalain degradation compounds. The orange/yellow pigment neobetanin was measured in particularly high quantities (providing 1·3 g in the 225 ml). A total of sixteen healthy individuals were recruited, and consumed the test meals in a controlled single-blind cross-over design. Results revealed a significant lowering of the postprandial insulin response in the early phase (0-60 min) (P < 0·05) and a significantly lower glucose response in the 0-30 min phase (P < 0·05) in the BEET treatment compared with MCON. Betalains, polyphenols and dietary nitrate found in the beetroot juice may each contribute to the observed differences in the postprandial insulin concentration.

Human absorption and metabolism of oleuropein and hydroxytyrosol ingested as olive (Olea europaea L.) leaf extract

Phenolic compounds derived from the olive plant (Olea europaea L.), particularly hydroxytyrosol and oleuropein, have many beneficial effects in vitro. Olive leaves are the richest source of olive phenolic compounds, and olive leaf extract (OLE) is now a popular nutraceutical taken either as liquid or capsules. To quantify the bioavailability and metabolism of oleuropein and hydroxytyrosol when taken as OLE, nine volunteers (five males) aged 42.8 ± 7.4 years were randomized to receive either capsulated or liquid OLE as a single lower (51.1 mg oleuropein, 9.7 mg hydroxytyrosol) or higher (76.6 mg oleuropein, 14.5 mg hydroxytyrosol) dose, and then the opposite strength (but same formulation) a week later. Plasma and urine samples were collected at fixed intervals for 24 h post-ingestion. Phenolic content was analyzed by LC-ESI-MS/MS. Conjugated metabolites of hydroxytyrosol were the primary metabolites recovered in plasma and urine after OLE ingestion. Peak oleuropein concentrations in plasma were greater following ingestion of liquid than capsule preparations (0.47 versus 2.74 ng/mL; p = 0.004), but no such effect was observed for peak concentrations of conjugated (sulfated and glucuronidated) hydroxytyrosol (p = 0.94). However, the latter peak was reached earlier with liquid preparation (93 versus 64 min; p = 0.031). There was a gender effect on the bioavailability of phenolic compounds, with males displaying greater plasma area under the curve for conjugated hydroxytyrosol (11,600 versus 2550 ng/mL; p = 0.048). All conjugated hydroxytyrosol metabolites were recovered in the urine within 8 h. There was wide inter-individual variation. OLE effectively delivers oleuropein and hydroxytrosol metabolites to plasma in humans.

Olive (Olea europaea L.) leaf polyphenols improve insulin sensitivity in middle-aged overweight men: a randomized, placebo-controlled, crossover trial

Background

Olive plant leaves (Olea europaea L.) have been used for centuries in folk medicine to treat diabetes, but there are very limited data examining the effects of olive polyphenols on glucose homeostasis in humans.

Objective

To assess the effects of supplementation with olive leaf polyphenols (51.1 mg oleuropein, 9.7 mg hydroxytyrosol per day) on insulin action and cardiovascular risk factors in middle-aged overweight men.

Design

Randomized, double-blinded, placebo-controlled, crossover trial in New Zealand. 46 participants (aged 46.4±5.5 years and BMI 28.0±2.0 kg/m²) were randomized to receive capsules with olive leaf extract (OLE) or placebo for 12 weeks, crossing over to other treatment after a 6-week washout. Primary outcome was insulin sensitivity (Matsuda method). Secondary outcomes included glucose and insulin profiles, cytokines, lipid profile, body composition, 24-hour ambulatory blood pressure, and carotid intima-media thickness.

Results

Treatment evaluations were based on the intention-to-treat principle. All participants took >96% of prescribed capsules. OLE supplementation was associated with a 15% improvement in insulin sensitivity (p = 0.024) compared to placebo. There was also a 28% improvement in pancreatic β-cell responsiveness (p = 0.013). OLE supplementation also led to increased fasting interleukin-6 (p = 0.014), IGFBP-1 (p = 0.024), and IGFBP-2 (p = 0.015) concentrations. There were however, no effects on interleukin-8, TNF-α, ultra-sensitive CRP, lipid profile, ambulatory blood pressure, body composition, carotid intima-media thickness, or liver function.

Conclusions

Supplementation with olive leaf polyphenols for 12 weeks significantly improved insulin sensitivity and pancreatic β-cell secretory capacity in overweight middle-aged men at risk of developing the metabolic syndrome.

Trial Registration

Australian New Zealand Clinical Trials Registry #336317.

Possible effects of dietary polyphenols on sugar absorption and digestion

Excessive post-prandial glucose excursions are a risk factor for developing diabetes, associated with impaired glucose tolerance. One way to limit the excursion is to inhibit the activity of digestive enzymes for glucose production and of the transporters responsible for glucose absorption. Flavonols, theaflavins, gallate esters, 5-caffeoylqunic acid and proanthocyanidins inhibit α-amylase activity. Anthocyanidins and catechin oxidation products, such as theaflavins and theasinsensins, inhibit maltase; sucrase is less strongly inhibited but anthocyanidins seem somewhat effective. Lactase is inhibited by green tea catechins. Once produced in the gut by digestion, glucose is absorbed by SGLT1 and GLUT2 transporters, inhibited by flavonols and flavonol glycosides, phlorizin and green tea catechins. These in vitro data are supported by oral glucose tolerance tests on animals, and by a limited number of human intervention studies on polyphenol-rich foods. Acarbose is a drug whose mechanism of action is only through inhibition of α-amylases and α-glucosidases, and in intervention studies gives a 6% reduction in diabetes risk over 3 years. A lifetime intake of dietary polyphenols, assuming the same mechanism, has therefore a comparable potential to reduce diabetes risk, but more in vivo studies are required to fully test the effect of modulating post-prandial blood glucose in humans.