Non-alcoholic fatty liver disease pathophysiology

NAFLD has a complex pathophysiology, which is described by the two-hit hypothesis⁽ Reference Berlanga, Guiu-Jurado and Porras ^{7 )}. In this model, the first hit describes the accumulation of fatty acids (FA) and TAG in hepatocytes leading to steatosis, which results from multiple mechanisms such as: (a) increased hepatic delivery and uptake of FA associated with increased lipolysis in adipose tissue and/or increased intake of dietary fat; (b) decreased FA oxidation; (c) increased hepatic de novo lipogenesis; and (d) decreased hepatic lipid export via VLDL⁽ Reference Berlanga, Guiu-Jurado and Porras ^{7 ,} Reference Tilg and Moschen ^{12 )}. The inability to regulate lipid partitioning leads to the second hit, whereby an overwhelmed FA β-oxidation produces mitochondrial dysfunction which increases reactive oxygen species resulting in sustained oxidative stress and a depletion of the antioxidant defences⁽ Reference Serviddio, Bellanti and Vendemiale ^{13 ,} Reference Gupte, Lyon and Hsueh ^{14 )}. FA intermediates and a compromised oxidative status activates Kupffer cells producing inflammatory mediators, and dysregulated insulin action leading to the progression from benign steatosis to NASH⁽ Reference Schuppan, Gorrell and Klein ^{3 ,} Reference Serviddio, Bellanti and Vendemiale ^{13 )}. Finally, chronic inflammation and oxidative stress induce hepatocyte apoptosis and injury which activates stellate cells which are central to the progression to liver fibrosis⁽ Reference Schuppan, Gorrell and Klein ^{3 ,} Reference Gupte, Lyon and Hsueh ^{14 )}.

Polyphenols: chemical structures and sources

Phenolic compounds are secondary metabolites of plants which are present in high amounts in fruits, vegetables, cereals and beverages such as red wine, tea or coffee. More than 8000 structures have been identified ranging from compounds with at least one aromatic ring with one or two hydroxyl groups, to polymers of up to fifty units with multiple hydroxyl groups. Generally, all phenolic compounds are commonly referred to as polyphenols, despite a group of them having only one aromatic ring. Polyphenols are divided into two main categories, namely flavonoids and non-flavonoids, based on the number of phenol rings and the way in which these rings interact⁽ Reference Del Rio, Rodriguez-Mateos and Spencer ^{15 )}.

Flavonoids have a common basic structure of fifteen carbons (C₆–C₃–C₆) with two aromatic carbon rings (A and B rings) connected by a three-carbon bridge (C ring). Flavonoids may be sub-classified according to the degree of oxidation of the C-ring, the hydroxylation pattern of the ring structure and the substitution of the three-position into: (a) flavonols (e.g. quercetin and kaempferol) whose sources include onions and broccoli, (b) flavones (e.g. luteolin, apigenin) found in celery and parsley, (c) isoflavones (e.g. genistein and daidzein) found in leguminous plants and in particular soyabeans and soya products, (d) flavanones (e.g. naringerin and hesperitin) abundant in citrus fruits, wine and herbs such as oregano, (e) anthocyanidins (e.g. cyanidin and peonidin) found in berry fruits and red wine, and (f) flavanols (e.g. (+)-catechin, (−)-epicatechin, epigallocatechin) abundant in cocoa and green tea⁽ Reference Rodriguez-Mateos, Vauzour and Krueger ^{11 ,} Reference Del Rio, Rodriguez-Mateos and Spencer ^{15 )} (Fig. 1).

Fig. 1. Polyphenol structures.

Non-flavonoids may be sub-classified into phenolic acids and stilbenes. Phenolic acid includes hydroxybenzoic acids (C₆–C₁) and hydroxycinnamic acids (C₆–C₃). Hydroxybenzoic acids (e.g. gallic acid) are found in pomegranate and raspberries. Hydroxycinnamic acids (e.g. caffeic acid) can be found in coffee beans and blueberries. Stilbenes have a C₆–C₂–C₆ structure. Resveratrol which is the main stilbene, can be found as cis or trans isomers as well as conjugated derivatives in grapes and red wine⁽ Reference Rodriguez-Mateos, Vauzour and Krueger ^{11 ,} Reference Del Rio, Rodriguez-Mateos and Spencer ^{15 )} (Fig. 1).

Polyphenols have been identified as powerful antioxidants in vitro ⁽ Reference Williams, Spencer and Rice-Evans ^{16 )}. However, given their extensive metabolism and relatively low tissue concentrations, their in vivo preventative properties are considered largely independent of conventional antioxidant activities⁽ Reference Williams, Spencer and Rice-Evans ^{16 )}. The ability of polyphenols to exert antioxidant properties in vivo depends on the extent of their phases 1 and 2 biotransformation and conjugation products during absorption in the gastrointestinal tract and post-absorption primarily in the liver. Although a full overview of polyphenols metabolism and its regulation is beyond the scope of the current review (see Rodriguez-Mateos et al. ⁽ Reference Rodriguez-Mateos, Vauzour and Krueger ^{11 )} for an extensive review), knowledge about their bio-kinetics (the composite of their distribution, biotransformation and elimination) alluded to throughout, is essential to understand the bioactivity of polyphenols in vivo ⁽ Reference Rodriguez-Mateos, Vauzour and Krueger ^{11 )}.

In vitro studies

Cell culture studies constitute a useful tool to elucidate the molecular mechanisms of action of polyphenols in the prevention of steatosis. Primary cultures of human hepatocytes are the optimal cell culture model for studying determinants of NAFLD. However their widespread use is limited by logistical factors such as liver samples availability. The main alternative model is the human hepatocyte-derived cell line, HepG2.

Palmitic (16 : 0) and oleic (18 : 1n-9) acids are the most abundant FA in the liver of both normal subjects and NAFLD patients⁽ Reference Gomez-Lechon, Donato and Martinez-Romero ^{17 )} and have been used (generally in a bovine serum complex) to induce lipid accumulation in HepG2 and reproduce the key cellular features of human NAFLD⁽ Reference Gomez-Lechon, Donato and Martinez-Romero ^{17 –} Reference Shimada, Tokuhara and Tsubata ^{19 )}. In addition, steatosis in HepG2 cells has been induced by high concentrations of glucose (25–30 mm)⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 ,} Reference Shang, Chen and Xiao ^{21 )} which through a multistep process, including glycolysis and the Krebs Cycle generates acetyl-CoA, a key substrate for de novo lipogenesis⁽ Reference Berlanga, Guiu-Jurado and Porras ^{7 )}.

Pure polyphenol compounds and polyphenol-rich extracts have been tested in both these in vitro models of steatosis (Table 1). Most studies are concordant with the fact that a range of polyphenols reduce hepatocellular TAG accumulation induced by FA⁽ Reference Vidyashankar, Sandeep Varma and Patki ^{18 ,} Reference Shimada, Tokuhara and Tsubata ^{19 ,} Reference Liu, Ma and Wang ^{22 –} Reference Wu, Lin and Wang ^{24 )} or by high glucose concentrations⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 ,} Reference Shang, Chen and Xiao ^{21 ,} Reference Guo, Li and Ling ^{25 )} with a range of reported mechanisms, including an inhibition of lipogenesis and a promotion of FA catabolism (Fig. 2).

Fig. 2. (Colour online) Possible mechanisms underlying the effect of polyphenols in non-alcoholic fatty liver disease (NAFLD). Polyphenols may prevent cellular damage in hepatocytes associated with NAFLD through different mechanism of action including: (a) reducing de novo lipogenesis through sterol regulatory element-binding protein 1c (SREBP-1c) down-regulation, (b) increasing β-fatty acid (FA) oxidation by PPARα up-regulation, (c) improving insulin sensitivity (d) reducing oxidative stress through increasing the antioxidant defence levels via nuclear factor-erythroid 2-related factor 2 (Nrf2), (e) attenuating the inflammatory pathways. Presumably SREBP-1c down-regulation and PPARα up-regulation are modulated by AMPK activation (by phosphorylation). TNFR, TNFα receptor; IL6-R, IL-6 receptor; IR, insulin receptor; CD36, cluster of differentiation 36/FA translocase; p-AMPK, phosphorylated AMP-activated protein kinase α; ACC, acetyl-CoA carboxylase; FAS, fatty acid synthase; SCD, stearoyl-CoA desaturase; GPAT, glycerol-3-phosphate acyltransferase; CPT-1, carnitine palmitoyl transferase 1; ACO, acyl-CoA oxidase; PGC-1, PGC1α, PPARγ coactivator-1α; JNK, c-Jun N-terminal kinase; PKC, protein kinase C; mTOR, mammalian target of rapamycin.

Table 1. Cell culture studies investigating the impact of polyphenols on non-alcoholic fatty liver disease

Arrow indicates an increase (↑) or decrease (↓) in the levels of gene expression, protein concentrations or activity.

p-AMPKα, phosphorylated AMP-activated protein kinase α; ACC, acetyl-CoA carboxylase; CPT-1, carnitine palmitoyl transferase 1; FAS, fatty acid synthase; ROS, reactive oxygen species; SREBP-1c, sterol regulatory element-binding protein 1c; p-ACC, phosphorylated ACC; SIRT-1, sirtuin-1; GSH, reduced glutathione; GSSG, oxidised glutathione; TNFα, tumor necrosis factor α; CAT, catalase; SOD, superoxide dismutase; GPx, glutathione peroxidase; ALT, alanine aminotransferase; HGMCR, 3-hydroxy-3-methylglutaryl coenzyme A reductase; GPAT, glycerol-3-phosphate acyltransferase; mtGPAT, mitochondrial GPAT; PKC, protein kinase C; p-JNK, phosphorylated, c-Jun N-terminal kinase; MDA, malondialdehyde; HFD, high-fat diet.

Sterol regulatory element-binding protein 1c (SREBP-1c) is the most important transcription factor regulating genes involved in FA synthesis and TAG metabolism in the liver⁽ Reference Jump, Tripathy and Depner ^{26 )}. A number of in vitro studies with polyphenols have shown a down-regulation of SREBP-1c and its main targets in lipogenesis⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 ,} Reference Liu, Ma and Wang ^{22 ,} Reference Wu, Lin and Wang ^{24 )}. In particular, Liu et al.⁽ Reference Liu, Ma and Wang ^{22 )} reported that luteolin induced a reduction of palmitate-stimulated lipid accumulation in HepG2 cells associated with decreased SREBP-1c and FA synthase gene expression and an attenuation of the activity of acetyl-CoA carboxylase. Acetyl-CoA carboxylase and FA synthase play an essential role in de novo lipogenesis converting the acetyl-CoA into palmitate that subsequently is esterified into TAG in the liver. Similar reduced expression of SREBP-1c and FA synthase were reported using a chlorogenic acid derivative (3-caffeoyl, 4-dihydrocaffeoylquinic acid) and rutin (quercetin-3-O-rutinoside) in a high glucose-stimulated and oleic-stimulated lipid accumulation HepG2 cell model, respectively⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 ,} Reference Wu, Lin and Wang ^{24 )}. In addition, treatment with 3-caffeoyl, 4-dihydrocaffeoylquinic acid, luteolin and rutin induced an activation (by phosphorylation) of AMP-activated protein kinase (AMPK), a well-known inhibitor of SREBP-1c and hence of lipogenesis⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 ,} Reference Liu, Ma and Wang ^{22 ,} Reference Wu, Lin and Wang ^{24 )}. Sirtuin 1 (SIRT-1) activation by polyphenols represents a downstream regulator of AMPK⁽ Reference Canto and Auwerx ^{27 )}. Pil et al. ⁽ Reference Pil Hwang, Gyun Kim and Choi ^{20 )} found that 3-caffeoyl, 4-dihydrocaffeoylquinic acid treatment increased SIRT-1 activity, suggesting that SIRT-1 may be involved in the AMPK-dependent reduction in SREBP-1c and FA synthase expression induced by polyphenols. Cyanidin-3-O-β-glucoside also attenuated de novo lipogenesis through an alternative pathway, increasing protein kinase C ζ activity and suppressing mitochondrial glycerol-3-phosphate acyltransferase 1 activation, the rate limiting enzyme which controls the first step of TAG synthesis from palmitate⁽ Reference Guo, Li and Ling ^{25 )}.

In the liver, PPARα plays a pivotal role in FA metabolism by up-regulating the expression of numerous genes involved in FA oxidation as well as other processes which regulate cellular FA status such as receptor mediated FA uptake and lipoprotein assembly and secretion⁽ Reference Contreras, Torres and Tovar ^{28 )}. As a consequence, activation of PPARα is associated with decreased hepatic fat storage⁽ Reference Berlanga, Guiu-Jurado and Porras ^{7 )}. Oxidation of FA occurs within the mitochondria, peroxisomes and the endoplasmic reticulum and is regulated mainly through key rate limiting enzymes such as carnitine palmitoyl transferase 1 (CPT-1) and acyl-CoA oxidase. In the outer membrane of mitochondria, (CPT-1) mediates the transfer of FA from the cytosol into the mitochondria prior to β-oxidation and acyl-CoA oxidase catalyses the first rate-limiting step in peroxisomal β-oxidation⁽ Reference Berlanga, Guiu-Jurado and Porras ^{7 ,} Reference Serviddio, Bellanti and Vendemiale ^{13 )}. Procyanidin B1 (an epicatechin-(4β→8)-catechin dimer) suppressed palmitic-stimulated lipid accumulation in HepG2 cells through an up-regulation of the acyl-CoA oxidase and (CPT-1) mRNA expression⁽ Reference Shimada, Tokuhara and Tsubata ^{19 )}. In addition to inhibiting lipogenesis, luteolin induced (CPT-1) gene expression in HepG2 challenged with palmitate⁽ Reference Liu, Ma and Wang ^{22 )}. Furthermore, rutin increased PPARα protein levels which was associated with a reduction in the lipid load in HepG2 cells⁽ Reference Wu, Lin and Wang ^{24 )}.

It is well known that a number of polyphenols can indirectly act as antioxidants by inducing phase II antioxidant defences enzymes⁽ Reference Masella, Di Benedetto and Vari ^{29 –} Reference Rodriguez-Ramiro, Martin and Ramos ^{31 )}. There is evidence suggesting that the antioxidant response can alleviate the cellular damage induced by oxidative stress during the progression of NAFLD⁽ Reference Gupte, Lyon and Hsueh ^{14 )}. Accordingly, Vidyashankar et al.⁽ Reference Vidyashankar, Sandeep Varma and Patki ^{18 )} reported that quercetin induced an increase in the activity of antioxidant cellular defences, such as catalase, glutathione peroxidase and superoxide dismutase and an increase of reduced glutathione levels. Likewise, rutin attenuated the cellular oxidative stress induced by oleic acid through raised superoxide dismutase, glutathione peroxidase and catalase protein levels which was associated with an increase in PPARα protein levels⁽ Reference Wu, Lin and Wang ^{24 )}. A sustained oxidative stress can induce hepatocyte apoptosis and accentuate the transition from simple steatosis to NASH. Jiang et al., showed that cyanidin-3-O-β-glucoside reduced oxidative stress and the apoptotic pathway activation induced by hyperglycaemia, preventing mitochondrial dysfunction through modulation of phosphatidylinositol-3-kinase/protein kinase B and c-Jun N-terminal kinase (JNK) -signalling pathways⁽ Reference Jiang, Tang and Zhang ^{32 )}.

Animal in vivo studies

Animal models of NAFLD can be divided in three major categories: those caused by a genetic mutation, by a dietary or pharmacological manipulation, or a combination of both models⁽ Reference Takahashi, Soejima and Fukusato ^{33 )}. The choice of model results in variability in the characteristics and severity of the NAFLD phenotype and its aetiological basis, with careful selection needed in order to address the specific research question in a meaningful way. For example, two of the most widely used dietary models of NASH, the high fat diet (HFD) and the methionine and choline deficiency (MCD) models display important differences in their metabolic characteristics. Although both present significant steatosis, mice fed a HFD develop obesity and insulin resistance which are characteristic of NAFLD and NASH in human subjects. Conversely mice fed a MCD exhibit atypical (for human subjects) weight loss and low serum insulin and leptin levels. However, the MCD model produces a more pathological form of NAFLD characterised by severe inflammation, oxidative stress, mitochondrial dysfunction, apoptosis and fibrogenesis, features which are only induced to a limited extent when using the HFD model⁽ Reference Takahashi, Soejima and Fukusato ^{33 )}. For evaluation of the efficacy of dietary approaches in NAFLD, the HFD may be chosen when evaluating the ability to prevent NAFLD development or for ameliorating steatosis, whereas the MCD model may be more appropriate to assess the therapeutic potential to reverse NASH associated liver injury.

Several studies have revealed that different subclasses of polyphenols ameliorate the severity and metabolic consequences of NAFLD in animal models. In general, liver biopsies (using haematoxylin/eosin staining) accompanied by semi-quantitative NAFLD activity scoring have shown that pure polyphenols or polyphenolic extracts reduced liver TAG accumulation⁽ Reference Ueno, Torimura and Nakamura ^{34 –} Reference Joven, Espinel and Rull ^{38 )}. However, the underlying molecular mechanisms associated with reduced steatosis are variable and dependant on the choice of animal model and the dose of phenolic compound of interest (Table 2 and Fig. 2).

Table 2. Rodent studies evaluating the impact of polyphenols on non-alcoholic fatty liver disease

Arrow indicates an increase (↑) or decrease (↓) in the levels of expression or activity.

EGCG, epigallocatequin-3-gallate; ALT, alanine aminotransferase; SREBP-1c, sterol regulatory element-binding protein 1c; IRS-1, insulin receptor substrate-1; p-IRS-1, phosphorylated IRS-1; p-AKT, phosphorylated AKT; p-IKKβ, phosphorylated inhibitor of κB kinase β subunit; HFD, high fat diet; AMPK, adenosine monophosphate-activated protein kinase; ACC, acetyl-CoA carboxylase; LXRα, liver X receptor α; FAS, fatty acid synthase; CPT-1, carnitine palmitoyl transferase; TBARS, thiobarbituric acid reactive substances; p-JNK, phosphorylated, c-Jun N-terminal kinase; ; IκB, inhibitor of κB; AST, aspartate aminotransferase; CAT, catalase; αSMA, α-smooth muscle actin-positive cells; TLR, toll-like receptor; p-AMPK, phosphorylated adenosine monophosphate-activated protein kinase; NOS, nitric oxide synthase, SOD, superoxide dismutase; GPx, glutathione peroxidase; ALP, alkaline phosphatase; UCP2, uncoupling protein 2; PGC1α, PPARγ coactivator-1α; GSH, reduced glutathione ; GSSG, oxidised glutathione; ACO, acyl-Coenzyme A oxidase; SIRT-1, sirtuin-1; p-ACC, phosphorylated ACC, ChREBP, carbohydrate-responsive element-binding protein; SCD-1, stearoyl-CoA desaturase; ACAT, acyl-CoA:colesterol acyltransferase; ME, malic enzyme; G6PD, glucose-6-phosphate dehydrogenase; MDA, malondialdehyde; FRAP, ferric reducing antioxidant power; miR, microRNA; Q3G, quercetin 3-O-β-D-glucuronide; GPAT1, glycerol-3-phosphate acyltransferase 1.

Clinical trials

To the best of our knowledge, only five human randomised controlled trials (all with a double-blinded placebo-controlled design) focused on polyphenols and NAFLD, have been published to date (Table 3). Three were undertaken with 500 and 600 mg resveratrol for 12 weeks⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 ,} Reference Chen, Zhao and Ran ^{64 )} or 3000 mg for 8 weeks⁽ Reference Chachay, Macdonald and Martin ^{65 )}. The other two studies were carried out using an HSE (about 150 mg polyphenols)⁽ Reference Chang, Peng and Yeh ^{66 )} or a bayberry juice (500 ml equivalent to 1350 mg polyphenols)⁽ Reference Guo, Zhong and Liu ^{67 )} for 12 and 4 weeks, respectively. Four out of the five studies have reported a significant impact of intervention on select characteristics of NAFLD. Chang et al. reported that anthropometric measures (body weight, BMI and waist:hip ratio) were significantly lower (1·4, 1·33 and 1·09 %, respectively) following intervention with HSE⁽ Reference Chang, Peng and Yeh ^{66 )} but no changes were observed with bayberry juice⁽ Reference Guo, Zhong and Liu ^{67 )}. For the two clinical trials using a similar dose of resveratrol (500 and 600 mg) only one observed a reduction in anthropometric measurements. This apparent discrepancy is likely due to the fact that in one of the trials resveratrol intervention was accompanied by a change in lifestyle with patients advised to follow physical activity guidelines⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 )}. With regard to hepatic function, two of the resveratrol interventions reduced the serum alanine transaminase concentrations by 15 %⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 ,} Reference Chen, Zhao and Ran ^{64 )} although no reduction was detected in the studies with other polyphenol extracts⁽ Reference Chang, Peng and Yeh ^{66 ,} Reference Guo, Zhong and Liu ^{67 )}. In addition, one of the interventions with resveratrol and the HSE showed a reduction in serum total- and LDL-cholesterol⁽ Reference Chen, Zhao and Ran ^{64 )} and NEFA⁽ Reference Chang, Peng and Yeh ^{66 )}. A significant reduction in the homeostasis model assessment insulin resistance index associated with lower serum glucose levels following resveratrol supplementation was also reported⁽ Reference Chen, Zhao and Ran ^{64 )}. The clinical trials using the bayberry juice and resveratrol reported anti-inflammatory effects, with a reduction in serum cytokines (TNFα, IL-6 and IL-8)⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 ,} Reference Chen, Zhao and Ran ^{64 ,} Reference Guo, Zhong and Liu ^{67 )} and increased serum adiponectin levels⁽ Reference Chen, Zhao and Ran ^{64 )}. In addition, one of the interventions with resveratrol reported a reduction in NF-κB activity in the peripheral blood mononuclear cells⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 )}.

Table 3. Clinical trials carried out with polyphenols in non-alcoholic fatty liver disease (NAFLD) subjects

Arrow indicates increase (↑) and decrease (↓) in the levels of expression or activity.

HOMA-IR, homeostasis model assessment insulin resistance index; TNFα, tumour necrosis factor alpha; CRP, C-reactive protein; AST: aspartate aminotransferase; ALT, alanine aminotransferase

None of the clinical trials conducted liver biopsies and therefore had histological data on the severity of NAFLD. Instead non-invasive approaches such as semi-quantitative liver ultrasound examinations were carried out. Employing this approach, Chang et al. reported that HSE supplementation significantly reduced (by about 15 %) the liver damage score⁽ Reference Chang, Peng and Yeh ^{66 )} and among the clinical trials with resveratrol, only the one accompanied by a change in lifestyle observed a significant reduction in steatosis⁽ Reference Faghihzadeh, Adibi and Rafiei ^{63 ,} Reference Chen, Zhao and Ran ^{64 )}. Finally, the non-beneficial effect of resveratrol observed at the higher supplementation dose⁽ Reference Chachay, Macdonald and Martin ^{65 )} is likely due to a hormesis phenomenon, characterised by a low-dose stimulation and inhibition and a potentially detrimental effect at high-dose, which has been described for a number of bioactive compounds including resveratrol⁽ Reference Calabrese, Mattson and Calabrese ^{68 )}.

Doses of polyphenols: from animals studies to clinical trials

As discussed earlier, animal studies have been widely employed to assess the effects of a variety of polyphenols in NAFLD. However, the majority of these studies have used supra-physiological doses of compounds, with little consideration given to human equivalent doses⁽ Reference Reagan-Shaw, Nihal and Ahmad ^{69 )}. Taking resveratrol as an example, most of the pre-clinical studies in rats have employed doses ranging from 10 to 100 mg/kg body weight. Following allometric scaling calculations⁽ Reference Reagan-Shaw, Nihal and Ahmad ^{69 )}, such doses would equate to 97 and 970 mg resveratrol for a 60 kg person, although an estimated consumption in human subjects is only about 0·93 mg/d⁽ Reference Zamora-Ros, Andres-Lacueva and Lamuela-Raventos ^{70 )}. Therefore from a dose perspective the majority of the rodent scientific literature provides little insight into the likely benefits of dietary sourced resveratrol in human NAFLD, although such higher doses may be achievable through the consumption of resveratrol rich supplements.

However, the estimated intake of total polyphenols in Western populations is about 1–2 g/d with other polyphenols occurring in much higher amounts in the diet than resveratrol⁽ Reference Rothwell, Perez-Jimenez and Neveu ^{71 )}, with most plant sources consisting of a combination of different compounds which collectively may have a much greater impact on liver health relative to the effect of each one in isolation. Thus, more studies assessing possible additive and synergistic effects of polyphenol combinations commonly found in the diet are needed.