Curcumin and Biochemical Parameters in Metabolic-Associated Fatty Liver Disease (MAFLD)—A Review

Metabolic-associated fatty liver disease (MAFLD), formerly non-alcoholic fatty liver disease (NAFLD), is characterized by excessive fat accumulation in hepatocytes. It is the most common chronic liver disease worldwide and is a significant public health problem. In the absence of pharmacological therapy, other treatments such as diet, physical activity, or supplementation are sought. Non-pharmacological therapies may include curcumin supplementation, which has been shown to have many health-promoting properties, including antioxidant, anti-inflammatory, and anti-cancer effects. For this reason, we reviewed available databases to analyze publications describing the effect of curcumin supplementation on biochemical parameters in MAFLD. Nine studies (eight RCTs and one CT) based solely on supplementation of patients with curcumin were included in this review. The results from the individual trials were varied and did not allow clear conclusions. Although they suggest that curcumin shows some potential in the treatment of MAFLD, further research is needed.


Introduction
Metabolic-associated fatty liver disease (MAFLD), formerly non-alcoholic fatty liver disease (NAFLD), is the most common chronic liver disease worldwide [1][2][3][4][5][6][7][8]. It is defined as a fat content of hepatocytes greater than 5% of the total weight of the liver, not caused by alcohol consumption, medication, or viral infection [9]. The prevalence of NAFLD has increased significantly in recent years, making it a serious public health problem. In 2013, Welsh et al. reported that up to half of obese men suffer from the disease [10]. Apart from obesity, MAFLD is often associated with other diseases such as insulin resistance, diabetes mellitus, metabolic syndrome and dyslipidemia [11][12][13][14][15]. In addition, it is also associated with an increased risk of liver-and cardiovascular disease-related mortality [16,17]. MAFLD is also associated with the risk of non-alcoholic steatohepatitis (NASH). It is estimated that 23-44% of MAFLD patients will develop NASH, which in approximately 37-41% of cases will lead to fibrosis, with 10-20% of these going on to develop cirrhosis. Within 5-7 years, cirrhosis will lead to liver failure in 40-60% of people, and hepatocellular carcinoma (HCC) in 2.4-12% of patients within 3-7 years [18]. The annual medical costs related to MAFLD have been estimated at €89 billion in the US and €35 billion in Europe. Early diagnosis, prevention, and treatment of risk factors as well as lifestyle modifications have been proposed as a cost-effective treatment strategy for MAFLD [19]. In 2016, the European Association for the Study of the Liver recommended the use of interventions leading to lifestyle changes in patients with MAFLD, specifically dietary changes and a gradual increase in aerobic exercise or resistance training [20].

Pathophysiology: Multiple Hits Hypothesis
The development of MAFLD can be influenced by dietary habits and genetic and environmental factors, making the pathogenesis of the disease complex, multifactorial, not yet fully understood, and therefore now referred to as a "multiple hits hypothesis". The key factor leading to MAFLD is insulin resistance (IR), resulting in an increase in de novo hepatic lipogenesis (DNL) as well as weaker inhibition of adipose tissue lipolysis, which leads to an increased flow of fatty acids to the liver and their accumulation in hepatocytes in the form of triglycerides. IR also affects the dysfunction of adipose tissue, resulting in altered production and secretion of adipokines and pro-inflammatory cytokines. Lipotoxicity is increased due to high levels of free fatty acids, free cholesterol, and other lipid metabolites, leading to the production of reactive oxygen species resulting in dysfunction of the mitochondria and endoplasmic reticulum. Moreover, changes in the intestinal microbiota leading to increased permeability of the small intestine, which results in greater absorption of fatty acids and increased levels of circulating molecules, have also been described as being involved in the pathogenesis. As a result, inflammatory pathways are activated and pro-inflammatory cytokines such as IL-6 and TNF-α are released [15].

Curcumin
Curcumin is a polyphenol classified as a curcuminoid. Its source of origin is turmeric (Curcuma longa), a plant from the ginger family found in Asia, mainly in India, where it is most often used as a spice due to its taste and aroma and intense yellow color. However, beyond its culinary use, it is valued for its health-promoting properties, with a history of use dating back several thousand years [21]. It has antioxidant, anti-inflammatory, and anticancer effects, among others, largely without adverse effects. Therefore, its use is being explored in the course of many diseases, such as allergic asthma; cardiovascular diseases; cancers of the lung, breast, colon, pancreas, and stomach; inflammatory bowel diseases; diabetes; and liver diseases [22][23][24][25][26].

Types of Participants
Studies were included if they were conducted in adult patients (aged > 18) of any gender or nationality with metabolic-associated fatty liver disease.

Types of Interventions
Interventions using curcumin supplementation alone, and presenting the results of biochemical parameter outcomes before and after supplementation, were included. Studies that recommended dietary changes and/or physical activity in addition to supplementation and animal trials were excluded.

Types of Comparisons
No specific comparison criteria were applied.

Types of Outcomes
Outcomes for at least one biochemical parameter were presented in the study, measured at baseline (pre-supplementation) and at post-supplementation.

Types of Studies
Any type of study (apart from case reports and reviews) was included if it was a study published in peer-reviewed journals in English. There were no restrictions on intervention length or follow-up measurement points. Exclusion criteria were as follows: non-human studies, use of an additional intervention beyond supplementation such as diet changes, lifestyle changes, or physical activity. The PICOS criteria for inclusion and exclusion of studies are shown in Table 1.

Search Strategy and Study Selection
We reviewed available publications using databases such as PubMed, Web of Science, and Scopus with the search words "NAFLD" or "MAFLD" or "metabolic-associated fatty liver disease" or "non-alcoholic fatty liver disease" and "curcumin" or "turmeric". We limited the results to papers in English or Polish ( Figure 1).

Data Presentation
The network graphs were created using Cytoscape software version 3.8.1 [27].

Study Selection
Nine studies (eight RCTs and one CT) that used only curcumin supplementation in MAFLD patients were included in this review.

Participant Characteristics
In most of the trials, the mean age of the patients was in the range of 37.41-49.4 years ( Table 2) [28][29][30][31][32][33][34]. In one study, the average age of the respondents was 66.72 years [35], in another, the mean age of the study group was not given, but ranged between 18-70 years [36]. The average BMI values were in the 27.6-31.81 kg/m 2 range.
Panahi et al. [33] were the only ones to control uric acid levels in their sample and observed a statistically significant decrease (5.9 ± 0.6 mg/dL vs. 5.2 ± 0.5 mg/dL, p < 0.001).
Nodes denoting biochemical parameters on which effects of curcumin supplementation were examined are color-coded. Open circles denote reviewed studies. The size of the nodes (shown as dots) is proportionally related to the number of participants examined in a particular study/intervention group (for open nodes) or the overall number of participants on which the effect of curcumin supplementation on a particular parameter was examined (colored nodes). Edges (arrows connecting nodes) are shown in gray if the effect on a parameter noted in the study was not statistically significant. Thicker, green edges denote statistically significant improvement (health benefit), while red edges denote worsening of a parameter (deleterious health effect) noted in response to curcumin supplementation. ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; EGF, epidermal growth factor; HbA1c, glycated hemoglobin; HDL-C, high-density lipoprotein cholesterol; hs-CRP, high-sensitivity C-reactive protein; IFNγ, interferon gamma; IL-1α, interleukin-1 alpha; IL-1β, interleukin-1 beta; IL-2, interleukin-2; IL-4, interleukin-4; IL-6, interleukin-6; IL-8, interleukin-8; IL-10, interleukin-10; LDL-C, low-density lipoprotein cholesterol; MCP-1, monocyte chemoattractant protein-1; MDA, malondialdehyde; non-HDL-C, non-high-density lipoprotein cholesterol; TAC, total antioxidant capacity; TC, total cholesterol; TG, triglycerides; TNF-α, tumor necrosis factor alpha; VEGF, vascular endothelial growth factor.

Discussion
To the best of our knowledge, this is the first review on the effects of curcumin on biochemical parameters in MAFLD focusing on supplementation-only interventions.
Curcumin is characterized by numerous pro-health (especially anti-inflammatory and antioxidant) properties. In addition, research has shown that it is able to influence many molecular mechanisms. Additional advantages are its natural origin and lack of serious side effects, indicating that it can be safely used in most patients. As a result, it is used in many diseases, such as cancer, neurological diseases, or inflammatory bowel diseases, as well as in the course of pathological changes in the liver [37,38].
Ghaffari et al. [30], after 12 weeks of using the supplement at a dose of 3 g/day turmeric (6 × 500 mg/ day) recorded a statistically significant increase in TAC.
Curcumin also exhibits antioxidant properties comparable to vitamins C and E. It increases, among others, the activity of catalase, superoxide dismutase (SOD), glutathione peroxidase (GPX), and heme oxygenase-1 (HO-1) [37,42]. It also inhibits superoxide anion radical synthesis induced by homocysteine [43]. In patients with MAFLD, mitochondrial damage can occur with disease progression, leading to the development of NASH. This is a result of excessive β-oxidation occurring in the mitochondria due to the large amount of incoming fatty acids, resulting in excessive production of reactive oxygen species. Exposure of lipids to excessive oxidation results in the production of their toxic metabolites [15,44,45].
Another important factor in the course of MAFLD is glucose. In studies comprising 8-week supplementation with 500 mg/day of an amorphous dispersion preparation comprising 70 mg curcuminoids (Rahmani et al.) [28]; phospholipid curcumin 250 mg/day (equivalent to 50 mg pure curcumin (Chashmniam et al. and Mirhafez et al.) [31,32], and 500 mg curcuminoids plus 5 mg piperine/day (Saberi-Karimian) [36] the results showed a decrease in the level of fasting glucose, but in three of these studies [28,32,36] the results were not statistically significant (Chashmniam et al. did not report statistical significance). Additionally, Rahmani et al. [28] reported a statistically significant decrease in HbA1c. Fasting glucose and HbA1c were not controlled. Lower levels of fasting glucose and HbA1c may have been the result of an improvement in the function of β-cells and an increase in insulin sensitivity. A study conducted among pre-diabetes patients using curcumin supplementation (3 × 250 mg of curcuminoids twice a day) for 9 months resulted in an improvement in the function of β-cells and a reduction in HOMA-IR, fasting glucose, and HbA1c levels [46]. A decrease in insulin resistance was also noted in a 4-week study among obese children taking 500 mg of curcumin daily [47].
Free fatty acids and free cholesterol also play an important role in the pathomechanism of MAFLD [15]. Chashmniam et al. [31] noted an increase in TG levels after the use of phospholipid curcumin at 250 mg/day (equivalent to 50 mg pure curcumin) (statistical significance not given). Rahmani et al. [28], Mirhafez et al. [32], and Saberi-Karimian et al. [36], using respectively for 8 weeks 500 mg/day of an amorphous dispersion preparation com-prising 70 mg curcuminoids; phospholipid curcumin 250 mg/day (equivalent to 50 mg pure curcumin), and 500 mg curcuminoids plus 5 mg piperine/day, observed non-statistically significant decreases in TG levels. Only Panahi et al. [33], after the use of 3 × 500 mg/day (100 mg curcuminoids per capsule) in patients for 8 weeks, noted a statistically significant decrease in TG levels. The effectiveness of curcumin in reducing TG levels has also been observed in people with metabolic syndrome [48], obesity [49], and type-2 diabetes [50].
In  [36] reported decreases in TC levels after 8 weeks of supplementation, but only Rahmani et al. [28] noted a statistically significant result. Panahi et al. (3 × 500 mg/day (100 mg curcuminoids per capsule)) [33] reported a statistically significant decrease in non-HDL-C levels. As shown by the studies carried out so far, curcumin exerts an effect on cholesterol metabolism. By increasing the expression of LDL receptors, it leads to an increase in the removal of LDL from the plasma and increased excretion of cholesterol in the bile. Curcumin is also responsible for the inhibition of Niemann-Pick C1-Like 1 (NPC1L1) protein. Moreover, curcumin also influences lipogenesis by inhibiting involved enzymes and receptors, including sterol regulators binding protein-1 element, apolipoprotein B-100, fatty acid synthase, acetyl CoA carboxylase, acyl coenzyme A, cholesterol acyltransferase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, peroxisome proliferator-activated receptor-α, cluster of differentiation 36, and adenosine monophosphate-activated protein kinase [28].
Uric acid is considered to be one of the factors involved in the MAFLD pathomechanism, as it exhibits pro-inflammatory and pro-oxidative effects by increasing the production of monocyte chemotactic protein-1 and activation of mitogen-activated protein kinase and nuclear factor κB pathways [51]. Panahi et al. [33] after using 3 × 500 mg/day (100 mg curcuminoids per capsule) in patients for 8 weeks, observed a statistically significant decrease in uric acid levels.
Mirhafez et al. (phospholipid curcumin 250 mg/day (equivalent to 50 mg pure curcumin)) observed a statistically significant decrease in leptin levels after 8 weeks of supplementation; a statistically significant increase in the level of adiponectin; and a statistically significant decrease in leptin:adiponectin ratio. This is important information in the context of future research, as increased levels of inflammatory cytokines, in addition to the factors mentioned above, may also be associated with leptin resistance, high leptin levels, and inhibition of adiponectin expression [32].
Considering that the curcumin content in turmeric is only a few percent and it also has a very low bioavailability after oral ingestion, in a study using a rat model, it was estimated at 1%. For this reason, supplements standardized for curcumin content are used for therapeutic purposes because the chances of obtaining a positive effect after using turmeric as a curcumin source are poor. Piperine from black pepper fruits can increase the bioavailability of curcumin by up to 2000% [52,53].
Our study has several strengths. First of all, only human studies were included. Studies with the exclusive use of curcumin supplementation were analyzed, without any other additional recommendations for patients, such as changing diet or lifestyle.
Because of differences in the study protocols and in the characteristics of supplemented patient cohorts in terms of age, BMI, and disease progression, more studies on the efficacy of curcumin supplementation in MAFLD patients of different ages and at different stages of disease progression should be conducted. In addition, further studies could compare effects of different dosages of curcumin supplementation and effects of various supplementation periods.

Conclusions
Curcumin has some therapeutic potential in MAFLD. However, the studies conducted so far do not allow us to unequivocally determine the positive effects of its action. It is also impossible to establish an effective dosing regimen. For these reasons it is necessary to conduct further studies using larger groups of patients and different doses of the supplement.