A fad too far? Dietary strategies for the prevention and treatment of NAFLD

Abstract

Nonalcoholic fatty liver disease (NAFLD) is a major health problem and its prevalence has increased in recent years, concurrent with rising rates of obesity and other metabolic diseases. Currently, there are no FDA-approved pharmacological therapies for NAFLD, and lifestyle interventions, including weight loss and exercise, remain the cornerstones for treatment. Manipulating diet composition and eating patterns may be a sustainable approach to NAFLD treatment. Dietary strategies including Paleolithic, ketogenic, Mediterranean, high-protein, plant-based, low-carbohydrate, and intermittent fasting diets have become increasingly popular due to their purported benefits on metabolic disease. In this review, we will highlight what is currently known about these popular dietary approaches in the management of NAFLD in clinical populations with mechanistic insight from animal studies. We will also identify key knowledge gaps to better inform future preclinical and clinical studies aimed at the treatment of NAFLD.

1.0. Introduction

Nonalcoholic fatty liver disease (NAFLD) is a progressive disease of the liver that ranges from hepatic steatosis (liver fat >5% of liver weight) to more severe steatohepatitis (NASH, hepatocellular inflammation), with progression to fibrosis and end stage cirrhosis (1). The definition of NAFLD also encompasses the absence of excessive alcohol intake (> 21 units/week for males and 14 units/week for females), hepatic viral infections, and the use of potentially hepatotoxic medications. NAFLD is one of the most common causes of chronic liver diseases in Western countries, affecting ~30% of the general US population, among whom nearly 25% may progress to NASH (2). Similarly, in Europe, about one-fourth of the general population suffers from NAFLD (3). NAFLD was typically thought to take three or more decades to progress; however, the prevalence of NAFLD is increasing at an alarming rate in children and adolescents with obesity, occurring as early as 10 to 20 years of age (4). Furthermore, the progression of NAFLD is the most rapidly increasing indication for liver transplantation and is predicted to surpass hepatitis C virus infection as the leading cause of liver transplantation in Western countries in the next decade (5). Patients with NAFLD are at greater risk for cardiovascular disease (CVD), type 2 diabetes and kidney disease (6). Moreover, NAFLD is an independent risk factor for liver-related and all-cause mortality, highlighting the need to curb disease progression.

2.0. Pathophysiology and Current Treatments for NAFLD

A number of factors are thought to contribute to the manifestation and progression of NAFLD. These include obesity, poor diet, sedentary lifestyle, genetic, epigenetic and environmental factors as well as insulin resistance and type 2 diabetes. These factors contribute to hepatic insulin resistance, hyperinsulinemia, inflammation, oxidative stress, mitochondrial dysfunction, imbalanced pro-inflammatory cytokines, and fibrosis (1). The initial clinical diagnosis of NAFLD is usually suspected in patients with excess weight or obesity and may, but not always, occur alongside mild elevations in liver enzymes, including elevated alanine aminotransferases (ALTs) and aspartate aminotransferases (ASTs). Liver imaging via ultrasound, MRS, and MRI are additional non-invasive measures used for screening patients with suspected NAFLD/NASH and to detect the presence of steatosis. However, histological assessment of the liver with liver biopsy is required for definitive diagnosis of NASH.

While hepatic steatosis is a reversible state, persistence of the underlying causes can result in progression to NASH - characterized by the presence of steatosis, inflammatory cell infiltration, hepatocellular injury, apoptotic hepatocytes, and Mallory-Denk bodies. As NASH progresses, the liver’s innate macrophages, Kupffer cells, become activated resulting in the secretion of pro-inflammatory cytokines. This response is also accompanied by infiltration of bone marrow derived macrophages, which further contributes to the activation of the inflammatory cascade. The progression of this inflammatory state promotes hepatic stellate cell activation and the onset and progression of hepatic fibrosis (7). The degree of inflammation varies considerably and does not always correlate with the degree of steatosis. Sustained inflammation and fibrogenesis can later contribute to the development of liver cirrhosis and hepatocellular carcinoma. While aspects of NAFLD pathophysiology are briefly described above, complexities related to disease development and progression are beyond the scope of this review but discussed in detail elsewhere (7, 8).

Currently, there are no pharmacological therapies or surgical procedures approved to treat NAFLD/NASH. Therapeutic approaches focus on the management of the disease through reducing related risk factors, including obesity, insulin resistance, hyperglycemia, dyslipidemia, inflammation and oxidative stress (2). Therapies aimed at slowing the development and progression of NAFLD and NASH have been met with some success, but further research is required. Pharmacotherapy studies have investigated the efficacy of several drugs including - weight reducing drugs, insulin sensitizers, lipid lowering agents, antioxidants, bile salts and co-factors increasing the mitochondrial transport of fatty acids are being considered, and have been reviewed extensively elsewhere (8). Those which have shown the greatest efficacy are the thiazolidinediones and vitamin E, in patients without type 2 diabetes; however, these therapies are not currently recommended to treat NAFLD in patients with type 2 diabetes (2).

In patients with obesity who are unresponsive to lifestyle changes and pharmacotherapy, bariatric surgery may be an option for reducing weight and metabolic complications of NAFLD (9). Currently, there are no randomized-controlled trials (RCTs) for bariatric surgery in the treatment of NAFLD; however, there are several retrospective and prospective cohort studies with follow-up liver biopsies which have explored the efficacy of bariatric surgery to treat NAFLD (10). The AASLD reports that bariatric surgery can be considered in patients with obesity and NAFLD whom are eligible for surgery (2). However, bariatric surgery has yet to be established as an option to specifically treat NAFLD/NASH.

Weight loss via caloric restriction remains the most viable option in the treatment of NAFLD and fibrosis (2). Reducing body weight by 7-10% with hypocaloric diets has been shown to be the most efficacious in the treatment of NAFLD/NASH and fibrosis (11, 12, 13), with greater weight loss (≥10%) being associated with the highest rates of NAFLD/NASH resolution and fibrosis regression (12). However, histological improvements are also observed with as little as 3-5% weight loss (12, 14). Further, both aerobic and resistance exercise training in the absence of weight loss has been shown to reduce intrahepatic lipid (IHL) in patients with NAFLD (15, 16, 17, 18, 19). However, whether exercise training without weight loss can reduce the histological features of NASH and fibrosis remains unknown (20). Clearly, more studies are required to further explore the molecular and cellular mechanisms involved and to define the optimal volume and intensity of exercise required and whether weight loss is required for histological improvement in NASH and fibrosis.

3.0. Significance of Current Review

Given that weight loss by dietary modifications is the only proven therapy to treat NAFLD, attention should be focused on various dietary options. The plethora of popular diets touted by fitness, health, and medical professionals leaves members of the public and clinicians to sift through the noise in order to attain accurate, non-biased information on dietary strategies for NAFLD. In this review, we aim to consolidate the most current information on popular diets including but not limited to - Paleolithic, ketogenic, Mediterranean, high-protein, plant based, low-carbohydrate, and intermittent fasting diets (Table 1; Fig. 1). Given the considerable popularity and media coverage these diets have garnered in recent years, this review will add important and timely updates on peer-reviewed evidence for their effects on reducing NAFLD development and progression.

Table 1.

Macronutrient characteristics of popular diets

Diet	Carbohydrate (%)	Protein (%)	Fat (%)	Comments
USDA	(45-65)	(10-30)	(25-35)	Current USDA Dietary Guidelines
Low-carbohydrate	↓ (≤40)	↑ (20-30)	↑ (≥25)
Ketogenic	↓ (≤5)	↔/↓ (10-15)	↑ (>70-90)	<20g carbohydrate per day
High-protein	↔↓ (≤45)	↑ (~40)	↔ (20-25)
Mediterranean	↔(50)	↔(15)	↔/↑ (30)	High in mono- and poly-unsaturated fats
Paleolithic	↓ (35-45)	↑ (20-30)	↔/↑ (30)
Plant-based	↔/↑ (50-70)	↔ (15-20)	↔/↓ (10-25)

↑ increase relative to USDA Dietary Guidelines; ↓ decrease relative to USDA Dietary Guidelines; ↔ moderate or no change relative to USDA Dietary Guidelines; USDA United States Department of Agriculture;

Figure 1.

4.0. Dietary Strategies for the Prevention and Treatment of NAFLD

4.1. Dietary Carbohydrate Content

Diets high in carbohydrates can prime the liver to increase hepatic de novo lipogenesis (DNL) in lean healthy individuals and to a greater extent in individuals with hyperinsulinemia and obesity (21). Hepatic DNL is a major contributor to NAFLD development (22). DNL refers to the biochemical process where fatty acids are synthesized from acetyl-CoA subunits derived from different sources, most notably carbohydrates. Further, the carbohydrate-induced increase in hepatic DNL has been shown to be more pronounced with higher consumption of simple sugars (23). Hence, simply reducing dietary carbohydrates may be an effective strategy for NAFLD prevention and treatment. Although low-carbohydrate diets are gaining in popularity as an efficacious approach to weight loss (24), there still remains significant controversy regarding their metabolic effects and long-term sustainability – particularly in the setting of NAFLD.

In one study comparing a low-carbohydrate or low-fat hypocaloric diet, patients at high risk of NAFLD were randomized to a 16-week low-carbohydrate (40% carbohydrate, 45% fat) or low-fat (60% carbohydrate, 25% fat) diet of equal energy deficiency (−750 kcal/day) (25). The low-carbohydrate group reduced serum ALTs to a greater extent compared with the low-fat diet group (−4.2 ± 8.3 U/L) despite equal weight loss. Similarly, short term carbohydrate restriction (<50 g/day) over a period of 48 hours reduced IHL to a greater extent than a low-fat, high-carbohydrate feeding in patients with NAFLD (−29.6±4.8% vs. −8.9±1.4%, low-carbohydrate vs low-fat, respectively) (26). However, maintaining these patients on low- or high-carbohydrate moderate caloric restriction diets for 3 months with similar weight loss (−7%) resulted in no significant differences in reductions in IHL (low-carbohydrate diet, −38.0±4.5% vs. low-fat diet, −44.5±13.5%). This suggests that total caloric deficit and weight loss may be the mediating factor for decreasing IHL. In fact, six months of either low-fat (<20% fat, 0.8 g protein/kg body weight, remaining energy content provided by carbohydrate) or low-carbohydrate (≥30% Fat, ≥90 g carbohydrate, 0.8 g protein/kg body weight) hypocaloric diet resulted in similar reductions in IHL (~7 fold decrease in IHL vs. baseline) (27). These data suggest a negative energy balance may be the mediating factor in decreasing IHL, independent of visceral fat loss or changes in insulin sensitivity, and that macronutrient content of the hypocaloric diet may be secondary in a prolonged intervention study. While evidence suggests that a hypocaloric, low-carbohydrate diet may be more beneficial to patients with or at risk of developing NAFLD, no rigorous longitudinal prospective trials have linked specific diet composition with histologic improvements in NAFLD, and it remains unclear whether consuming low-carbohydrate or low-fat diets produce long-term benefits in the absence of significant weight reduction.

The quality and type of carbohydrate also likely influences the development and progression of NAFLD (28). The glycemic index (GI) is a quantitative measure of a food’s carbohydrate content through its postprandial glycemic effects; high GI foods cause a rapid rise in blood glucose while lower GI foods result in a blunted glucose response (29). High GI carbohydrate consumption is associated with worsening metabolic outcomes, including insulin resistance and increased IHL in healthy individuals (30) and individuals with insulin resistance (31). Conversely, regular consumption of low-GI foods is associated with reduced risk of metabolic disease including diabetes, CVD and NAFLD (32). In one RCT, patients on a low-GI Mediterranean diet displayed the greatest reductions in hepatic fat at six months compare to those assigned a standard low-fat control diet (33). In addition, a recent meta-analysis reported that low-GI diets may reduce hepatic liver mass and serum ALTs in children and adults with NAFLD (34). The authors suggest that the alterations of high and low-GI diets on hepatic fat deposition may act through differing effects on fatty acid synthesis pathways and further research is required. Long-term RCTs using isoenergetic, low-GI diets to treat NAFLD are required to further explore their efficacy in the treatment of NAFLD.

4.2. Ketogenic Diets

Ketogenic diets, very low in carbohydrate and very high in fat, have been used traditionally to treat epileptic disorders and more recently touted as therapy for a multitude of health conditions including neurodegenerative diseases, cancer, obesity, heart failure, type 2 diabetes, and NAFLD. In epileptic disorders, these diets consist of 75-80% fat, 10-15% protein and ≤ 5% carbohydrate (35). Despite gaining popularity, there are limited data to support the efficacy of a ketogenic diet in humans to treat metabolic disease, particularly NAFLD.

Short-term carbohydrate restriction (20-30 g/day), emulating a ketogenic diet, has been shown to significantly reduce IHL (36, 37) and a marked decrease in de novo lipogenesis (37). However, it is important to note that these changes occurred in the presence of weight loss of ~2%. Carbohydrate restriction (<20 g/day) was shown to reduce IHL to a greater extent than a total macronutrient restricted diet alone (1,200-1,500 kcal/day) in patients with biopsy proven NAFLD, despite similar weight loss between groups (36). The authors highlight that this may be due to enhanced hepatic and whole body mitochondrial oxidative capacity and fat oxidation (36). Similarly in a more recent study, ketogenic diet feeding (23 ± 1 g carbohydrate/day) for 6 days decreased IHL by 31% in the presence of a 3% reduction in body weight in men and women with obesity and insulin resistance (38). However, while this study decreased carbohydrate intake, there was no change in protein and fat intake, resulting in an overall reduction in energy intake and likely influencing IHL outcomes. Further, in a long-term pilot study of five patients with biopsy-proven NASH, six-months of ketogenic diet feeding (limiting carbohydrate intake to <20 g/day without overall caloric restriction) reduced body weight by 13 kg and improved histological measures of hepatic steatosis, hepatocellular inflammation, and fibrosis in follow-up biopsies (39). While promising, given the absence of a comparison group and the small sample size, it is difficult to determine whether the observed effects are specifically due to the salutatory benefits of ketogenic diet or to the weight loss during the intervention. There is a clear need for well-controlled studies with respect to effects of a ketogenic diet on NAFLD. This includes the need for evaluation of efficacy and safety with the dramatic shifts in dietary macronutrient content and studies where weight loss is a controlled factor.

Given the dearth of studies examining ketogenic diets on NAFLD outcomes in humans, we briefly highlight some work conducted in rodent studies, fully acknowledging the disparaging findings in rodents (35, 40, 41, 42, 43, 44, 45, 46) and the difficulties in comparing to humans. Rodents fed a ketogenic diet exhibit a lean, ketotic, euglycemic, and hypoinsulinemic phenotype. These favorable changes were observed despite the presence of glucose intolerance and an increase in hepatic steatosis in ketogenic diet fed mice (41). Further, ketogenic diet feeding is also associated with a reduction in markers of DNL (41, 44), improved mitochondrial quantity and quality and increased antioxidant defense systems (44). In contrast, livers of ketogenic diet fed mice have also been shown to develop hepatic endoplasmic reticulum (ER)-stress, inflammation, macrophage accumulation, and hepatocellular injury (41). A number of potential factors may influence the differing hepatic phenotypes. In particular, the reduced choline and methionine content of a rodent ketogenic diet, in which deficiencies have been shown to induce NAFLD (45, 46). Further, it is indicated that diets containing <14% protein result in growth retardation in rodent models and may also contribute to the observed NAFLD phenotype (46). These potential confounding factors should be considered in future rodent research and when translating these findings to humans.

Capacity for ketogenesis may also play a role in adaptation to a ketogenic diet and metabolic outcomes. Hepatic ketogenesis may feedback and regulate TCA cycle flux, anaplerotic flux, phosphoenolpyruvate-derived gluconeogenesis and glycogen turnover (40). Conversely, impaired ketogenesis has been shown to be associated with impaired hepatic metabolism and NAFLD severity in both animal models and humans (40, 47). Impaired ketogenesis directs excess acetyl-CoA to increase TCA cycle flux, which in the liver can result in increased oxidative stress-mediated injury and divert excess carbons toward de novo lipogenesis (40, 48, 49). These findings highlight that ketogenesis plays an important role in hepatic and whole-body homeostasis and is not a passive overflow pathway. Future experiments are required to explore the mechanisms by which ketogenic insufficiency may become maladaptive, contribute to hyperglycemia, provoke steatohepatitis, and if these mechanisms are exacerbated in the setting of a ketogenic diet and metabolic disease including NAFLD.

Ketone bodies, while used for energy in other tissues, are not oxidized by hepatocytes due to the lack of expression of succinyl-CoA:3-ketoacid CoA transferase; however, they can contribute to lipogenesis and serve as signaling molecules in hepatocytes and neighboring cells, independent of their oxidation. Interestingly, a recent report suggests the ketone body acetoacetate metabolism in mitochondria in liver tissue macrophages plays a role in protecting against the pro-fibrogenic response to hepatic injury (50).

In summary, ketogenic diets are being recommended more frequently to treat metabolic diseases such as NAFLD, type 2 diabetes, and obesity. While they appear beneficial, their macronutrient impact and signaling effects on metabolism are not yet completely understood, and rodent and patient responses to these diets are variable. Future studies are required to further elucidate the therapeutic potential of a ketogenic diet in the setting of NAFLD and should consider the effects of low protein content and dietary fatty acid composition.

4.3. High-Protein Diets

High-protein diets have been successful in reducing body weight and lowering markers of metabolic syndrome, while increasing satiety, thermic effect of food, and are shown to be superior to lower protein diets at retention of muscle mass (51). Retention of muscle mass may be important for patients with NAFLD, as sarcopenia is an independent risk factor for NAFLD (52). However, the role of high-protein diets specifically in the treatment of NAFLD remains understudied. The current protein recommended daily allowance for adults is 0.8 g/kg body weight, although higher protein intake (1.0 – 1.6 g/kg body weight) is associated with lower adiposity and CVD risk (53).

Increasing dietary protein intake on the background of a hypocaloric diet has been shown to improve serum lipids, glucose homeostasis and liver enzymes in patients with NAFLD, independent of changes in BMI or body fat mass (54). Similarly, four weeks of protein supplementation (60 g/day) in ad libitum fed young-adult women with obesity decreased IHL by 20% without changes in body weight, although fat mass decreased slightly (55). In addition, increasing dietary protein in a high-fat hypercaloric diet blunted the increase in IHL by 22% when compared to a high-fat diet alone, in the absence of differences in carbohydrate intake (56). Furthermore, IHL was shown to be decreased by 30% with high vs normal protein during four weeks of high-fat overfeeding in healthy men and women (57); however, it is important to note that carbohydrate intake decreased in the high-protein diet group, which may have influenced IHL outcomes. Taken together, these data suggest that higher protein content in the diet may offset the detrimental effects of a short-term high-fat diet on hepatic steatosis, but additional studies controlling for carbohydrate and fat intake are needed. Further, data suggest that higher protein diets regardless of source may be a viable option for reducing NAFLD. Both diets high in plant and animal protein (30%) have been shown to reduce IHL to similar levels in individuals with type 2 diabetes and NAFLD (58). These beneficial changes in IHL were seen with isocaloric diets and in the absence of weight loss or alteration in resting energy expenditure. Notably, dietary protein was increased at the expense of fat, suggesting that the reductions in liver fat cannot be simply attributed to the reduction in dietary carbohydrate. On the other hand, it was recently demonstrated that acute high protein feeding rich in glutamate increased DNL associated triglycerides and liver-derived VLDL particles in healthy male subjects (59). The authors demonstrated that the amino acids glutamate, glutamine, and lysine were stimulatory for DNL pathways in hepatocytes. Consequently, the popularization of high-protein diets to combat NAFLD may require more careful consideration and greater understanding of the role of certain amino acids in their contribution to hepatic steatosis.

Similar to findings in existing studies in humans, high-protein diets lower liver fat compared to normal protein diets in male rats, with further reductions when combined with caloric restriction (60). Gene and protein expression of key players in hepatic lipogenesis (ACC, FAS, SCD, SREBP-1c) are decreased with higher protein diets regardless of caloric intake, suggesting independent hepatic metabolic advantages with higher dietary protein despite elevating hepatic gluconeogenesis, likely from the increased oxidation of amino acids (61, 62). However, these studies replaced dietary carbohydrate with higher protein, and this may be the driver behind reduced hepatic DNL. Others have also reported that high-protein diets both prevented and reversed hepatic steatosis development in male mice, regardless of dietary carbohydrate, fat, or overall caloric intake (63, 64). Both studies found increased amino acid oxidation and mitochondrial oxidative capacity, indicating further potential benefits of elevated substrate oxidation in the liver with high-protein diets.

Overall, high-protein diets consistently demonstrate reductions in hepatic steatosis in both human and rodent models of NAFLD. In some cases, this phenomenon is also seen independent of reductions in body weight. While we have already stated that reductions in dietary carbohydrate may mediate beneficial effects on IHL, shown here is that the addition of dietary protein or increasing protein at the expense of dietary fat also exerts beneficial effects on NAFLD outcomes (56, 65, 66). This suggests that increasing dietary protein may have beneficial effects on liver outcomes, independent of dietary carbohydrate. Further research on increasing dietary protein to combat NAFLD is warranted, but high-protein diets may be a safe and practical dietary strategy for patients with NAFLD.

4.4. Mediterranean Diet

Over the last several decades, many epidemiological studies have examined the Mediterranean Diet (MD) and its positive effects on cardiovascular health and cancer (67). The diet most often described consists of fresh fruit and vegetables, unrefined cereals, olive oil and nuts, replacing red meat with fish and legumes, and moderate alcohol consumption in the form of red wine. The macronutrient composition of the diet includes 40-50% of the energy intake from carbohydrates (~80% coming from complex carbohydrates such as whole grain bread, pasta, and rice), 10-20% protein (particularly white meat and fish), and 30-40% fat – mainly from monounsaturated fats and polyunsaturated omega-3 fatty acids (67). The benefits of such a diet on insulin sensitivity, glucose metabolism, and long-term weight reduction has substantially increased interest in this diet particularly for the treatment of metabolic syndrome (67).

The MD is the only diet recommended by the European Association for the Study of the Liver (EASL), European Association for the Study of Diabetes (EASD), and European Association for the Study of Obesity (EASO) as a potential therapy for patients with NAFLD (68). Indeed, several cross-sectional studies have reported that adherence to a MD is associated with more favorable outcomes in NAFLD, including reduced hepatic steatosis and improved glycemic control (69, 70). Specifically, diets enriched with omega-3 polyunsaturated fatty acids, as is the case with a MD, have been shown to increase insulin sensitivity, reduce hepatic steatosis, and reduce NASH (71). In addition, studies have shown that compared to low-fat/high-carbohydrate diets, patients with NAFLD on an ad libitum Mediterranean-style diet significantly reduced hepatic steatosis (39 ± 4% vs. 7 ± 3%, as measured by MRS) and improved insulin sensitivity (72). Similarly, individuals with type 2 diabetes on an isocaloric Mediterranean-style diet displayed reductions in serum ALTs and IHL, as well as improvements in other markers of cardiovascular health compared to a low-GI diet (73, 74). Collectively, these studies suggest that a MD may in fact be superior to a low-fat/high-carbohydrate or low GI diet in ameliorating NAFLD development and progression.

The benefits of a MD on NAFLD outcomes is thought to be due to the diet’s lipid lowering effects, as well as antioxidant and anti-inflammatory properties of the high monounsaturated and polyunsaturated fats (75). Polyunsaturated fatty acids act on major regulatory pathways in hepatic lipid and carbohydrate metabolism to enhance fatty acid oxidation (PPARα) and suppress glycolysis and DNL (ChREBP, SREBP-1)(76, 77). Polyunsaturated fatty acids may also induce an independent anti-inflammatory effect in the setting of NASH via suppression of TNFα and IL-6 (78). Whether the use of MD is superior to a standard low-fat diet on NAFLD outcomes remains unclear. Following a 12-week intervention, patients with NAFLD displayed similar improvements in IHL regardless of whether they followed an ad libitum MD (35-40% kcal fat, 40% kcal carbohydrate, 20% kcal protein) or low-fat diet (30% kcal fat, 50% kcal carbohydrate, 20% kcal protein) (79). Conversely, it has recently been shown that a low-carbohydrate MD (<70 g/day carbohydrate) more effectively reduced hepatic fat content than a low-fat diet (<30% kcals from fat), in the presence of moderate weight loss in individuals with obesity (80). Larger RCTs are needed to assess the long-term benefit of a Mediterranean-style diet on patients with NAFLD, and whether this diet is unique in its ability to ameliorate progression of the disease.

Overall, the benefits of the MD on IHL, glucose metabolism, and insulin sensitivity are apparent, though it does remain unclear the exact mechanism by which this is accomplished. Regardless, ample evidence exists supporting the continued use of the MD in preventing and treating NAFLD.

4.5. Paleolithic Diets

In an effort to combat the rise of hyper-processed Westernized diets, unprocessed whole food diets have increased in popularity. A widely known diet in this category is the Paleolithic diet (also known as the Hunter-Gatherer or Paleo diet). The Paleolithic diet aims to replicate the eating patterns of humans during the Paleolithic era which typically included lean meats, roots, vegetables, seasonal fruits, nuts and seeds, while limiting foods unavailable during that era, such as dairy, grains, and legumes (81). Compared to guideline-based control diets, Paleolithic diets have shown promise in improving glucose homeostasis and serum lipids in patients with metabolic syndrome (81).

Paleolithic diet may be a potential therapeutic for the hepatic manifestation of the metabolic syndrome, NAFLD. Despite the potential benefits of a Paleolithic diet based on the nutrient profile, there are relatively few studies which have examined the effects on NAFLD outcomes. Short-term (5 weeks) Paleolithic diet feeding has been shown to reduce IHL in post-menopausal women (82), an effect likely driven by reduced food intake and weight loss (−4.5 kg). This effect is not surprising, given that Paleolithic diets are reported to be more satiating compared to Mediterranean or low-fat diets (83). In addition, six months of a Paleolithic diet decreased IHL by 64% compared with a 43% reduction with a low-fat control diet in healthy, postmenopausal women with obesity (84). Interestingly, hepatic insulin sensitivity was improved only in the Paleolithic diet group; however, the added benefits of the Paleolithic diet were lost after 2 years of follow-up. There is also limited evidence that a Paleolithic diet alone was as effective as combining a Paleolithic diet with aerobic exercise training for 12 weeks in reducing IHL, suggesting no additional benefit of adding exercise to this dietary strategy (85). However, the lack of a control diet group makes it difficult to infer any inherent benefits on a Paleolithic diet beyond the significant weight lost by the participants.

The potential benefits of a Paleolithic diet on NAFLD may be due to its macronutrient composition, typically consisting of a moderate amount of carbohydrate and fat (22-40% and 28-47% of total energy, respectively), and high protein (19-35% of energy) when compared to other popular diets (86). Paleolithic diets also often contain high amounts of mono- and poly-unsaturated fatty acids (86); as highlighted above, these fatty acids have been shown to be beneficial on NAFLD outcomes. The lower amount of carbohydrate, particularly refined carbohydrates and fructose in Paleolithic diets, likely also imparts beneficial effects in patients with NAFLD by reducing glycemic load and stimulus for hepatic DNL. Additionally, the high protein content in Paleolithic diets may contribute to the amelioration of fatty liver as high-protein diets have been shown to reduce liver fat with and without weight loss, as previously mentioned (55, 56, 57). Collectively, these studies suggest that Paleolithic diets may have beneficial effects on reducing IHL beyond just reductions in body weight. However, interventions comparing Paleolithic diets with diets similar in carbohydrate and protein amounts are needed to determine if there are inherent benefits to Paleolithic diets themselves. Longer-term and better-controlled studies are required before Paleolithic diets should be recommended as a dietary strategy specifically for the treatment of NAFLD.

4.6. Plant-Based Diets

Recently, plant-based diets have risen in popularity both socially and experimentally due to their purported health benefits and risk reduction for chronic disease. Plant-based diets emphasize a high intake of plants and plant-derived food (i.e. grains, legumes, vegetables, and fruits) and low intake of animal-derived food; whereas vegetarian and vegan diets are characterized by varying levels of abstinence from animal products. A significant amount of evidence supports healthful, plant-based diets as well as vegetarian and vegan lifestyles in reducing the risk of CVD, insulin resistance and type 2 diabetes (87, 88), all of which are risk factors of NAFLD. Therefore, it is logical that plant-based dietary strategies would also be effective at reducing risk and treating NAFLD. Accordingly, multiple recent reports demonstrate the beneficial effects of plant-based diets on NAFLD risk.

In 18,000+ U.S. adults, healthy, plant-based diets were associated with 21% reduced risk of NAFLD via measurements of ALT, AST, and the fatty liver index (89). Similar results were reported in a cohort of Chinese adults consuming a ‘grain-vegetable’ diet using ultrasound to grade steatosis (90). In a comparison of the quartiles with the highest and lowest grain-vegetable intake, the former had lower prevalence of NAFLD while the greatest intake of animal foods was associated with the highest prevalence of NAFLD. An analysis of metabolic risk in 773 subjects demonstrated those who were classified as vegetarian had lower metabolic syndrome components and reduced risk for metabolic syndrome (91). Many reports also show beneficial effects of plant-based diets on the prevention and treatment of type 2 diabetes and CVD, based on the plant-based diet index in large cohort trials (92, 93, 94). While these data are promising, the aforementioned studies were strictly observational and lacked objective dietary composition measurements as well as a robust NAFLD diagnosis.

The Ornish diet, a very low-fat (<10% kcal from fat) vegetarian-style diet was originally developed as a dietary approach for the treatment of CVD, alongside an exercise and stress-relief lifestyle intervention (95, 96, 97). The diet consists of unlimited consumption of fruit, vegetables, grains, beans, and legumes and restricts all meats (including fish), oils and fats, nuts, avocados, dairy products, and simple carbohydrates. Several studies have demonstrated the effectiveness of the Ornish Diet in preventing and even reversing CVD at 1 and 5 year follow-up vs standard of care (97). Whether a diet of similar macronutrient and food content would be beneficial in treating NAFLD is unknown. Further, given its restriction of fats, the Ornish Diet is inherently low in poly- and mono-unsaturated fats which have been associated with improvements in insulin resistance, inflammation, blood lipids and NAFLD outcomes, as discussed previously.

In terms of plant-based diet interventions, a 16-week vegan plant-based diet was more effective than a control diet in lowering body weight, fat mass, insulin resistance (HOMA-IR), and beta-cell function improvements in subjects with obesity (98, 99). The same study also concluded that an increase in fiber was associated with these beneficial outcomes (100), in agreement with other reports on NAFLD risk factors (89, 90). Long-term intervention trials investigating plant-based diets compared to alternative diets with rigorous dietary control, NAFLD classification, and mechanistic designs are required especially given existing data which do not support beneficial effects of plant-based diets on NAFLD risk (58, 101).

4.7. Intermittent fasting

Dietary macronutrients clearly play an important role in NAFLD, but it is possible that the timing and/or frequency of eating meals is a factor to consider. Consuming the majority of calories in the evening or at night is associated with higher risk of NAFLD, while shifting the bulk of daily caloric intake towards earlier in the day is associated with decreased risk for hepatic steatosis (102). Further, long feeding windows can perturb metabolic pathways that are entrained by circadian rhythms, and disruption of these pathways increases susceptibility to metabolic dysfunction, including NAFLD (103, 104). This lends support to the notion that fasting periods, where little to no food is consumed, may have potential benefit for NAFLD prevention. Indeed, given the difficulty of patient adherence with caloric restriction, intermittent fasting (IF) diets have gained in popularity as an alternative approach to traditional caloric restriction. IF is a broad term that encompasses any eating pattern that periodically limits food consumption for a predetermined amount of time. IF regimes can include but are not limited to: alternate day fasting or every other day fasting, consuming little or no food on two consecutive days (5:2) followed by normal eating patterns on non-restricted days (105, 106), periodic fasting (2-21 days of minimal calories), and periods where food consumption is restricted to certain hours during the day (typically 4-8 hours), known as time restricted feeding (TRF).

Both IF and caloric restriction diets have shown to be equally effective in managing weight loss and improving cardiometabolic outcomes (106, 107). Alternate day fasting in healthy subjects reduced fat mass, body weight, and increased β-hydroxybutyrate, likely via the observed 37% reduction in calories (108). Similarly, a well-controlled clinical trial demonstrated that early TRF improved insulin sensitivity, β cell responsiveness, and oxidative stress in men with pre-diabetes without weight loss (109). A recent prospective observational trial investigated the effects and safety of periodic fasting in subjects with and without type 2 diabetes (110). After an average of 8.5 ± 4.0 consecutive days of fasting (maximum total energy intake of 250 kcal/day), almost half of the 246 subjects with fatty liver decreased their fatty liver index score. Reductions in the fatty liver index correlated with the number of days fasting and the magnitude of weight lost and BMI reduction. It is also important to note the fasting therapy was accompanied by a physical exercise program with moderate walking and gymnastics as group activities, which may have also influenced fatty liver index outcomes. Unfortunately, there is limited other data on the benefits of fasting protocols on measures of NAFLD. Ramadan fasting (1 month of 15-16 hour fasts) reduces body mass and body fat percentage, along with improvements in plasma glucose, insulin, and HOMA-IR in people previously diagnosed with NAFLD (111), and also reduces serum ALTs in individuals with metabolic syndrome (112). Conversely, others have shown that Ramadan fasting actually increased serum ALTs in patients with NAFLD (111). While Ramadan fasting provides a unique opportunity to study the effects of fasting on NAFLD outcomes, multiple confounders with this approach include meals that break the fast are typically high in fat and sugar, sleep and circadian rhythms are often disrupted, and physical activity levels differ during these periods (113). While various intermittent fasting protocols in humans show promise as a NAFLD therapeutic, better controlled studies patients with NAFLD and/or liver specific outcomes are required.

Rodent literature provides more mechanistic insight into the potential benefits of different fasting protocols on NAFLD development. Alternate day fasting in mice is associated with decreased body weight, IHL, serum ALTs, and suppression of genes involved in hepatic lipogenesis, despite no difference in total energy intake compared to ad libitum fed mice (114). Restricting feeding to the active period in mice for two weeks resulted in a ~50% reduction of liver fat compared to ad libitum fed mice, despite no differences in total food intake between the groups (115). Similarly, mice undergoing TRF had increased hepatic AMP-activated protein kinase (AMPK) and improved oscillations of the circadian rhythm, while suppressing hepatic lipogenesis (116). However, despite comparable energy intake, TRF mice had lower body weights than those with ad libitum feeding, likely due to slight elevations in energy expenditure. Interestingly, switching high-fat diet-fed mice to a TRF regime prevented further hepatic lipid accumulation, indicating the potential for TRF as a rescue and prevention for fatty liver (117). Further, 18 weeks of TRF (3-5 hours) in mice fed a high- or low-fat diet attenuated body weight gain and epididymal fat mass even in the face of equivalent calories consumed vs controls, although no changes in liver fat were observed (118). Unfortunately, the feeding time was during the light cycle as opposed to the nocturnal feeding schedule of mice which may have influenced hepatic outcomes.

Collectively, these data outline a potential benefit for fasting protocols in attenuating NAFLD development. However, results are much more robust in animal models, and appear to be driven by reductions in body weight, with possible mechanisms including re-entrainment of hepatic circadian rhythms and a switch to more hepatic lipid oxidation over synthesis. While rodent studies of intermittent fasting are important to tease out potential mechanisms of action, fasting times for rodents and humans are difficult to compare physiologically. The nocturnal-circadian rhythms and elevated resting metabolic rate of rodents are potential confounding factors that should be acknowledged when extrapolating these findings to humans. Overall, fasting protocols appear to show promise as a therapeutic for patients with NAFLD as an alternative to caloric restriction diets, although longer-term fasting randomized controlled trials examining NAFLD outcomes in humans are warranted.

Concluding Remarks

Current guidelines recommend lifestyle modifications including a caloric restricted diet and weight loss in the treatment of NAFLD/NASH. However, there is little agreement regarding whether diet composition or following a particular dietary pattern provides greater benefit. Here we summarize the existing literature on several popular dietary approaches and what is currently known about their role in the management of NAFLD/NASH and highlight gaps in the current literature. The Mediterranean diet has received the most attention with the most promising results and is recommended by the EASL-EASD-EASO organizations for patients with NAFLD. The other dietary strategies summarized here, including Paleolithic, ketogenic, high-protein, plant-based, low-carbohydrate, and intermittent fasting approaches all have shown some promise in beneficially impacting hepatic outcomes. However, it is clear that prospective, long-term, randomized trials with liver histopathological endpoints and controlling for body weight and adiposity changes are required before recommendations should be made regarding the majority of these dietary approaches in the specific treatment of NAFLD/NASH.

What is already known about this subject?

• Currently, caloric restriction and weight loss remains the cornerstone of NAFLD treatment in patients. However, there is little consensus on particular dietary options and strategies that may be most effective in the treatment of NAFLD.

What are the new findings in your manuscript?

• Current evidence-based data state that compliance with a long-term calorie-restricted diet, independent of specific recommendations for diet composition, results in the resolution of NAFLD/NASH.

• Low-carbohydrate, high-protein, Mediterranean, Paleolithic, ketogenic and plant-based diets, as well as certain fasting protocols, appear to have some promising outcomes for patients with NAFLD; however, prospective trials comparing these various strategies in NAFLD patients are limited and further research is required.

How might your results change the direction of research or the focus of clinical practice?

• The existing literature on many of the dietary strategies outlined in this review have received limited attention in a clinical NAFLD population.

• Here we summary that a range of diets with varied macronutrients may be a viable option for NAFLD patients.

• Future research utilizing large randomized-controlled trials with pre- and post- outcome measures of NAFLD/NASH/fibrosis should be pursued.