NAFLD (Non-Alcoholic Fatty Liver Disease) is a prevalent liver condition where fat accumulates in the liver cells of individuals who consume little to no alcohol. While weight loss can help alleviate NAFLD, improving the quality of the diet, especially focusing on higher-quality carbohydrates like whole grains, fruits, and vegetables, may also offer benefits for NAFLD management.
Gut Microbiota
The gut microbiota is a crucial factor in how the food we eat can lead to NAFLD. Metabolites produced by gut bacteria, including LPS (lipopolysaccharides), are carried to the liver through dietary fats and chylomicrons, leading to chronic liver inflammation [1]. In contrast, short-chain fatty acids produced by probiotics can reduce inflammatory cytokines secreted by the liver. Leaky gut can also be a risk factor for NAFLD, as studies have found a connection between its severity and NAFLD [2].
In 2019, a study found that certain NAFLD patients experienced symptoms resembling alcohol consumption, even though they hadn’t drunk alcohol [3]. Researchers discovered a bacterium called Klebsiella pneumoniae (Kp) in their guts, which can turn carbohydrates into alcohol. Further analysis of 32 NASH patients’ feces showed that 21 of them also had Kp. Transplanting Kp into mice led to the development of NAFLD, similar to how alcohol-induced NAFLD occurs.
Gut bacteria’s impact on the liver, similar to alcohol effects, might be just one of the pathways in developing fatty liver disease. Ongoing research may uncover more about how gut microbiota influences this condition, explaining why the same macronutrient proportions can have varying effects due to their impact on gut bacteria.
Fructose and Sucrose Worsen NAFLD
Among high-carbohydrate diets, those containing fructose may have the most significant impact on liver fat accumulation. When carbohydrates are digested, they turn into glucose and other monosaccharides. Glucose is absorbed into the circulatory system to provide energy, regulated by insulin to control blood sugar levels and enter cells as an energy source. In contrast, fructose metabolism differs entirely from glucose. It is not regulated by insulin, so when consumed, it bypasses insulin secretion and directly reaches the liver. Fructose undergoes fructokinase metabolism, rapidly converting into triglycerides and fat stored in the liver, leading to NAFLD and other metabolic-related diseases.
In a study published in 2016, researchers found that fructose is more likely to cause NAFLD than a high-fat diet, with both involving de novo lipogenesis. Sugary beverages like soft drinks, which contain fructose, are the main contributors to NAFLD [4].
In another study conducted in 2015, eight participants underwent an 18-day experiment [5]. For nine days, they consumed a large amount of fructose, followed by another nine days where other carbohydrates replaced fructose while keeping the same caloric intake. During the fructose consumption period, the participants experienced de novo lipogenesis, where the liver converted excess energy into fat storage, leading to NAFLD formation. This trial showed that despite constant calories and carbohydrate intake, fructose consumption was linked to the development of NAFLD.
Increasing fructose exacerbates NAFLD, while reducing fructose can reverse it. In a study published in 2017 [6], involving 41 obese children aged 8 to 18 who habitually consumed large amounts of fructose (with 63% having excessive liver fat), their fructose intake was restricted to 4% of their required energy, and the reduced calories were replaced with starchy foods, keeping the total energy and carbohydrate intake unchanged. After nine days, the participants showed reduced liver fat, decreased body fat, lowered insulin resistance, and improved various health indicators. The researchers concluded that this study supports reducing sugar intake.
In another controlled clinical study in 2019 [7], without changing the overall dietary structure, simply eliminating sugary foods (sucrose and fructose) and sugary drinks (including fruit juices) to less than 3% of calories significantly improved NAFLD in adolescents after eight weeks.
Added sugars, especially fructose, have a greater impact on NAFLD than a typical starchy diet. Cutting back on sugar and sugary beverages is the most effective and straightforward way to improve NAFLD.
Improving NAFLD with Fruits
While fructose is unfavorable for NAFLD, fruits, on the other hand, can help improve the condition. A 2020 epidemiological study from South Korea [8], analyzing 2130 NAFLD cases, found that fruits and vegetables can reduce the risk of NAFLD by about 25%. Specifically, fruits alone reduced the risk by 23% and vegetables by 29%.
Another cohort study conducted in 2020 also discovered a significant inverse relationship between fruit consumption and NAFLD risk, indicating that fruits can lower the risk of NAFLD [9].
In a 2017 study from Spain, 70 obese NAFLD subjects underwent a calorie-restricted (CR) diet intervention, divided into two groups. After 6 months, both groups showed similar macronutrient intake, but the group with a higher fruit intake demonstrated more significant improvements in NAFLD. Researchers attributed the positive effects to the abundant dietary fiber in fruits, which significantly improved liver function. [10].
While fructose may be unfavorable for NAFLD, fruits have a beneficial impact on the condition due to their content of dietary fiber and polyphenolic antioxidants, proving that the “quality” of diet may be more critical for NAFLD than simply focusing on macronutrient ratios.
Improving NAFLD with Whole Grains
A 2014 study found that refined grain consumption increased NAFLD risk, while whole grain consumption lowered the risk [11]. In a 2020 clinical study, replacing half of grain starch foods with whole grains for 12 weeks in obese NAFLD patients improved liver inflammation and enzymes, possibly due to improved gut microbiota and increased short-chain fatty acids from probiotics [12].
In a clinical study from 2018 [13], increasing dietary fiber was found to improve liver function in NAFLD patients. During a six-month period, 32 NAFLD participants incorporated whole wheat bread, brown rice, and other high-fiber foods into their diet, raising their daily fiber intake from 19g to 29g. This change resulted in significant reductions in sensitive indicators of liver cell damage, such as serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST), indicating improved liver function. The study suggested that dietary fiber improved liver function by addressing gut permeability. Replacing refined grains with whole grains is a healthy practice.
Summary
Improving the quality of the diet, especially focusing on higher-quality carbohydrates like whole grains, fruits, and vegetables, can benefit NAFLD management.
Gut microbiota plays a crucial role in NAFLD development, with metabolites from gut bacteria contributing to liver inflammation.
Fructose and sucrose worsen NAFLD, while reducing sugar intake, especially sugary beverages, is an effective way to improve the condition.
Whole grains and dietary fiber positively impact NAFLD, as studies show they can improve liver function and reduce liver fat accumulation.
References:
[1] Ni, Yinhua et al. (2020). The Gut Microbiota and Its Metabolites, Novel Targets for Treating and Preventing Non-Alcoholic Fatty Liver Disease. Molecular Nutrition & Food Research. 64. 10.1002/mnfr.202000375.
[2] Ji, Yun et al. (2020). The Molecular and Mechanistic Insights Based on Gut–Liver Axis: Nutritional Target for Non-Alcoholic Fatty Liver Disease (NAFLD) Improvement. International Journal of Molecular Sciences. 21. 3066. 10.3390/ijms21093066.
[3] Yuan, Jing et al. (2019). Fatty Liver Disease Caused by High-Alcohol-Producing Klebsiella pneumoniae. Cell Metabolism. 30. 1172. 10.1016/j.cmet.2019.11.006.
[4] Softic, S. et al. (2016). Role of Dietary Fructose and Hepatic De Novo Lipogenesis in Fatty Liver Disease. Digestive diseases and sciences, 61(5), 1282–1293. https://doi.org/10.1007/s10620-016-4054-0
[5] Schwarz JM, Noworolski SM, Wen MJ, et al. Effect of a High-Fructose Weight-Maintaining Diet on Lipogenesis and Liver Fat. J Clin Endocrinol Metab. 2015;100(6):2434–2442. doi:10.1210/jc.2014-3678
[6] Schwarz, Jean-Marc et al., Effects of Dietary Fructose Restriction on Liver Fat, De Novo Lipogenesis, and Insulin Kinetics in Children With Obesity , Gastroenterology, 2017. Volume 153, Issue 3, 743 – 752
[7] Schwimmer, J. B. et al. (2019). Effect of a Low Free Sugar Diet vs Usual Diet on Nonalcoholic Fatty Liver Disease in Adolescent Boys: A Randomized Clinical Trial. JAMA, 321(3), 256–265. https://doi.org/10.1001/jama.2018.20579
[8] Kim, S. A., & Shin, S. (2020). Fruit and vegetable consumption and non-alcoholic fatty liver disease among Korean adults: a prospective cohort study. Journal of epidemiology and community health, 74(12), 1035–1042. https://doi.org/10.1136/jech-2020-214568
[9] Noureddin, M. et al. (2020). Diet Associations With Nonalcoholic Fatty Liver Disease in an Ethnically Diverse Population: The Multiethnic Cohort. Hepatology (Baltimore, Md.), 71(6), 1940–1952. https://doi.org/10.1002/hep.30967
[10] Cantero, I. et al. (2017). Fruit Fiber Consumption Specifically Improves Liver Health Status in Obese Subjects under Energy Restriction. Nutrients, 9(7), 667. https://doi.org/10.3390/nu9070667
[11] Georgoulis, M. et al. (2014). The impact of cereal grain consumption on the development and severity of non-alcoholic fatty liver disease. European journal of nutrition, 53(8), 1727–1735. https://doi.org/10.1007/s00394-014-0679-y
[12] Dorosti, M. et al. (2020). Whole-grain consumption and its effects on hepatic steatosis and liver enzymes in patients with non-alcoholic fatty liver disease: a randomised controlled clinical trial. The British journal of nutrition, 123(3), 328–336. https://doi.org/10.1017/S0007114519002769
[13] Krawczyk, M. et al. (2018). Gut Permeability Might be Improved by Dietary Fiber in Individuals with Nonalcoholic Fatty Liver Disease (NAFLD) Undergoing Weight Reduction. Nutrients, 10(11), 1793. https://doi.org/10.3390/nu10111793
