Like obesity, NAFLD and NASH are closely linked to nutrition and the "Western diet" which is rich in saturated fats and refined sugars. Although fat consumption has remained relatively stable, the marked increase in dietary fructose consumption (more than doubling in the past 30 years alone) supports the role of fructose in NAFLD and the metabolic syndrome. Although the mechanism(s) for fructose-related liver injury is not yet well defined, fructose-related hepatic adenosine triphosphate (ATP) depletion may contribute to liver injury. Observations in animals suggest that fructose induces metabolic syndrome and NAFLD independent of energy intake. One key difference in fructose metabolism (as opposed to glucose) relates to ATP depletion and the necessity of adenosine monophosphate (AMP) kinase to replenish ATP stores. As opposed to glucose, initial fructose metabolism involves phosphorylation of fructose to fructose-1-phosphate by fructokinase (ketohexokinase, KHK) using the substrate ATP. Unlike glucokinase, the phosphorylation of fructose by KHK is specific for fructose and not rate limited. Replenishment of ATP stores requires phosphorylation of AMP back to ATP via AMP kinase (which is inhibited in insulin resistance (common in patients with NAFLD) or conversion to uric acid via xanthine dehydrogenase resulting in hyperuricemia. The high activity of KHK in phosphorylating fructose to fructose-1-phosphate in the liver, could result in hepatic ATP depletion with habitual fructose consumption.

Published animal and human studies support our hypothesis that fructose is a risk factor for NAFLD and NAFLD-related liver disease progression. In animal models, diets high in fructose induce features of the metabolic syndrome including weight gain, insulin resistance, hypertriglyceridemia, and hypertension. Similar effects are not observed with the administration of other simple sugars such as glucose. Fructose (or sucrose) administration to humans also causes features of metabolic syndrome which are quite typical of patients with NAFLD. Fructose is lipogenic, stimulates triglyceride synthesis and causes hepatic steatosis. As previously reported in animals, our group reported that increased fructose consumption (assessed as fructose-containing beverages only) is a risk factor of metabolic syndrome and biopsy-proven NAFLD and that patients with NAFLD consume 3-4 times more fructose than age, gender, and mass index (BMI) matched controls without liver disease.

In addition to increased fructose consumption being a risk factor for NAFLD, fructose has been implicated in NAFLD disease progression. The administration of a diet with 25% of total energy as sucrose (which contains 50% fructose) resulted in a rise in liver aminotransferase levels within 18 days. This study, performed nearly 25 years ago, is all the more alarming as current sugar intake of Americans is in this same range. In our study of 427 patients with biopsy-proven NAFLD, increased consumption of fructose-containing beverages was univariately associated with decreased age (P \< 0.0001), male sex (P \< 0.0001), hypertriglyceridemia (P \< 0.04), low high density lipoprotein (HDL) cholesterol (\<0.0001), decreased serum glucose (P \< 0.001), increased calorie intake (P \< 0.0001), and hyperuricemia (P \< 0.0001). After controlling for age, sex, BMI, and total calorie intake, daily fructose consumption was associated with lower steatosis grade and higher fibrosis stage (P \< 0.05 for each). Being that triglyceride synthesis requires ATP, we hypothesize that lower hepatic steatosis may reflect deceased ATP availability. Additionally, in older adults (age ≥ 48 years), daily fructose consumption was associated with increased hepatic inflammation (P \< 0.05), and hepatocyte ballooning (P\< 0.05). However, the mechanism(s) by which fructose causes liver injury remains unknown.

In support of our hypothesis that ATP depletion underlies liver injury in patients with NAFLD, our group has demonstrated that patients with biopsy-proven NAFLD have increased hepatic mRNA (messenger ribonucleic acid) expression of KHK compared to matched controls. Indeed, in human pilot studies, intravenous (IV) fructose administration is associated with hepatic ATP depletion which can be assessed by 31P magnetic resonance spectroscopy (MRS). Reduced hepatic ATP stores are more prevalent in overweight and obese subjects than in lean subjects. Furthermore, recovery from fructose-induced ATP depletion was found to be delayed in patients with NAFLD (n=8). However, a limitation to this existing work is the small sample size and the inability to assess a cause-effect relationship(s) between BMI, NAFLD, energy homeostasis, and histologic features of liver injury. In liver cells, ATP depletion could perpetuate chronic liver injury by making fatty hepatocytes less proliferative. Hepatic ATP depletion also encourages the expansion of liver progenitor populations, causes arrest in protein synthesis, induces inflammatory and prooxidative changes, increases endoplasmic reticulum stress, promotes activation of stress-related kinases, induces mitochondrial dysfunction, and increases apoptotic activity. This supporting data suggests that fructose may be associated with NAFLD, NASH, and progressive fibrosis. Further, a study by Loguercio et al. demonstrated that increased uric acid levels above the basal level after IV fructose infusion was significantly higher (p \< 0.01) in patients with cirrhosis (3 mg/dl) and NASH (1.9 mg/dl) than in healthy controls (1.2 mg/dl). This effect was completely reversed by fructose 1,6-diphosphate which could replenish the ability to resynthesize ATP (adenosine triphosphate) from ADP (adenosine diphosphate). Therefore, an IV fructose challenge could effectively differentiate healthy subjects, from chronic hepatitis, from cirrhosis.

NAFLD lacks accurate and robust non-invasive biomarkers to grade and stage histologic disease activity. This is a critical barrier to understanding the influence of this important environmental risk factor (increased/habitual fructose consumption) on the pathogenesis and progression of NAFLD. Currently, reliable assessment NAFLD requires liver biopsy and interpretation of histology. Serum aminotransferase levels and conventional imaging methods can detect liver fat but cannot grade or stage NAFLD. Furthermore, current developments in biomarker are cross-sectional in nature and do not characterize the dynamic changes which underlie liver injury in patients with NAFLD. In vivo 31P MRS permits the evaluation of dynamic changes of individual phosphorus-containing metabolites in the liver parenchyma, such as phosphomonoester (PME), ATP, and inorganic phosphate (Pi). Intravenous fructose load alters phosphorus metabolites and allows assessment of liver function by 31P MRS. Other investigators have demonstrated that fructose loading could be used effectively as a tool to investigate change in metabolic steps of hepatic metabolism in humans with alcohol-related liver disease. Further, IV fructose loading causes significantly higher ATP degradation and uric acid production in cirrhotic patients than in healthy controls. The associations between fructose, increased uric acid, and hepatic ATP depletion has been previously described. Increased uric acid is an independent risk factor for NAFLD and in keeping with our hypothesis, hyperuricemia may be a surrogate marker of impaired hepatic energy homeostasis in patients with NAFLD. The proposed mechanism for fructose-related hepatic ATP depletion, NAFLD, NASH and the associated hyperuricemia is depicted in Figure 1 is novel, innovative, scientifically rigorous and address an important public health concern-the impact of fructose on the rising epidemic of NAFLD.