Added Sugars in the Body: Fructose Metabolism, Ketohexokinase, Fatty Liver, and Health

Added sugars comprise refined monosaccharides and disaccharides (sucrose, high-fructose corn syrup / HFCS, dextrose, concentrated fruit purees) integrated into food products during industrial processing or culinary assembly. Unlike intact whole fruit where carbohydrates reside within an insoluble pectin matrix, added sugars deliver an immediate, unbuffered metabolic impact.

Table sugar (sucrose) represents an equimolar disaccharide of glucose (50%) and fructose (50%). The physiological processing of these two hexose sugars diverges fundamentally.

Metabolic Divergence: Glucose Sparing vs. The Hepatic Fructose Influx

• Glucose Distribution: Serves as a universal substrate metabolized throughout the body by insulin-regulated GLUT-4 transporters in muscle and adipose tissue. Within hepatocytes, glucose phosphorylation by glucokinase is tightly restrained by phosphofructokinase and high intracellular ATP via classic negative feedback.
• Hepatic Fructose Monopoly: Enterocytes absorb fructose via GLUT-5 transporters, funneling over 85% to 90% of the entire load directly to the liver via the portal vein.
• The Ketohexokinase (KHK) Trap: Hepatocytes express ketohexokinase (fructokinase). Unlike glucokinase, KHK lacks any ATP negative feedback mechanism. It phosphorylates incoming fructose into fructose-1-phosphate instantaneously and without limit, draining massive stores of intracellular ATP.

The Pathophysiological Triad: Uric Acid, DNL, and Steatosis

1. Hepatic ATP Depletion and Hyperuricemia: The uncontrolled phosphorylation cascade leaves hepatocytes temporarily starved of ATP, generating surges of intracellular ADP and AMP. This upregulates AMP deaminase, directing purines into rapid catabolism and generating uric acid. Cytosolic uric acid suppresses endothelial nitric oxide, triggers mitochondrial oxidative stress, and elevates blood pressure.
2. Accelerated De Novo Lipogenesis (DNL): Fructose-1-phosphate bypasses the primary rate-limiting gate of glycolysis (phosphofructokinase), flooding mitochondria with triose-phosphate precursors. This unrestrained carbon surplus directly enters de novo lipogenesis, driving the rapid synthesis of palmitic acid and triglycerides.
3. Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD / NAFLD): Intrahepatic triglyceride accumulation generates microvesicular steatosis. Diacylglycerol intermediates activate protein kinase C epsilon (PKC-ε), impairing insulin receptor substrate-1 (IRS-1) signaling. This precipitates severe hepatic insulin resistance, compensatory hyperinsulinemia, and atherogenic dyslipidemia.

Liquid Sugar: The Supreme Metabolic Disruptor

Sugars dissolved in beverages (sodas, juices, sweetened coffee) impose the most severe metabolic harm:

• Lack of Satiety Circuitry: Liquid calories bypass gastric mechanoreceptors, failing to suppress hunger hormones or stimulate GLP-1 and PYY.
• Kinetic Shock: Liquid sugar empties from the stomach in under 15 minutes, slamming the liver with a concentrated fructose bolus.

Diagnostic Biomarkers and Guidelines

• WHO Recommendation: Limit free sugars to under 10% of total caloric intake, with an optimal target of under 5% (approximately 25 grams or 6 teaspoons daily for an adult).
• Clinical Biomarkers of Sugar Toxicity:
  • Fasting insulin and HOMA-IR index ($>2.0$ indicates insulin resistance).
  • Serum triglycerides ($>150$ mg/dL or $>1.7$ mmol/L).
  • Serum uric acid ($>6.0$ mg/dL in women, $>7.0$ mg/dL in men).
  • Elevated ALT and GGT indicating early hepatic steatosis.