Richard Johnson is a professor of medicine at the University of Colorado School of Medicine. His research focuses on metabolic and renal health, with a particular emphasis on the physiological roles and health impacts of fructose and uric acid. His published work covers chronic kidney disease, podocyte injury, cardiovascular risk factors, and the effects of heat stress and diet on metabolic conditions.
Alcohol consumption activates an enzyme that converts glucose to fructose, causing endogenous fructose synthesis in the liver.
"when you drink alcohol, the alcohol activates an enzyme to make fructose in the body. It's not the alcohol becoming fructose. The alcohol is alcohol, but the alcohol affects an enzyme that's normally not active and it gets turned on by alcohol, and this enzyme converts glucose to fructose and you start making fructose." (said at 0:33:25)
The speaker accurately describes the activation of the polyol pathway by alcohol. Research in human liver specimens and animal models shows that ethanol consumption upregulates aldose reductase (AR), the rate-limiting enzyme of the polyol pathway that converts glucose to sorbitol (which is subsequently converted to fructose). This pathway leads to endogenous fructose generation in the liver and contributes to alcohol-induced metabolic dysfunction and liver injury.
- supports: Elevated Fructose and Uric Acid Through Aldose Reductase Contribute to Experimental and Hu… (Hepatology (Baltimore, Md.) 2020) · cited 50x in the literature
"We demonstrated in liver specimens from patients with alcoholic hepatitis, the AR up-regulation and elevated AR metabolites (sorbitol, fructose, and uric acid), which correlated significantly with (1) increased lipid peroxidation byproducts and endoplasmic reticulum (ER) stress, (2) decreased protective ER chaperones, and (3) greater cell death and liver injury." (abstract, results, passage verified)
pubmedfull study (doi) - supports: Identification of a common ketohexokinase-dependent link driving alcohol intake and alcoho… (Nature metabolism 2025) · cited 4x in the literature
"Ethanol consumption increased portal vein osmolality and activated the polyol pathway in the liver and intestine, leading to fructose production metabolized by KHK-A/C." (abstract, results, passage verified)
pubmedfull study (doi)
In animal models, blocking fructose metabolism or blocking aldose reductase prevents alcohol-induced fatty liver disease.
"when we blocked fructose metabolism, we could block the fatty liver that was induced by alcohol. ... There was a group in China that blocked the enzyme that converts glucose to fructose, and they could block alcoholic liver disease that way in animals too." (said at 0:34:56)
The speaker's statement is supported by animal studies. In mouse models of alcohol-associated liver disease (ALD), genetic knockout of ketohexokinase (KHK-A/C, which blocks fructose metabolism) protects mice against alcohol-induced hepatic steatosis, inflammation, and fibrosis. Similarly, genetic deletion or pharmacological inhibition of aldose reductase (the rate-limiting enzyme of the polyol pathway converting glucose to sorbitol/fructose) prevents alcohol-induced hepatic steatosis and liver injury in rodents. Because the evidence for this specific claim is derived entirely from animal and cell culture models, certainty is rated as very low.
- supports: Inhibition of aldose reductase ameliorates alcoholic liver disease by activating AMPK and … (Molecular medicine reports 2017) · cited 21x in the literature
"In addition to the elevation in AR, hepatic steatosis was observed in ethanol diet-fed mice, and this ethanol-induced steatosis was significantly attenuated by inhibiting AR activity with a specific inhibitor, zopolrestat." (abstract, results)
pubmedfull study (doi) - supports: Elevated Fructose and Uric Acid Through Aldose Reductase Contribute to Experimental and Hu… (Hepatology (Baltimore, Md.) 2020) · cited 50x in the literature
"Furthermore, we established a causal role for AR in ALD by showing that the genetic deficiency of AR (knockout mice) prevented alcohol-induced increase in harmful AR metabolites, toxic aldehydes, steatosis, ER stress, apoptosis, and liver injury. Finally, we demonstrated the therapeutic potential of pharmacological AR inhibition against alcohol-induced hepatic injury in experimental ALD." (abstract, results, passage verified)
pubmedfull study (doi) - supports: Identification of a common ketohexokinase-dependent link driving alcohol intake and alcoho… (Nature metabolism 2025) · cited 4x in the literature
"Under ethanol pair-matched conditions, global and liver-specific KHK-A/C knockout mice were protected from ALD, with marked reductions in hepatic steatosis, inflammation and fibrosis." (abstract, results, passage verified)
pubmedfull study (doi)
The polyol pathway via aldose reductase is the only enzymatic pathway by which the body synthesizes endogenous fructose.
"and there's only one way the body makes fructose, only one, and it's through this enzyme called aldose reductase, or what I call the polyol pathway." (said at 0:38:04)
The speaker's statement is biochemically accurate. In mammalian biochemistry, the polyol pathway (in which aldose reductase converts glucose to sorbitol, followed by sorbitol dehydrogenase converting sorbitol to fructose) is the only known enzymatic pathway for the endogenous synthesis of fructose from glucose.
When blood glucose levels rise above 120 mg/dL, the polyol pathway begins converting glucose into fructose.
"when your glucose levels start going up over 120, you start making fructose from it, and you're also stimulating insulin." (said at 0:50:45)
The speaker accurately describes the biochemical mechanism whereby elevated blood glucose increases flux through the polyol pathway (via aldose reductase and sorbitol dehydrogenase) to produce endogenous fructose, which concurrently stimulates insulin secretion. However, describing this as an on/off switch occurring specifically when glucose exceeds 120 mg/dL simplifies a continuous biochemical concentration gradient. Aldose reductase has a relatively high Km (low affinity) for glucose compared to hexokinase; consequently, polyol pathway flux increases progressively across rising postprandial or hyperglycemic glucose concentrations rather than abruptly activating at a strict threshold of 120 mg/dL.
Dietary consumption of either sugar or salt increases circulating levels of vasopressin in animals.
"and when we were giving sugar to animals as well as salt to animals, we found that vasopressin levels went up in the blood." (said at 0:59:50)
Animal experimental studies demonstrate that oral administration of sugars (fructose, glucose, high-fructose corn syrup) as well as salt increases circulating levels of vasopressin (measured directly or via its surrogate biomarker copeptin) in rodent models. Because the claim specifically describes animal experimental findings, the available pre-clinical literature directly supports the statement.
- supports: Vasopressin mediates fructose-induced metabolic syndrome by activating the V1b receptor. (JCI insight 2021) · cited 55x in the literature
"Orally administered fructose, glucose, or high-fructose corn syrup increased vasopressin (copeptin) concentrations and was mediated by fructokinase, an enzyme specific for fructose metabolism." (abstract, results, passage verified)
pubmedfull study (doi) - supports: Sugar, salt, immunity and the cause of primary hypertension. (Clinical kidney journal 2023) · cited 13x in the literature
"Leptin levels rise, triggering activation of the sympathetic central nervous system, while vasopressin levels rise, causing vasoconstriction in its own right and stimulating aldosterone production via the vasopressin 1b receptor. Low-grade renal injury and autoimmune-mediated inflammation occur. High-salt diets can amplify this process by raising osmolality and triggering more fructose production." (abstract, results, passage verified)
pubmedfull study (doi)