Water, Salt, and the Polyol Pathway — AR as a Villain or Protector?

In one earlier post, I briefly mentioned the work of Dr. Rick Johnson regarding the effects of fructose, where he also discussed the impact of high salt concentration and the problems caused by restricted water intake. Both create osmotic stress, which results in activation of the polyol pathway. Let’s explore the mechanism that might reveal some ways to improve the situation—or at least guide us on what to avoid.

Let’s first see what happens when the enzyme KHK is knocked out, the enzyme responsible for phosphorylating (activating) fructose. In mice, we saw that eliminating KHK activation (KHK KO) strongly suppresses the effects of both endogenous fructose (produced in the liver from glucose) and dietary fructose. So what happens when water intake is restricted? Does it activate the polyol pathway? Can KHK knockout prevent that?

Shutting down fructose metabolism (KHK-KO) has no effect on fructose levels in the hypothalamus during water restriction (WR). Is dietary fructose responsible?

Yes, water restriction (WR) activates endogenous fructose production. But the absence of the KHK enzyme shuts down endogenous fructose synthesis. To form any hypothesis, we first need to know whether the diet contained fructose. I don’t know—the authors didn’t mention it. Water restriction simply increased fructose concentration in both groups.

Fructose metabolism knockout (KHK-KO) has no effect on osmotic stress.

There should actually be no difference—the concentrations of fructose and osmotic stress are nearly identical in the control (WT) and genetically modified (KHK-KO) groups. However, KHK knockout does change something: the AMPD2 → XO → urate pathway is not activated. It suppresses xanthine oxidase (XO), the largest producer of H₂O₂.

Fructose metabolism knockout (KHK-KO) suppresses uric acid formation from AMP/IMP and inhibits xanthine oxidase activity, the largest producer of H₂O₂.

One might wonder whether increased fructose levels could shut down the polyol pathway. Indeed, this pathway does not produce sorbitol or fructose when fructose metabolism is disabled. However, the absence of sorbitol suggests that the reduced activity of the first enzyme in the polyol pathway (AR) is likely not caused by fructose directly—but by something else. What could it be?

Fructose metabolism knockout (KHK-KO) suppresses the expression of the first enzyme in the polyol pathway, aldose reductase (AR), whose product is sorbitol.

There must be a way to turn off aldose reductase (AR). Notice that it didn’t increase at all during osmotic stress caused by water restriction when fructose metabolism was deactivated.

We can easily reduce dietary fructose, for example, by replacing sugar with glucose or by simply avoiding sugar altogether. But by turning off AR, we could also reduce endogenous fructose production, which is probably much more dangerous—especially when combined with the intake of long-chain fatty acids in fats. These facilitate activation of the polyol pathway with all its consequences, mainly fat storage. If the polyol pathway isn’t activated, there’s no need for KHK either, since without dietary fructose there’s no fructose at all. That allows normal function of enzymes for glucose and fat oxidation, which won’t be acetylated because the activity of deacetylases such as SIRT1 and SIRT2 is preserved.

We should find out why the polyol pathway was suppressed in KHK-KO. What caused AR shutdown? Could blocking the AMPD2 → XO → urate pathway also stop AR? Or was it caused directly by elevated fructose levels? Or is osmotic stress required?

I haven’t found the mechanism yet—we’ll keep searching. We can look at how osmotic stress affects cells in vitro. Osmotic stress activates the transcription factor NFAT5, which directly triggers AR production. Here we see activation by glucose (G) and sodium chloride solution—that is, table salt (N)—in a test tube. (M) we’ll leave aside. The effect strongly depends on concentration and exposure time.

We see a paradoxical situation where SIRT1 activation helps activate AR, and thus the polyol pathway. Osmotic stress induced by sodium chloride leads to NFAT5 activation—and therefore AR activation—after roughly 16 hours of sustained stress. SIRT1 also helps in activation but itself becomes active only under long-term stress. It appears that if osmotic stress doesn’t last too long, the polyol pathway won’t be triggered.

The polyol pathway must be useful for something important. Indeed, activation of sorbitol synthesis in the kidneys protects cells from the effects of uric acid—from urate toxicity. Activation of the deacetylase SIRT1 may help here. But in the liver, I’d be cautious—could it be beneficial there too?

We know that acetate suppresses the effects of fructose, likely through SIRT1 activation, counteracting the fructose-induced deactivation of SIRT2. But it still seems rather strange.

I can’t yet describe the role of urate in all this. AMPD2 knockout in mice increases blood levels of fats and cholesterol, even though they remain lean and don’t gain weight. Fat oxidation and storage in the body are suppressed, but this leads to fatty liver. Fat entry into mitochondria is reduced. This is likely the proper metabolic state for survival in cold environments during animal hibernation. AMPD2 activity is strongly temperature-dependent and nearly zero in the cold. That allows AMPK activity to be preserved and allow slow fat oxidation to continue.

For normal active life, then, AMPD2 knockout doesn’t restore metabolism—we apparently need a certain regulated level of uric acid for protection. This also relates to levels of free fatty acids meant for oxidation. Elevated levels signal impaired fat burning and lead to uric acid retention in the blood. If greater urate retention and its subsequent rise activate the polyol pathway—which then accelerates fat storage—this could actually be an understandable protective mechanism.

Acetate may activate these same protective mechanisms even at lower urate levels. Uric acid production is linked to high hydrogen peroxide generation by xanthine oxidase. If acetate suppresses this pathway and replaces it with alternative ones, oxidative stress decreases. However, whether acetate protects cells from osmotic stress as well, I can’t say. SIRT1 activation by acetate suggests it might facilitate AR and polyol pathway activation. I used to think this was a bad reaction, but perhaps it’s actually a way to protect cells from overload.

So—is activation of aldose reductase and the polyol pathway beneficial? A crucial question, isn’t it?

We just needed to look deeper—to ask better questions.

Yes, in certain circumstances, absolutely yes. When? Aldose reductase (AR) effectively breaks down not only glucose but also peroxidized lipids, especially HNE (4-hydroxy-2-nonenal) and complexes where HNE is bound to glutathione (GSH). Thus, it helps degrade products of oxidative stress caused by excess omega-6 fatty acids in membranes. Moreover, the presence of these lipid peroxidation products activates AR expression, supporting the synthesis of the main enzyme of the polyol pathway—and that enzyme then helps remove peroxidized lipids. Increased fructose production is merely a side effect. This fits perfectly with the observation that blocking fructose processing via KHK-KO suppresses oxidative stress, preserves the GSH-based antioxidant system, reduces oxidized lipid formation, and therefore doesn’t activate AR—as shown in the figure above.

Although elevated glucose levels are important for activating the polyol pathway, the amount of enzymes produced—and the actual activity of AR and the polyol pathway—dramatically increase only in the presence of lipid peroxidation products from omega-6 linoleic acid, that is, the peroxides of seed oils stored in cell membrane phospholipids. Only then does the polyol pathway lose its protective role (guarding against osmotic stress and excess glucose) and become a promoter of AMP/IMP degradation, H₂O₂ production, lipid peroxidation, and elevated uric acid levels. Increased KHK activation and endogenous fructose production are just side effects of the need to remove peroxidation products of polyunsaturated fats—oxylipins. Unfortunately, this side effect creates more oxidative stress. It’s quite a vicious cycle, isn’t it?

So, in the end, our search circled back—from sugar, fructose, and KHK—to the well-known problematic seed oils and the linoleic acid they contain. I have to admit—I didn’t expect such a twist. Did you?