The Human Brain Converts Glucose Into Fructose

stargazer1111

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Feb 16, 2017
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JCI Insight - The human brain produces fructose from glucose

I have a short e-mail correspondence with one of the lead authors of the study.

My hypothesis is that fructose is a nutrient and that the brain does this because it requires fructose for some biological function(s). She seems to agree, tentatively. She also admits this is early and many more studies are needed.

This is in stark contrast to the majority of experts I have heard claim that fructose's only potential benefit is to lower the glycemic response to a meal and improve glycogen storage.

I also wonder if some people have mutations in this fructose conversion pathway in the brain. In that case, they would need to consume sucrose in some form to get the fructose the brain needs. This could explain why I feel lousy on a fructose-free diet but feel much better with a small amount of fructose at each meal.
 

Amazoniac

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It's an interesting find indeed, but it came from excess glucose:

"While it is possible that plasma fructose may cross the BBB, we observed that the rise of intracerebral fructose occurred much earlier (by 20 minutes) than the modest increase in peripheral fructose levels (at 180 minutes). Furthermore, in rats, a fructose infusion raising plasma fructose nearly 140-fold only increased brain fructose concentrations by 2-fold (3). Taken together, it is highly unlikely that plasma fructose crossing into the brain significantly contributed to the rising brain fructose levels."

"fructose is almost never consumed in isolation, and our finding that glucose levels drive brain fructose levels suggests that any sugar consumption that raises circulating glucose may in large part mediate fructose’s effects in the brain."

"fructose is produced in the human brain in response to hyperglycemia"
 
OP
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stargazer1111

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True. But, this doesn't really make sense in the larger biochemical picture. Fructose is more unstable and more toxic than glucose. It would not make sense for neurons to have evolved a function to convert glucose into fructose to lower glucose toxicity.

There must be more to this pathway that we have not discovered yet.
 

Peater Piper

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I remember reading about this last year. The study used 80 healthy volunteers. I'd like to see what happens in diabetics. Do higher blood levels in diabetics result in even higher amounts of fructose conversion in the brain? What about in Alzheimer's and Parkinson's patients?
 

Travis

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I think this goes to show how much isomerism takes place between the two, and how little attention we should pay between glucose–fructose debates. Much of the information in the popular press about this seems to come from toadies of the beef industry (i.e. Lustig, WAPF) or that gimmicky David Asprey, who try to shift the blame for obesity, fatty liver, and diabetes to fructose ostensibly just to exonerate their often linoleic‐acid‐laden and high‐methionine foods they promote. This is somewhat absurd considering fructose is naturally found in fruit and breast milk; even more absurd when you realize that ω−6 fatty acids and lack of B‐vitamins must play a larger role. My high fructose consumption certain hasn't given me any of these issues; quite the opposite really (I weigh as much as I did at age 20, and probably always will.) I would say the trend is more in line with animals (and people) who eat the most fructose having the least obesity.
 

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