Why avoid fructose and galactose in von Gierke disease (type I glycogen storage disease)? - USMLE Forums
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Old 07-30-2015
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Default Why avoid fructose and galactose in von Gierke disease (type I glycogen storage disease)?

In von Gierke disease, glucose-6-phosphatase is defective. This screws up the final steps of both glycogenolysis and gluconeogenesis.

One of the recommended treatments is to eat glucose frequently (after all, you won't be able to use glycogen or make neo glucose when you're fasting), and the other is to avoid fructose and galactose, because "they require G6Pase to be utilized"

But doesn't fructose eventually get made into glyceraldehyde-3-phosphate in fructolysis? That can just join the later parts of glycolysis, can't it? No G6Pase required.

Same thing for galactose. It gets converted to Gal-1-P -> Glu-1-P -> Glu-6-P, which can then join the rest of glycolysis, right?

So it seems neither one REQUIRES G6Pase to be utilized. Why, then, should patients avoid them?
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Old 07-30-2015
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In a nutshell,
galactose and fructose can both - go down the glycolysis and TCA cycle and produce ATP, or they can be converted to glc-6-P and glc.
When you have von Gierke, you will have hypoglycemia. This means increasing glucagon/insulin ratio --> lipolysis, glycogenolysis and GLUCONEOGENESIS.
Unlike glucose, fructose doesn't stimulate insulin and therefore, you will try to make new glucose from it.
TCA cycle won't work properly, because you will use up oxaloacetate for GNG. Therefore, all energy you get is from glycolysis. This will cause excess pyruvate and lactic acidosis.

You need glucose to increase insulin to stimulate glycolysis.
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Old 07-30-2015
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That was a great explanation. Thank you!
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Old 07-30-2015
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But in von gierke glucoeogenesis and glycogenolysis both are defective as g6phosphatase wont be there for make a glucose by glycogen breakdown and gluconeogenesis. So how oxaloacetate will get depleted
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Galactose converts to glucose 1 phosphate, then to glucose 6 phosphate. Needs glucose 6 phosphatase to become glucose again and be sent out into the blood. G6P thus accumulates in liver.

Fructose becomes DHAP and glyceraldehyde, then eventually glucose 6 phosphate. Conversion to glucose requires glucose 6 phosphatase. This accumulates G6P in the liver.

Lactate is formed by RBC, muscle and sent to the liver to undergo Cori Cycle to become glucose again. This cannot occur when glucose 6 phosphatase is deficient, and the lactate simply builds up.
If fructose/galactose was given, it worsens lactic acidosis - more G6P build up in liver stops more lactate from attempting gluconeogenesis - therefore more lactate that spills out into the blood
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