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I thought before, in general, when we have sugar in our food, then the sugar goes in our bloodstream, and if there is too much sugar, then insulin is secreted by our pancreas, and the sugar is pushed into our cells for storage, and to lower the blood sugar (and the insulin may over do it, causing our blood sugar to be too low).

But then recently I heard that sugar in fruit goes through a different route: it goes directly to the liver, and not moderated by the pancreas and insulin? How does it work?

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    Hello, and welcome to the site! Note that not only don't we do personal health question, but asking what caused somebody's cancer is completely futile, as nobody in the world, not even his doctors, can tell that. Getting cancer is the combination of hundreds of stochastic processes happening in the same time, nobody can say what exactly happened. – rumtscho Sep 13 '15 at 09:44
  • Also, I'm flagging your question for migration to Biology, because the metabolic routes of nutrients are quite complicated and not really a matter of medicine practice. They are typically studied by biologists, not medics. – rumtscho Sep 13 '15 at 09:45
  • I have to second @rumtscho 's words, that we can't answer the question about someone's cancer and since you called it a side note yourself, the question would be better off if you deleted that last paragraph. As for the metabolism of sugar: the word sugar can refer to a whole class of chemical compounds - there are different sorts of those in fruit and they have different metabolic paths - so if the question is indeed migrated, I would suggest breaking it down to several questions or narrowing it a bit to a particular sugar: i.e. glucose, fructose etc. –  Sep 13 '15 at 11:08
  • @Lucky as far as I'm aware, there are only two monosaccharides a human can digest, glucose and fructose. All other sugars are either broken down to these two, from where the two metabolism paths start (example: sucrose), or are indigestible. But I think the people on Biology will be better informed whether there are indeed only two sugar pathways. – rumtscho Sep 13 '15 at 11:28
  • @rumtscho now that I think of it you are right (there are some enzymes for galactose, but that one isn't found in fruit). Depending on the amount of detail this wouldn't be a whole book, but it can definitely be a whole chapter. Still you are right about the second thing as well - if/when the question is migrated, the community at biology SE would have a better way to solve this :-). –  Sep 13 '15 at 13:17

3 Answers3

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Do you mean fructolysis?

Here what I found on the textbook by John L. Tymoczko:

Fructose can take one of two pathways to enter the glycolytic pathway.Much of the ingested fructose is metabolized by the liver, using the fructose 1-phosphate pathway. The first step is the phosphorylation of fructose to fructose-1-phosphate by fructokinase. Fructose-1-phosphate is then split into glyceraldehyde and dihydroxyacetone phosphate, an intermediate in glycolysis. This aldol cleavage is catalyzed by a specific fructose 1-phosphate aldolase. Dihydroxyacetone phosphate continues into stage 2 of glycolysis, whereas glyceraldehyde is then phosphorylated to glyceraldehyde 3-phosphate, a glycolytic intermediate, by triose kinase. In other tissues, fructose can be phosphorylated to the glycolytic intermediate fructose 6-phosphate by hexokinase. enter image description here

sunboyharry
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This refers to metabolism of fructose. Many fruits contain high amounts of fructose (hence the name :) in addition to glucose. So does ordinary table sugar (sucrose, which contains equal parts glucose and fructose), and most other sweets you can find --- candy, soft drinks and so on.

Dietary fructose is rapidly metabolized, mostly into glucose, glycogen and fat. This happens mostly in the liver, but also in the intestines and kidneys; these are the only tissues known to express the enzymes involved (see the answer given by sunboyharry), as well as the transporter protein that brings fructose into the cells. Neither the transporter nor these enzymes are controlled by insulin, as far as we can tell, they are always active and ready to convert any fructose that comes around. This differs markedly from glucose metabolism, where both the transporter and key metabolic enzymes are activated by insulin. A review article is found here.

This arrangement makes sense given that the "goal" is to rapidly convert fructose to glucose (the "standard" blood sugar in mammals), and into storage forms (glycogen, fat). But since the glucose obtained from fructose is controlled by insulin as usual, in the end the effects of eating glucose or fructose are similar. Also, ingesting fructose does lead to an insulin response, probably because it is converted to glucose. While there is some research that suggests high fructose intake is more dangerous than glucose, particularly for the liver, I'm not sure there is any conclusive evidence or a scientific consensus about this yet.

Roland
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This is my speculation. Converting modified sugars to sugars which are immediately available could take some time. So, blood sugar, which is mainly glucose, might not elevate immediately.

The same reason is suggested when you take starch from plants. Starch does cause less trouble than equivalent amount of glucose.

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