Fructose is described to have a different metabolic pathway (a more fat-inducing one) than glucose (see: http://healthyeating.sfgate.com/difference-between-sucrose-glucose-fructose-8704.html) as it can only be metabolized by the liver and does not impact insulin.

What I don't understand, is that in glycolysis, a paramount step of metabolizing glucose, it is very quickly converted to fructose-6-phosphate. Why does the body want to convert glucose into fructose?


The metabolic pathway you are talking about is how fructose is converted into energy and how its concentration in the blood is regulated. It is indeed true that blood fructose level does not affect blood insulin level, and this is why it is more "fat-inducing", as it cannot be effectively regulated like glucose.

It is also true that in glycolysis, glucose is converted to fructose-6-phosphate. However, this should not be confused with fructose itself. The fructose phosphate that fructolysis produced is fructose-1-phosphate, not fructose-6-phosphate.

  • $\begingroup$ It still feels strange because it is essentially a fructose molecule (although an isomer) so it must possess some of the same properties? $\endgroup$
    – Paze
    Mar 18 '15 at 10:30
  • 3
    $\begingroup$ @BjarniJohannsson in biochemistry, structure matters. Enzymes and receptors depend on the molecular structure to recognize and differentiate substances. Also, the "properties" of fructose that make them "fat-inducing" requires it to be in the blood, and glycolysis is entirely carried out in the cytosol, and there likely isn't any membrane protein that allows fructose-6-phosphate to exit the cell as it is an essential intermediate product in cellular respiration. $\endgroup$
    – busukxuan
    Mar 18 '15 at 10:38

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