Can anyone explain why fructose is more readily stored as bodyfat?

[slash347]

I've always heard that fructose is more readily converted into triglycerides than glucose, but I never got a clear reason why. I'm looking for a good, scientific explanation.

One explain I've heard is that muscle tissue lacks the necessary enzymes to convert fructose into glycogen (no argument there); only the liver can convert it into glycogen. Since liver glycogen stores are limited (I've read 100-300g), only a small amount of fructose would "fill up" the glycogen stores and the rest would be converted to triglycerides. I don't argue with any of this; however, it just seems like a total cop-out explanation.

Another reason I've heard is that fructose enters the glycolysis pathway "futher down the chain" than glucose, and this is why it is converted to bodyfat easily. Can anyone explain this in more detail, and how it relates to the question at hand? I have an intermediate-level understanding of CHO metabolism, so don't be afraid to go into detail (I would prefer it). Thanks in advance guys.

[K_Y]

I actually just read a really interesting article about this.

"Dietary fructose induces a wide range of genes with distinct shift in carbohydrate and lipid metabolism in fed and fasted rat liver"
Hyun-Young Koo a, Matthew A. Wallig a, Byung Hong Chung b, Takayuki Y. Nara a, B.H. Simon Cho a,c, Manabu T. Nakamura a,⁎

Biochimica et Biophysica Acta 1782 (2008) 341?348

a b s t r a c t
Dietary fructose has been suspected to contribute to development of metabolic syndrome. However, underlying mechanisms of fructose effects are not well characterized. We investigated metabolic outcomes and hepatic expression of key regulatory genes upon fructose feeding under well defined conditions. Rats were fed a 63% (w/w) glucose or fructose diet for 4 h/day for 2 weeks, and were killed after feeding or 24-hour fasting. Liver glycogen was higher in the fructose-fed rats, indicating robust conversion of fructose to glycogen through gluconeogenesis despite simultaneous induction of genes for de novo lipogenesis and increased liver triglycerides. Fructose feeding increased mRNA of previously unidentified genes involved in macronutrient
metabolism including fructokinase, aldolase B, phosphofructokinase-1, fructose-1,6-bisphosphatase and carbohydrate response element binding protein (ChREBP). Activity of glucose-6-phosphate dehydrogenase, a
key enzyme for ChREBP activation, remained elevated in both fed and fasted fructose groups. In the fasted liver, the fructose group showed lower non-esterified fatty acids, triglycerides and microsomal triglyceride transfer protein mRNA, suggesting low VLDL synthesis even though plasma VLDL triglycerides were higher. In conclusion, fructose feeding induced a broader range of genes than previously identified with simultaneous increase in glycogen and triglycerides in liver. The induction may be in part mediated by ChREBP.


PM me if you don't have access to it.

[Blindead]

Alan Aragon posted a study where people fed 500g of fructose above maintenance or an isocaloric solution of something, forget what, had the same fat gains.

[slash347]

I've seen one or two where fructose was shown to increase VLDL and increase lipogenesis over glucose, but I still don't understand the mechanism.

I appreciate the replies, but I still don't know the answer to the question. Any more ideas?