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Why is it that fat can't be converted directly to glucose?

Answered on August 19, 2014
Created May 19, 2011 at 6:12 PM

In an evolutionary sense, why is it that only amino acids from protein can be turned into glucose for fuel through gluconeogenesis? Why couldn't the body have evolved to be COMPLETELY protein-sparing and just turn fat into glucose for energy?

47a42b6be94caf700fce9509e38bb6a4

(9647)

on June 02, 2011
at 11:55 PM

Though fat in meat, say, will be mostly in the form of triglycerides (since they store it like we do), so we're getting some from the animal. Some? Most? I wouldn't know how to go about discovering the proportion of our glycerol from outside us and the proportion from inside us.

B3e7d1ab5aeb329fe24cca1de1a0b09c

(5242)

on June 02, 2011
at 11:11 PM

Nicely put! .

510bdda8988ed0d4b0ec0b738b4edb73

(20898)

on June 02, 2011
at 10:46 PM

The beauty of evolutionary arguments is that you can go the other way with them. I'd argue that because the lipid->carbohydrate conversion is so hard we evolved to not need much glucose and to burn fat directly.

510bdda8988ed0d4b0ec0b738b4edb73

(20898)

on June 02, 2011
at 10:44 PM

Yeah and the original glycerol probably came from glucose when we made the triglyceride in the first place.

47a42b6be94caf700fce9509e38bb6a4

(9647)

on June 02, 2011
at 09:14 PM

Of course technically we can get a little bit of glucose from the glycerol part of a triglyceride -- but it's a bit sneaky to say that since the glycerol part isn't really *fat*, and anyhow it's not very much. See here for example: http://paleohacks.com/questions/26415/how-much-glucose-can-we-synthesize-from-glycerol-in-our-dietary-triglycerides

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3 Answers

best answer

2
Cf626d3fba66c18297b3f1116a920e58

(3417)

on May 19, 2011
at 06:29 PM

Because a common pool of chemical-group carriers and associated enzymes evolved a pretty efficient (not perfectly efficient) system of interconverting proteins, carbs, and fats right around the crux between glycolysis and the TCA cycle. As important as it was in archaic lifeforms, it was conserved almost as is from bacteria right on up (bad choice of word?) to humans with a few minor alterations because it worked pretty well from the start.

Asking why something didn't evolve a different way is kind of a question without an answer since evolution is a force of order emerging through chaos. You could always ask the great designer in the sky why he designed this system with some inefficiencies. Now that's an answer I'd like to hear myself.

2
510bdda8988ed0d4b0ec0b738b4edb73

(20898)

on June 02, 2011
at 08:20 PM

One reason is that they're so similar, it's easy to interconvert amino acids to carbohyrates (and hard/impossible?) with fats.

First, here's a list of the glucogenic amino acids in humans. Go to each page and look at their structures:

http://en.wikipedia.org/wiki/Glucogenic_amino_acid

Next, here's what glucose looks like

http://en.wikipedia.org/wiki/Glucose

Notice that they're similar, chains of carbons, lots of OH bonds

Now here's a fatty acid, I chose Oleic acid, but they all look the same just different lengths

http://en.wikipedia.org/wiki/Oleic_acid

Note how it's completely different in structure. Long chain of C's and H's, no OH groups to be found except at the hydrophilic end.

It would take too much energy, even if it is possible, to convert fats to carbohydrates. That's probably why we never evolved to that.

47a42b6be94caf700fce9509e38bb6a4

(9647)

on June 02, 2011
at 09:14 PM

Of course technically we can get a little bit of glucose from the glycerol part of a triglyceride -- but it's a bit sneaky to say that since the glycerol part isn't really *fat*, and anyhow it's not very much. See here for example: http://paleohacks.com/questions/26415/how-much-glucose-can-we-synthesize-from-glycerol-in-our-dietary-triglycerides

47a42b6be94caf700fce9509e38bb6a4

(9647)

on June 02, 2011
at 11:55 PM

Though fat in meat, say, will be mostly in the form of triglycerides (since they store it like we do), so we're getting some from the animal. Some? Most? I wouldn't know how to go about discovering the proportion of our glycerol from outside us and the proportion from inside us.

510bdda8988ed0d4b0ec0b738b4edb73

(20898)

on June 02, 2011
at 10:44 PM

Yeah and the original glycerol probably came from glucose when we made the triglyceride in the first place.

1
B3e7d1ab5aeb329fe24cca1de1a0b09c

(5242)

on June 02, 2011
at 10:24 PM

Good answers thus far. Just another point on the evolutionary selection pressure:- The human body needs bugger all glucose to operate. We can make what we need from protein (gluconeogenesis) and everything else runs perfectly well on fat & ketones.

If for whatever reason we needed a lot more glucose chances are the selection pressure would of seen us evolve with more mechanisms to generate it from other macro nutrients (or become extinct, of course).

But we don't and we didn't.

B3e7d1ab5aeb329fe24cca1de1a0b09c

(5242)

on June 02, 2011
at 11:11 PM

Nicely put! .

510bdda8988ed0d4b0ec0b738b4edb73

(20898)

on June 02, 2011
at 10:46 PM

The beauty of evolutionary arguments is that you can go the other way with them. I'd argue that because the lipid->carbohydrate conversion is so hard we evolved to not need much glucose and to burn fat directly.

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