Wednesday, December 15, 2010

The Human Body Cannot Make Glucose

Only plants can make glucose from scratch.* Humans must eat the plants, or eat animals that ate the plants, to obtain glucose. And humans absolutely need glucose to survive. This simple sugar is the sole source of energy for our red blood cells and the preferred source for other cells. We are utterly dependant on plants for our existence. (Not to mention that they release oxygen in the process of manufacturing glucose - oxygen that we also need to survive.)

* And some algae like seaweed, and some bacteria.

Glucose is a molecule with 6 carbon atoms bound together. Humans cannot harness the immense amount of energy needed to get 6 carbon atoms to bind together. Plants, however, can. It's quite a feat actually. They harness the energy from the sun to do this.

Plants take in carbon dioxide, string together 6 carbons to make glucose for their fuel (starch is just a chain of glucoses), and give off the excess oxygen.

Humans take in the oxygen given off from plants and use it to extract the energy from those bonds within the glucose molecule. (One pathway to extract that energy is called glycolysis. I'll return to glycolysis later.) The waste product, if you will, from our energy-extraction process is carbon dioxide. We exhale it. This is the same carbon dioxide that the plant takes in to make glucose - it needs those carbons. Humans and plants have a cyclical relationship.

At night, plants respire just like us. They use oxygen to extract energy from the glucose they made during the day, giving off carbon dioxide in the process.

One last point...

There is a process our bodies evolved to supply glucose in a pinch. It's called gluconeogenesis ... the new making of glucose. Since we can't make glucose from scratch, this process allows us to reassemble preformed 3-carbon and 4-carbon molecules to make the 6-carbon glucose. These precursor molecules' carbons were bound via photosynthesis, via plants' harnessing of energy from the sun. We still can't isolate ourselves from this fuel that plants make - that we eat.

Gluconeogenesis is essentially the glycolysis pathway (recall above) in reverse - not quite though. It uses different enzymes (enzymes make it go), and where glycolysis provides us energy by breaking down glucose, gluconeogenesis uses up energy. From where does the energy come to reassemble those precursors for gluconeogenesis? From plants!

Glucose From Fat?

If we eat too much glucose, we convert some of it to fat for storage. Since we can make fat from glucose, can we make glucose from fat? No.

When we disassemble the fat (fatty acid chain), we're left with a small 2-carbon molecule. Recall that gluconeogenesis requires at least a 3-carbon molecule.


Mike said...

"Since we can make fat from glucose, can we make glucose from fat? No."

Actually, yes. Triglyceride = glycerol w/ 3 fatty acids. The glycerol can be converted to glucose.

Hence the reason athletes on high fat, mod pro, low carb diets excel.

Bix said...

Some wrote to say they can use glycerol to make glucose. As I said in my post, 3-carbon molecules such as glycerol can be used in gluconeogenesis.

But glycerol is not a fat. We cannot make glucose from fat.

Perovskia said...

...So it pays to go vegetarian? ;)

Bix said...

You know, I'm not big on vegan diets. But I sure do respect what plants give us. If we ever want to establish a colony on another planet, we better make sure the environment is hospitable to plants.

Jonathan said...

Mike is right. Bix, your definition of "fat" is a little off.

A fat is a triglyceride. Not a fatty acid. Triglycerides contain three fatty acids (two in phospholipids) and one glycerol molecule. As you said yourself, glycerol can be turned into glucose. Therefore, we can make glucose from fats.

Bix said...

Jonathan, a fat is a hydrophobic compound, as I think you may know. Fat may therefore be either a triglyceride or a fatty acid, among other things.

Glycerol, which may be used to make glucose, is not a fat, it is not hydrophobic. It dissolves in water because of its many hydroxyl or OH groups.

While it is true that glycerol can derive from a triglyceride, it is not itself a fat.

Bix said...

I think the question to ask is ... can the human body make glucose from an acetyl group? COCH3? A two-carbon compound?

Jonathan said...

Hydrophobia does not make a molecule a fat. Every scientific definition I have seen (from textbooks, journals and web sources) defines a fat specifically as a triglyceride. A fatty acid, then, would not be considered a fat - it's just a fatty acid. The names are *not* interchangeable.

So I agree with you that we probably cannot make glucose from fatty acids, but we certainly can make glucose from fats.

Bix said...

Thank you for your input, Jonathan.

I am not narrowly defining the word fat, as was evident in my definition. I am using it for lipid here. The human body does not make glucose from fat.

To address your point... The motherlode of energy in a triglyceride is stored in the fatty acid chain, not in the token glycerol. Even when you use glycerol in gluconeogenesis it's an endergonic reaction, it requires energy to move forward instead of releasing it. The yield is low.

You can get less than 40 ATP from oxidizing a glucose molecule you eat, less if the body has to spend energy to make a glucose molecule, from, say, glycerol. While you can get over 100 ATP from oxidizing one typical fatty acid chain. There is a *lot* of energy in 3 fatty acid chains.

So, when you eat too much glucose, you can store some of that energy by making fat (as fatty acid, as lipid). Can humans make glucose from energy stored in fat? No.

As I said, there is discussion as to how physiologically important production of carbohydrate is from acetyl-CoA, the breakdown product of a fatty acid. This was not your assertion though.

Bix said...

"Because animals cannot run the glyoxylate cycle, they cannot make glucose from acetyl-CoA in net amounts, but plants, yeast, and bacteria can. That may go a long way to explaining why you don't see fat plants, yeast, and bacteria. When they have excess acetyl-CoA, they have the option of either using it to make glucose or fatty acids. When we have excess acetyl-CoA, about all we can do with it is make fatty acids."


Jonathan said...

I see what you meant, but it becomes a little misleading if you refer to lipids in general as fats, which of course are only one subset of lipids. Clarity is very important, and if people never use the same definitions... well, you can imagine what kind of mess would occur.

Anyway, the issue in question (at least, for me and Mike) is not whether we can use the energy from fat to make glucose, it is simply whether we can use triglycerides to make glucose. We're not really concerned about the low energy yield in gluconeogenesis - that's to be expected, since we can't photosynthesize and must obtain the energy from another source. We just care about whether we can get enough glucose to places that need it without needing to take in glucose or other carbohydrates, since that is essentially the main goal of any low-carb or no-carb diet. And, as you said, we *can* use the glycerol from triglycerides to make glucose.

I do agree with your other points. But please be more clear about what type of lipid you're talking about.