Last question from Jeff says, "I thought, I remember you saying in one of your podcasts
that fat burning doesn't yield as much energy output, ATP maybe, than glucose burning does,
but I'm not sure about that. Could you clarify that for me? Also, if so, how does that compare
with fat containing more calories per gram than glucose shouldn't fat yield, more energy?"
First of all, you should never, ever, ever, ever think of metabolism through the perspective
of calories per gram. You want to I think from the perspective of ATP yield.
I mean, you could think of the perspective as ATP yield per molecule,
I mean, I don't know why you would even look at it per molecule. What you really want to look at
is, you could look at ATP yield per calorie, if you're trying to say that.
It's conceivable that you get a different level of ATP yield per calorie,
but generally calories are going to be roughly equivalent to ATP yield. But I mean,
calories per gram is so totally irrelevant because your body doesn't care about grams.
For example, if you're looking at burning the carbon dioxide produced
between a carbohydrate and a fat, that's some by-product of energy metabolism that differs,
right? So the way you would want to look at that is really per ATP yield,
or you could look at it per calorie to be more general.
But it would be silly to look at it per molecule because the
molecules have different amounts of energy. It would be silly to look at it per gram
because your body doesn't calculate grams of anything. If you put in nine grams of fat,
your body doesn't know that it's burning nine grams of that. It knows that it's burning
X amount of ATP yield of fat, and that's what it regulates things by. But fat burning doesn't
yield... Okay. So you said, "I thought, I remember you saying on one of your podcasts, that fat
burning doesn't yield as much energy output, ATP maybe, than glucose burning does." I don't
remember saying that.
I don't remember saying that, and I'm not sure what that refers to. But
I'm trying to think of what I might've said.
So glucose can generate ATP in more diverse ways.
So glucose can generate ATP in the cytosol through glycolysis and fat can't, that's one thing.
Then there's also lots of things you can do with glucose that you can't do with fat. So glucose can
support the pentose phosphate pathway, fat mostly can't. Glucose can support anaplerosis, which is
providing citric acid cycle backbones in order to keep it running, fat can't do that. So glucose can
do more things to support energy metabolism than fat can. Glucose can yield ATP in the cytosol.
It's possible that in something in the past, maybe I did do something about yielding...
I don't know if I did anything on ATP yield per calorie.
I might've said that. Let me just see if there's something, let me see. One second.
So I just pull up this paper. "How does the ratio of ATP yield from the complete
oxidation of palmitic acid to that of glucose compared to the relative energy contents of fat
and carbohydrate?" So this seems to answer the question as it should be stated. This paper is
from 1998, and it says, "The authors of many recent popular textbooks of biochemistry quote
values for the energy content of triacylglycerol and dry carbohydrate on a weight basis, as well
as presenting calculations for the yield of ATP obtained when a molecule of glucose or palmitic
acid is completely oxidized to CO2 and H2O. "By extending these calculations and computing
the yield of ATP in terms of the weight of glucose or palmitic acid oxidized to CO2 and H2O, it can
be shown that the value for the ratio of the energy content of fat to that of carbohydrate
is almost identical to the ratio of the yield of ATP per gram of palmitic acid oxidized, compared
with that of glucose. Therefore, the efficiency on a per gram basis by which energy is made
available as ATP is comparable for both the oxidation of fat and carbohydrate,
thus underscoring the fact that the catabolic pathways for both fat and carbohydrate
ultimately use the same means of generating ATP, namely oxidative phosphorylation."
So they said comparable, see if they have exact numbers. You know what? This paper's real short,
let's just read it. It's two paragraphs. "The following calculation makes use of the more
recent figures of 2.5 molecules of ATP form per molecule of mitochondrial NADH oxidized by the
electron transport chain, that is coupled to oxidative phosphorylation and 1.5 molecules of
ATP form per molecule of an enzyme bound FADH2 oxidized by the same process. The complete
oxidation of glucose to carbon dioxide water, either 30 or 32 molecules of ATP can be formed,
depending on whether cytosolic and NADH is oxidized via the glycerol phosphate shuttle or
the malate aspartate shuttle respectively. For the complete oxidation of palmitic acid, 106 molecules
of ATP can be formed. If we averaged ATP yield for glucose and use a value of 31 ATP molecules, then
on a weight basis, we find that 0.17 molecules of ATP and 0.41 molecules of ATP are formed when
one gram of glucose and one gram of palmitic acid are completely oxidized, respectively."
So I hate people that... I don't hate the people. I hate the use of
this sentence with "respectively" at the end, it just makes it harder to read. So what that
means is, "We find that 0.17 molecules of ATP are formed when one gram of glucose is oxidized
and 0.41 molecules of ATP are formed, when one gram of palmitic acid are completely oxidized.
Thus, the ratio of the yield of ATP per gram of palmitic acid oxidized to that of glucose
oxidized is 2.4. Thus, over twice as much chemical energy in the form of ATP is obtained per gram of
palmitic acid that is completely oxidized to carbon dioxide and water compared with
oxidizing glucose. If we make use of the values usually quoted for the energy content of fat,
37 kilojoules per gram dry weight and carbohydrate 16 kilojoules per gram dry weight."
Of course that's nine and four-ish Calories per gram.