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There’s one last question that we need to address before launching into the next module—we
know that carbohydrates and fats are both important sources of calories in our diet,
but they are also different kinds of molecules. How is it possible for someone to eat a lot
of carbs—say lots of rice and pasta and bread—and have this result in weight gain
through the addition of body fat?
Conceptually it would be easy enough to account for the conversion of dietary fats into body
fat—you could hypothesize that if you ate too much fat, the excess would simply be put
into your body’s own store of fat in adipose tissue. As far as carbs are concerned, you
know that our bodies have stores of a polysaccharide called glycogen in muscle and liver cells,
so an excess of glucose in the bloodstream could be made into glycogen by regular dehydration
synthesis. But on the subject of a person (or a doggie) getting chunky by eating a lot
of carbs—how could an excess of glucose be used to synthesize a molecule as different
as a triglyceride?
In order to answer this question the best place to begin is to think about how we normally
use triglycerides as the starting point for the fueling reactions that so far we have
addressed only in the context of glucose metabolism.
When you catabolize a fat molecule—specifically a triglyceride (and remember the structure
of a triglyceride is a glycerol attached to three fatty acids)—the first step is lipolysis:
lipo- lysis, the splitting of a fat. Here’s where the triglyceride is broken into its
component glycerol and fatty acids. This typically happens in digestion, and the fatty acids
are then taken up by the blood.
A fatty acid, once it gets taken up by your cell, is taken into a mitochondrion more or
less directly—that is, there’s no glycolysis step like in the metabolism of glucose.
And in the mitochondrion it undergoes beta-oxidation, where the longish chain of a fatty acid is
broken into small, two carbon bits, called Acetyl CoA, which are then taken up by the
Krebs cycle, and from this point on everything is exactly the same pathway as what we discussed
earlier for the aerobic metabolism of glucose.
I neglected to mention Acetyl CoA before, but now that we’re talking about the molecular
crossroads of carbohydrates and fats, we need to identify this as the place where the two
catabolisms meet. The pyruvic acids produced by glycolysis are converted into AcCoA, as
are the fatty acids as I said earlier.
This is where you want to think about what happens when you eat more food molecules than
you need. Let’s say you have loaded all these food molecules—glucoses and fatty
acids—into your cell from your last meal, but you don’t have a need to make that much
ATP. Things are going to back up somewhere, right? So what happens is that all of the
catabolic processes keep chugging along until it gets to the point of AcCoA, which then
piles up. When there is an overabundance of AcCoA the excess gets taken up by enzymes
and synthesized into fatty acids. The fatty acids then get esterified or converted into
triglycerides, which then get stored in your adipose tissue.
It doesn’t matter whether the extra AcCoA came originally from the dinner roll or the
butter. It’s going to become part of a fat molecule on you either way.