Energy Metabolism Explained
or
'How your body uses, stores and burns energy'.
NOT MEDICAL ADVICE - CONSULT A DOCTOR FOR QUESTIONS
ABOUT YOUR HEALTH
Conversion of energy
Once the energy ingredients are converted into fuels and stored, they are
available for expenditure or conversion.
The energy needs of your body are not fixed. They depend on the 'attitude'
chosen by your metabolism, your activities and your composition. First about
how energy is made available to your cells.
The actual energy currency: ATP
Nearly all bodily processes do not run on the fuels mentioned earlier. They
run on the conversion of ATP to ADP, which makes ATP the energy currency of
choice. You can see the fuels mentioned as things you could barter with and
ATP as actual money.
So it all revolves around ATP. Why doesn't your body store ATP directly
then? Well, who knows. ATP might be too active to be suitable for long term
storage, or too agressive. As molecules go, ATP is a real firecracker.
Conversion
As said, your body stores its energy in the form of the three different
fuels. To be used, these fuels must be converted to ATP. This can be done
in lots of places.
- Glucose
Glucose, stored in the form of glycogen, can be converted into ATP by all
cells containing mitochondria, which means nearly all of them. Muscles can
even burn glycogen directly.
- Fatty acids
Much the same goes for fatty acids, with the very notable exception of the
brain. Fatty acids cannot cross the barrier into braincells.
The brain uses loads and loads of energy which is a major challenge for your
metabolism - the organ which uses most energy prefers to run on the fuel
that can be stored worst.
Fatty acids can be converted to ketones, which can partly power the brain.
More about the brain later, it is not that simple.
- Amino acids
Can be converted by the liver into glucose, or even into fatty acids.
We could write a matrix of conversions - almost everything can be converted
to everything else by the body. But not always, and not everywhere.
Important conversions are:
- From glucose to glycogen to stored fat
Mentioned earlier, this is done when your sugar intake has exceeded the
storage capacity. This happens a lot.
- From stored fat to glucose
Creating glucose from non-glucose parts is called gluconeogenesis and is
very important. It helps you power the brain from long-term energy storage
(fat).
You will read everywhere that this is not possible. Why people keep
repeating this is a complete mystery. To stamp out this myth, I will spell
it out here in gory detail:
-
Adipose tissue (stored fat) contains a lot of TAGs, triacylglycerol
molecules. Glycerol, released from TAGs by either triacylglycerol lipase or
lipoprotein lipase, can be taken up by the liver and converted into glycerol
3-P by glycerol kinase. Glycerol 3-P can be converted into DHAP, an
intermediate of gluconeogenesis, which is the creation of glucose by the
liver from non-glucose precursors.
If you ever encounter anybody stating that you can't make sugar from fat,
point them to this paragraph. Discard any book stating this impossibility.
Continue reading about how your body expends
energy.