It seems that for many people, the word ‘metabolism’ conjures up an idea of how fast we can burn off the calories we eat.  Certainly, if I let slip that my job is to study human metabolism, the most common response is ‘Oh, can you tell me why mine is so slow?’.  But that’s a very restricted view of what metabolism involves.  Everything that we are made of is the product of a metabolic process: even if it’s something that we have eaten and then incorporated into our tissues, it will have been made by metabolism.  To take just one example: our brains contain a large amount of the fatty acid called by scientists docosahexaenoic acid (DHA).  This is one of the so-called ‘Omega-3 fatty acids’.  We cannot make it ourselves, but we need it for many purposes, brain and immune function especially.  The DHA in our bodies has mostly come from fish.  But neither can fish make it (a common misconception) – they themselves obtain it from smaller organisms.  Ultimately it is made by algae, seaweeds etc, using energy from sunlight.  (I will write more about the Omega-3 fatty acids, or as scientists call them n-3 polyunsaturated fatty acids, in a later blog.)

In fact energy from sunlight is the ultimate source of everything within us.  Plants use that energy to combine carbon dioxide, CO₂, from the air into more complex molecules – sugars, in fact.  From sugars can be made starches and other carbohydrates, and also fats.  We may eat these plant products directly, or via another animal such as a cow.  (We tend not to eat carnivorous animals, but we may well eat carnivorous fish, so there may be several steps before these compounds get into our own bodies.)  The nutrients that we use to derive energy, sugars, fats and proteins (in the form of their building blocks, amino acids), all come to us in this way.  We then derive energy to move our muscles, make our nerves function, drive our immune system, etc, by breaking these substances down, in combination with oxygen (the process called oxidation) to liberate CO₂ again.  It’s one big cycle.  But don’t lose sight of the fact that, when you jog round the block or pedal your bike up a nearby hill, you are, in the end, powered by solar energy.  It’s the ultimate in green energy provision.

In future blogs I will discuss sugars, fats and amino acids in more detail, and also look at how hormones control our metabolism, and how enzymes make metabolism happen.

Title: Understanding Human Metabolism

Author: Keith Frayn

Paperback ISBN: 9781009108522