It is often difficult to get a handle on what molecules are and what they do. In biological systems, there are many that carry out and are involved in several unrelated processes, as we have seen with cholesterol. This means the work that scientists need to do to uncover a molecule’s true importance usually requires contributions from several scientific disciplines. However, cholesterol does not quite fall into the category of the molecule I want to discuss here, because unlike cholesterol, we cannot make all that we need of this molecule. It is a fatty acid called docosahexaenoic acid.
Fear not if its name looks incomprehensible, it is a name invented by organic chemists to state unambiguously what the molecule is and how it is built atomically. The name is often shortened to DHA, which I think you will agree is more manageable. It is given the trivial name cevronic acid. Whatever we choose to call it, there is evidence that without it, several of our bodily systems would suffer. Its presence in the nervous system is well documented . The amount of DHA in a neurone’s membrane influences the action of proteins that control whether potassium, sodium and calcium ions are inside or outside of a cell. These are proteins are known as ion channels, and although what they do might sound a bit trivial, their behaviour is particularly important for all nerves. The impulses involved in walking, talking and touching, all rely upon ion channels in order to get from or to the brain, along sensory or motor nerves. And this is just DHA in its ordinary form, as shown in Figure 1. DHA is also found in our eyes, in the retina in fact, and helps us to see things properly: it helps turn the light that falls on our eyes into the nerve impulses that go to the brain. So the fact that you are reading this is thanks, in part, to DHA.
DHA also has a measurable effect on the lipids it is used to make. The best published example to date is also concerned with vision. DHA can be used to make phosphatidylcholine , whose name is also shortened, this time to PC. It is found in the membranes of cells in the eye and its job is a physical one. The PC that contains DHA makes the membranes more ‘fluid’. This is the opposite of the effect of the presence of cholesterol [link], that stiffens the membranes. The simple advantage of a more fluid membrane is that the proteins in it, such as ion channels, do not get squashed. Biophysicists describe this in terms of membrane pressure, and it is pretty easy to understand. Imagine you are holding hands with the love of your life. It is nice to know that s/he is there, but if the grip is too little, it is not really happening, and if it is too much, it hurts. This is rather true with a membrane: if it is too fluid, the membrane cannot resist changes in the environment and is open to the risk of falling apart. If the membrane is too stiff, the proteins are under pressure and cannot work properly. DHA is one of the molecules that contributes to this balance. In terms of your vision, it allows the plasma membrane of the cells at the back of your eye to be fluid enough for the proteins, called rhodopsins, to do their stuff.
But humans have a problem; we cannot make this fatty acid. It is nothing to panic about as we can get much of what we need from easily available sources – fresh fish being the richest natural one. An even better one in terms of mass, is popping a cod liver oil tablet, packets of which can be bought from just about every vaguely food shop or pharmacy in the country. The larger supermarkets have their own brands of it; it is not a difficult product to put together and sell. In the shops it is often placed alongside vitamin and mineral supplements. However, as it is a dietary supplement it is subject to much of the same sort of scientific and media attention as vitamin pills. Recent evidence indicates that too much of certain vitamin and mineral goodies in our diet, can be counter-productive. So, if you find your joints are less flexible than they once were, or your eyesight is worsening, DHA might give you a bit of a hand. However, unlike vitamin and mineral supplements, it will not shorten your life just like that. As you are consuming a fatty acid, if you eat too much, you will put on weight instead.
 N. Salem, B. Litman, H. Y. Kim K. Gawrisch, Lipids, , 36, 945-959. http://www.springerlink.com/content/r2326m7553u64258/.
 B. J. Litman, S. L. Niu, A. Polozova, D. C. Mitchell, Journal of Molecular Neuroscience, 2001, 16, 237-242. http://web.pdx.edu/~drakem/papers/J_mol_neuro-2001.pdf