Fat is fat, right? We all used to think this until it started appearing on food labelling a few decades ago. Now pretty much anyone in the developed world has heard the terms saturated, monounsaturated, and polyunsaturated in reference to fat.
The term saturated fat has become synonymous in our culture with clogged arteries and heart attacks. The very word makes us think of food dripping with…. well, dripping!
This has arguably been a huge mistake, as millions are finding out. But let’s stay on topic, because on this page we’re going to look at what fats are made of. It’s important to respect the chemistry.
A little fat chemistry
Fats consist of three elements – carbon, hydrogen and oxygen. In other words, they are a hydrocarbon. In fact, fats are incredibly similar in structure to a family of basic fossil fuels known as alkanes. So, lets see how these hydrocarbon alkanes are built.
Atoms consist of a nucleus with protons and neutrons, and whizzing around this nucleus are various numbers of electrons. Atoms sometimes share electrons with each other, which makes them stick together – until a more attractive atom comes long and… we all know how it feels. Anyway, while atoms share electrons they are said to be bonded.
Without getting into too much depth, within simple hydrocarbons the element Carbon (represented by a red C in the diagrams below) is predisposed to form 4 bonds with surrounding atoms, and Hydrogen (represented by a blue H) is predisposed to forming a single bond.
So, looking at the simplified hydrocarbon structures below, you can see how these bonding requirements are all satisfied.
Methane and ethane make up what we call natural gas – what you burn on your gas stove. Propane is the king of BBQ fuels. Butane is what cigarette lighters burn, and Octane is an important component in automotive fuel. Although we don’t consume these alkanes, in a moment you will see how similar they are to a typical fat.
When we refer to a fats, we are really talking about fatty acids. It is actually fatty acids that come in saturated, monounsaturated and polyunsaturated forms.
What makes a fatty acid?
A fatty acid is basically an alkane (as seen above), but the last carbon atom is reconfigured slightly. This odd little structure stuck on the end of the carbon chain is called a carboxyl group (COOH) which consists of one carbon, two oxygen and a hydrogen atom, stuck on the end. Oxygen is predisposed to forming 2 bonds.
The example below shows Lauric Acid – a 12 carbon fatty acid found in coconut oil, palm kernel oil, cow and goat milk, and human breast milk. It’s very similar to the 12 carbon alkane called dodecane.
So, staying with Lauric acid, what makes it saturated? Just like the simple hydrocarbons (alkanes) we burn for fuel, every carbon atom in a saturated fat is saturated with hydrogen. You just can’t fit any more hydrogen in there! It’s as simple as that.
If you wanted to force another hydrogen atom into Lauric acid then somewhere or other you would have to break the rules regarding the number of bonds between atoms, or as Scotty would remind us “I cannae change the laws of physics!”
Monounsaturated and polyunsaturated fat
A monounsaturated fatty acid (MUFA) is one which contains a single double bond in the carbon backbone – thus the term monounsaturated.
Palmitoleic acid (pictured below) is a 16 carbon fatty acid with a missing hydrogen near the middle. You can see that each carbon atom still has 4 bonds, and each hydrogen a single bond – so Scotty has nothing to complain about.
You might also notice that the whole thing is bent. Unless they are bonded together, atoms usually repel one another. Because there are now two hydrogen atoms missing on the bottom, the ones on the top will push themselves apart, bending the carbon backbone out of line.
A polyunsaturated fatty acid (PUFA) has more than one double carbon bond along the backbone, and so the shape can become even more bent out of shape. It should be noted that the missing hydrogen atoms are all from the same side. This is an important feature, and any unsaturated fat where the hydrogen atoms are missing from the same side is known as a cis fatty acid.
Unsaturated fats usually have lower melting points than saturated fats. A simple way to think of it is to imagine trying to pack pencils into a box. If the pencils are all straight, you can get more in, and the result is a more solid, dense package. If the pencils are all bent, then you won’t get so many pencils in the box, and they will tend to jiggle about more in a more liquid manner.
Likewise, because unsaturated fatty acids are ‘bent’, they don’t stack easily next to one another, and so tend to remain liquid at temperatures where saturated fats tend to solidify. This is easy to see when you consider butter and lard are high in saturated fat, and vegetable oils are usually high in unsaturated fat.
Omega-3 and Omega-6 Polyunsaturated fats
Marketing people love these compounds – they sound sciency and special, so you will find it plastered all over margarine and cooking oil bottles throughout the land. Omega-3 and Omega-6 are just different types of polyunsaturated fatty acids.
- Omega-3 – These provide anti-inflammatory benefits, helping manage cholesterol, joint care, and blood pressure.
- Omega-6 – Again, blood cholesterol and supporting healthy skin – but too much can have inflammatory effects.
They are also called essential fatty acids because the body cannot produce them internally, and so they must be consumed in dietary fat. We don’t need a great deal of either of these, and they can almost be thought of as the fatty equivalent of vitamins. You can read more about these essential fatty acids and the amounts you should be consuming here.
The name omega (last letter of the greek alphabet) refers to one end of the fatty acid molecule – the other end is called the alpha end. The name Omega-3 simply refers to the position of the first double carbon bond in the fatty acid’s backbone. It’s easier to understand with a picture.
Using the above example, you can see just how bent a polyunsaturated fatty acid can get. Alpha linolenic acid (Omega-3) is one of two essential fatty acids we can’t produce internally – the other is linolenic acid (Omega-6). Sounds similar? It is!
You can visualise how linolenic acid acid is structured (because I’m too lazy to draw it) by simply getting rid of that first double bond and putting two hydrogen atoms back onto the carbons and straightening out that end of the molecule a bit. Then, the first double carbon bond you would come to would be on the 6th carbon atom – thus Omega-6!
A trans fat is an unsaturated fat where some hydrogen atoms have been swapped from one side to the other in an effort to ‘straighten’ the carbon backbone. This is achieved via a process known as hydrogenation. You can also spot tran fats on food labels as they appear as hydrogenated fat or oil in the ingredients. This helps lengthen the shelf life of the fat, and increases it’s melting point, so that it behaves more like a saturated fat. The food industry loved the idea and started pumping it into everything from margarine to potato snacks.
Some years ago it was discovered that the consumption of so called trans fats increased the risk of coronary heart disease (CHD) by adversely affecting both good (HDL) and bad (LDL) cholesterol. As a result, you will be hard pressed to find hydrogenated fats on food labelling in the supermarket, but some foods do still contain them.
However, don’t be lured into a false sense of security. A lot of catering suppliers still use the stuff, and it can pop up in burger and fried chicken outlets (mentioning no names!). Oh, and microwave popcorn! and until pretty recently, Elmlea cream (well, I say cream). The golden rule is that if you are eating out, don’t buy anything containing oil unless you know for sure where it came from.
If you have ever had a blood test then you have probably heard the term triglyceride mentioned. You are probably also aware that you don’t want too many of these little buggers floating freely around in your blood. However, dietary fat has very little to do with serum (blood) triglycerides in individuals who are adapted to a low carbohydrate diet – but I’ll write more about this paradox in the blog.
Triglycerides are the form in which fat is stored and transported around the body. Essentially, a triglyceride molecule is a compound consisting of Glycerol (a sugar alcohol) and three (thus the name Triglyceride) fatty acids. You can think of glycerol like a hat stand on which you can hang three fatty acid molecules.
The diagram below shows the basic structure of glycerol on the left, and on the right the structure of a triglyceride comprising three Lauric acid molecules.
If you scroll back up and count the atoms in one of the fatty acid carboxyl groups and compare that to the ester bond it becomes, you will see that an atom of hydrogen has gone missing. Likewise you will notice that the hydrogen and oxygen atom on the left of the glycerol molecule has also gone missing (although the actual oxygen that goes missing is the one on the end of the fatty acid, but this is already reminding you of school, and your eyes are starting to glaze over). Anyway, the upshot is that each time a fatty acid joins to glycerol, then one molecule of water (H2O) is released.
This little bit of magic of joining fatty acids to glycerol is called esterisation and it goes on all the time. Likewise, triglycerides are constantly being broken down again through a process called hydrolysis which relies on pancreatic lipase. Everything is in a constant state of flux!
A triglyceride can carry a different fatty acid on each of it’s ‘hangers’. As you now know, there are all kinds of fatty acids of various lengths, so you can imagine there are thousands of different combinations that could make up a triglyceride.