 |
| HDLs are like customers around a kaiten sushi bar, picking up bundles of cholesterol like sushi from the turning belt and then returning the empty plates quickly. Photos: The Star |
They have some things in common – for example, every 100g of each fat provides around 900 calories of energy and the body needs several types of dietary fat to function properly. All complete fats are forms of triglycerides – and each triglyceride is a collection of three fatty acid molecules bonded together by the compound glycerol.
Heat and digestion break down triglycerides into their component fatty acids – and there are different types of fatty acids making up various triglycerides. The glycerol backbone usually accommodates a mix of different types of fatty acids in a single triglyceride.
However, there are rarer triglycerides made up of three molecules of the same fatty acid – a triglyceride of only stearic acid molecules is called a tristearin, as an example.
Fatty acids can detach themselves from triglycerides under various conditions forming free fatty acids (FFA), and FFAs are much more readily created when high heat is applied or when digestive processes dismantle the triglycerides.
By the way, dismantled triglycerides can also end up as di- or monoglycerides and are generically called lipids (a classification which also includes FFAs and triglycerides themselves). Sunlight also decomposes triglycerides so that is why you should never leave cooking oils in the sun.
As for the differences between saturated, monounsaturated and polyunsaturated fats, there is a lot of perplexing data on the Internet about them, especially on health websites – so one can be excused for being confused. However, the good news is that the explanation in chemical terms is really very straightforward. A saturated fat is simply a fatty acid that contains the maximum possible number of hydrogen atoms that can be attached to every carbon atom in the fat – hence it is “saturated” with hydrogen.
A monounsaturated fat has less hydrogen bonds (due to the configuration of a double-carbon bond in the fatty acid) and the mono prefix comes from the fact that it also has only one such double-carbon bond.
Therefore, a polyunsaturated fat is similar to a monounsaturated fat except that it has more than one double-carbon bonds. All these fats play a role in our well-being (as explained later) and the following diagrams clearly indicate the differences:
Hydrogenated fats (and other stories)
You have probably heard of hydrogenated fats – however, the critical fats to be wary of are the partially hydrogenated plant fats (often labelled as trans-fats) but hopefully these fats will get rarer over time (some developed countries have already banned them).
There are a few natural trans-fats but the ones you see on food labels are almost certainly man-made. There is one very significant difference between fully hydrogenated fats and partially hydrogenated fats – fully hydrogenated fats do not contain trans-fats and why this is so is explained a bit later.
Why we focus on plant trans-fats (PTF) is because decades of research have indicated that these fats are really bad for health.
In fact, PTFs are so bad that the FDA in the United States finally issued a directive in June 2015 that plant trans-fats need to be removed from all processed foods in the US by the year 2018 – the only wonder is why it took them so long to issue the ban.
Hydrogenated fats and PTFs are synthesised under heat by the addition of hydrogen atoms via a nickel catalyst to unsaturated fats – the aim of hydrogenation is actually to harden or create saturated fats from unsaturated fats, for this process stabilises, preserves and also raises the melting point of the fats to more convenient temperatures – it’s a lot nicer to spread margarine on bread than liquid sunflower oil.
If you’re curious about the difference between fully-hydrogenated fats, unsaturated fats and PTFs, the answer is in the final structure of the fats after hydrogenation. Normal unsaturated fats have what is termed a cis configuration of its fatty acid molecules – the word “cis” derives from the Latin for “on this side”. In contrast, the word “trans” in Latin means “across”.
In simple chemical terms, it means that the paired carbon-hydrogen molecules in an unsaturated fatty acid which were originally arranged in a cis formation have been flipped across into a trans formation – as structures, they are known as geometric isomers and the cause of the flip is the action of the nickel catalyst.
So the original unsaturated cis fats become unsaturated trans-fats – in proportional terms, after catalysation, around 66% of the original cis fats are turned into trans-fats (mainly because a trans-fat requires a lower energy configuration when returning from the catalytic process).
For fully-hydrogenated fats, all the unsaturated fats have been converted into saturated fats so there is no issue with unsaturated cis or trans structures.
A simplified representation of the carbon-hydrogen molecules in fatty acid chains is as follows:
Structural formulas of fats.
A horror movie called The Thing
One reason why scientists believe PTFs are bad for health is simply because plant-based trans-fats are artificially-created geometric isomers of normal unsaturated fats – basically, they just don’t exist in natural foods. They are a little like the aliens in that great John Carpenter horror movie The Thing, which look like real people, talk like real people, even act like real people – but are actually bloodthirsty parasites from another planet.
In a vaguely similar but much less violent fashion, these plant trans-fats also profoundly confuse the body – and one significant effect is that they mess with the balance of lipoproteins (the actual carriers of cholesterol) in the blood.
There are five kinds of lipoproteins: chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL).
But often you only hear of two types: low-density lipoprotein (LDL) and high-density lipoprotein (HDL) – LDLs are the most prolific carrier of the cholesterol that attends arterial wall damage, which if unchecked, causes atherogenesis.
By the way, all lipoproteins contain sub-classes of proteins known as apolipoproteins – LDL holds apo-B while HDL uses apo-A (more on this very soon).
A kaiten sushi bar
Firstly, the good news: HDLs help regulate cholesterol by extracting it from cells via special proteins on cell membranes – how this works is a little ingenious and involves an ABC, or an ATP-binding cassette transporter protein (if you’re curious, ATP is the acronym for adenosine 5’-triphosphate).
ABCs are like transit points for the exchanging of fats, peptides, mineral ions, steroids, etc – the specific type targeted by HDLs is the ABCA1 family. As mentioned, HDLs contain apo-A – and these proteins are like empty containers for fats, specifically cholesterol.
So if a cell is loaded with cholesterol and if a HDL is passing by, the ABCA1 protein on the cell membrane will bind with apo-A and dump its cholesterol into the HDL – the HDL in turn passes the cholesterol to the liver where it is oxidised.
As a little analogy, HDLs are like customers around a kaiten sushi bar, picking up bundles of cholesterol like sushi from the turning belt and then returning the empty plates quickly.
HDLs are also the smallest of the lipoproteins – and are termed high-density because they have the highest protein to fat ratio in their cell structure.
And now, the other side: LDLs operate in quite a different way, but the principle is the same in reverse – LDLs are larger than HDLs (and can therefore hold more cholesterol molecules), and interestingly, normally also hold a single molecule of the apo-B protein.
The presence of this molecule is picked up by the LDL receptors on cell membranes, and the LDL and the receptors then combine to form a cage (made from the protein clathrin) just underneath the cell membrane – this is where the cholesterol ends up.
Actually, the normal function of an LDL is not at all problematic – when cells have had their fill of cholesterol, they just turn away new LDLs. Where LDLs might become a nuisance is when they get transformed in some way.
There had been some studies done about how oxidised LDLs can turn nasty, but the research data is not wholly conclusive as it is quite hard to actually define what oxidised LDL means – scientists had even called it “the elephant that is described by blind men”.
So it may be only certain types of “oxidised” or damaged LDLs are pertinent to the development of arteriosclerosis – more on this later.
What is more established is that white blood cells called macrophages which have been affected by a body protein called PMA (or phorbol 12-myristate 13-acetate) will engage with LDLs – white blood cells are an integral defence mechanism against foreign bodies and other detritus in the blood – and macrophages are the bulkiest of these white blood cells.
The name macrophage is derived from the Greek for “large eaters” – and the problem about macrophages messing with LDLs is that the cholesterol in LDLs is not easily processed, and will cause the macrophages to turn into mushy foam cells and die.
Even so, foam cells are not necessarily problematic unless their cholesterol-laden decomposing parts get trapped in the structure of impaired tissues, such as damaged arterial walls – where they accumulate and turn into atheroma.
The probability of macrophages turning into foam cells appear to be a simple function of the quantity of LDLs floating around in the blood – an increased number of LDLs means more foam cells are likely to be formed. It is not known why PMA would cause macrophages to target LDL in this manner.
PTFs are undesirable in that they have been implicated in the proliferation of LDLs (while possibly reducing HDLs), thereby adding significantly to the problems of having arterial wall damage.
PTFs are also reported to interfere with the metabolism of other essential fatty acids. The physiological or chemical reasons why PTFs affect these metabolic processes are not really known.
Another noted negative health aspect of PTFs is that they cause the liver to produce more C-reactive protein (CRP) – this is a blood protein which normally forms as a response to inflammation and its role is to bind with dead or damaged cells, such as those found in damaged arteries.
It would seem that this action further aggravates the build-up of atheroma in arteries. It is unclear if the liver is producing more CRP due to some triggering mechanism in PTFs, or if it is a defensive bodily reaction to PTFs – the latter would probably be the more reasonable conjecture.
PTFs have also been linked to insulin tolerance (and therefore diabetes) – and more worryingly, the Institute of Medicine in the United States has stated that there is no known safe level for the consumption of these trans-fats.
There are claims by the Harvard School of Public Health that for every 2% of calories derived from PTFs, the risk of heart disease increases by 23%. This is all rather sobering food for thought – especially if you are munching on a packet of crisps while reading this (PTFs are generally used to improve and retain the crispiness of fried crisps).
If you’re now wondering why processed oils are not fully-hydrogenated instead of partially hydrogenated, the answer is simple: cost. It is very much faster and cheaper to partially hydrogenate unsaturated oils to achieve the right characteristics for cooking and consumption rather than creating expensive blocks of saturated fats via full hydrogenation.
So be careful with the labelling of fats in food – if a food simply states it contains “hydrogenated fats”, almost certainly it means partially hydrogenated trans-fats.
Even if it lists fully hydrogenated fats, look for other ingredients which may contain PTFs, such as “shortening”, “emulsifiers” or “E471” – it is not unusual to see food labels which list fully hydrogenated fats and then various partially hydrogenated fats represented under different names; for example, diglycerides and monoglycerides are very probably PTFs.
Also be very wary of labels that claim zero trans-fats “per serving” – in some countries, there is a loophole which allows food manufacturers to disclose zero trans-fats if there is less than a certain amount of trans-fats per serving.
One last important thing to check: if a food label gives the total fat content and also lists the quantities of saturated, monounsaturated and polyunsaturated fats, sum up the individual fats listed – if the sum is less than the label’s listed total fat content, then the difference is probably trans-fats.
It’s sad but if you want to eat safe processed foods, then it really helps to be a paranoid nerd. I’m not really clever or determined enough to be a nerd, so I tend to buy fresh food and cook at home – in this way, many of the problems with PTFs are avoided.
