In this first post of my Fat series I’d like to go back to the basics of fat biochemistry. If you have spent enough time in lecture theatres scribbling down the chemical formulae for beta-oxidation of fats, feel free to take a nap. If you don’t know your linoleic from your myristic, here is your chance!
Fats have received a pretty bad rep over the years. One of the most common misconceptions comes from the word itself: fat! Fat is bad! Fat makes you fat! We see these images in the media: an obese person gorging themselves on something that is considered unhealthy. You can see the glistening cheeks and mouth, thick fingers dripping oil, fat from the stomach traveling directly to their thighs. Your inner Puritan is horrified, judgement is formed in your mind even before you have consciously acknowledged it to yourself.
The media do a great job perpetuating the myth of “saturated fat clogging your arteries”. Here is a recent addition to the shock and awe campaign on fat, courtesy of the British government. As much as you want to imagine your blood vessels as pipes where you are pouring soft sticky fat mass every time you eat a burger, it bears absolutely no resemblance to what actually happens.
Fats, or better put fatty acids (FAs), are a diverse family with its own overachievers and black sheep. We mostly think of fats as something that provides calories: 9 calories per each gram of fat, the highest value out of all other macronutrients. They tell us that’s what makes us fat. But the caloric value of fats is only important if they are actually “burned” for energy. Dietary fats are used for much more than just providing calories: they are a vital component of the cellular membranes for each of the 50 trillion cells in the body. When you don’t receive enough cholesterol in your diet, your liver makes some by using the saturated fats from your food (why would it do that? Doesn’t it know that cholesterol causes heart disease?). Your brain is 60-80% fat and every neuron in your body has a fatty insulating myelin shield.
Most of us though think of fat as this stubborn orange-peel substance on our thighs, sticky soft sludge pouring through our arteries or the thick yellow deposit packing our tired liver.
So how can something so good be so very bad?
Let’s start with deciphering the common terms surrounding fats.
Triglyceride is the term commonly interchanged with fat, it consists of 3 fatty acids on glycerol backbone. Most dietary fats come into the body in form of triglycerides. Just like other animal fat, our own fat is also stored in triglycerides.
In biochemistry fatty acids are classified on the basis of saturation. The fatty acid is called saturated if carbons in its chain have a hydrogen atom at each one of the 4 available bonds. This arrangement makes for a chain which is straight and stable like a string of pearls. And because these can be easily packed together, they are solid at room temperature (think a block of butter).
An unsaturated fatty acid has at least one carbon which is missing a hydrogen. This makes the carbon form a double bond with its neighbour. Monounsaturated FAs have only 1 double bond in their chain. Polyunsaturated have obviously more than one. The double bonds in the chain give the fatty acid some interesting properties. Firstly, they are more vulnerable to the free radical attack, or oxidation, which can make them rancid. Secondly, double bonds change the shape of the fatty acid molecule: instead of a string of pearls you get a chain with a kink or two. This lowers the melting point of unsaturated fats, making them liquid at room temperature.
Here is what we are up to at the moment.
Fatty acids can be further classified by their length. Most of the saturated fat coming from animal sources is in the form of long chain saturated fatty acids: LCSFA, with >12 carbons each, for example palmitic, myristic and stearic. The carbon-carbon bond has the potential to release energy, which makes these FAs a very effective source of fuel. This is also the way your body stores them in your fat (adipose) tissue.
I think it’s important to understand the storage issue. We are so used to seeing stored fat as an inconvenience or an indicator of disease that we have forgotten why it is there in the first place. It is there to be burned. Think of it as fuel for your fireplace. You don’t collect dry timber and then get annoyed that it takes up too much room in your backyard. It is their for one reason only: to use it for energy (warm up your house) when there is a dire need. Humans have evolved to have extra energy stored away for further use.
Next question: when you stock firefuel for winter, do you use timber or rubber tyres? Both will burn, both will give off heat. But you will still choose something which is more benign and will not poison your entire family with fumes. If saturated fatty acids are so very dangerous, does it make sense that our body will carry a toxic load which will “clog your arteries” the moment it is released into the bloodstream. The fact that we have evolved to store saturated fats should be the first clue that they are the preferred source of energy and that the body considers them harmless.
If you are still with me, congratulations, you have high tolerance for dry science. Let’s plow along. Next we have MCSFAs, or the medium chain saturated fatty acids (the more common abbreviation is MCTs for medium chain triglycerides). They have between 8-12 carbons in their chains and the major dietary sources of those are breast milk (estimates range from 10-25%) and tropical oils (coconut oil and red palm). Lauric acid is a common example of a medium chain fatty acid. Unlike LCSFA, which are transported via lymphatic system in little vehicles called “chylomicrons”, MCTs travel via the portal vein from the intestine directly into the liver where they are preferentially used to form ketones. This is interesting because the conventional wisdom tells us that ketosis is always bad. So it’s kind of strange that a newborn baby would naturally receive a load of fat which encourages ketone production. Add to that the fact that your brain functions just as well (or maybe even better) on ketones and that your heart likes them as well, and there is another myth busted.
Finally, the SCSFA are obviously the short chains. Butyric acid is predictably enough in butter. But overall the dietary sources of these are very few. However, your intestinal bacteria happily produce them through fermenting fibre. Like MCSFAs they go straight to the liver.
If you are a male and therefore likely a visual learner, here is an updated pic :
Monounsaturated fatty acids, MUFA, (like oleic acid) are considered healthy by conventional standards. I, for once, agree with their assessment but for a completely different reason. The lipid hypothesis proponents support oleic acid because of their love affair with olive oil. I find it hard to believe that olive oil must be an essential part of our diet, mainly because there are plenty of regions in the world that do not naturally grow olives and somehow are still doing ok. However, monounsaturated fatty acids make up a big portion of animal fat, as much as 44% in pork lard, so it makes sense that we have evolved to process them. So if you want to increase monounsaturated FAs in your diet, fatty chunks of meat will get you much further than a tablespoon of olive oil on your salad.
Finally we come to polyunsaturated fatty acids, PUFA. They are subdivided into omega-3, omega-6 and omega-9 by the position of their first double bond if you count from the methyl (or omega) end of the chain. The FA with the first double bond in position 3 is therefore omega-3 PUFA and so on.
Animals do not tend to store large amounts of polyunsaturated fatty acids, mainly because they are not a very good source of energy. Therefore traditional animal fats are quite low in PUFA. Fish products are widely promoted for their PUFA content but while the relative PUFA composition are high compared to SFA, it is still a minuscule number in absolute terms. “Vegetable” oils (if you believe that seeds are from the same family as, let’s say, carrots) come on the top of that list. You have to eat 900 grams of smoked salmon to get the same amount of PUFA as in 1 tablespoon on sunflower oil kindly provided by the industrial processing technology. All PUFA tend to be lumped together in one big “healthy” basket but there are significant differences in their effect on the body.
To finish off just a quick summary of fat composition of common foods. Click on the table for better resolution.
And here is an updated diagram for you to memorise before the next post. Consider this your homework.
In the next post I will go a little deeper and talk about the “essentiality” of omega-3 vs omega-6. I will also address some clinical implications, health claims and health myths. Now go and have some fat!
Edit: a few errors which I blame on the brain fog commonly experienced after a hospital shift…