Just recently I started my diet review series and the very first diet I reviewed was the Western Diet or Standard American Diet. While reviewing all the items in the Western Diet that may lead to adverse health effects, I realized that we need to talk about fat again, this time in much more detail.
In my macronutrient review series, I spoke about fats in general – their structure, namely ,that fats are primarily represented by triglycerides (See Fig.1), which contain glycerol and 3 fatty acids, and the fats’ caloric yield of approximately 9 kCal per gram or 3,500 kCal per pound. Today, I would like to dig deeper and to talk about different types of fatty acids and their impact on our health.
The general structure of a fatty acid
All fatty acids are chains of carbon atoms that have a carboxyl group on one end. This group is what makes a fatty acid an acid. The other side of the molecule, the carbon chain (denoted as R on Fig.2) is called aliphatic group and it is the part of the molecule that varies a lot in terms of its length and saturation and conveys specific properties to different fatty acids. In some fatty acids the chain may be branched, but these are not too relevant for our today’s discussion as they are primarily found in bacteria. As a side note, valproic acid that we use in psychiatry is a branched chain fatty acid. In any case, I would like to focus on the length and saturation of these molecules’ carbon chains in this blog.
The length of the carbon atom chain
The length of a carbon atom chain may vary from 1 to 35 carbons (or more), so they can get quite long. We can divide fatty acids into several categories based on their length:
- Short chain fatty acids: 5 or less carbons
- Medium chain fatty acids: 6-12 carbons
- Long chain fatty acids: 13-21 carbons
- Very long chain fatty acids: 22 and more carbons
I put some examples of these fatty acids with the schematic representation of their chemical structure in Figure 3 so that you can have a better understanding of their structure and size.
These fatty acids differ not only in their length, but also in the way our bodies are using them and accordingly – their health effects. Short chain fatty acids are typically produced when our gut bacteria ferment fiber (yes, it can be fermented in human gut1) and are important for colon health and the cells of the colon lining called colonocytes use them as the major source of fuel. Some of the short chain fatty acids are being absorbed from the gut along with the medium chain fatty acids.
Medium chain fatty acids are part of several food groups such as milk (10-20%), palm kernel oil and coconut oil. There is something special about the medium and short chain fatty acids – they can be absorbed directly into the blood stream and used for energy. Notably, being readily usable lipids, they tend to stimulate fat burning (more on that in future blogs).
The long chain fatty acids are not being absorbed directly into the bloodstream. Instead, they are being absorbed into the lymphatic system in a form of chylomicrons, which are then taken to the liver, which in turn processes, repackages and releases them into the bloodstream to be picked up by other tissues. This process might seem to be quite tedious, but these fatty acids represent the majority of fats we consume, and they also have the highest energy yield.
Just for the sake of completeness, I need to mention the very long chain fatty acids – these are not very common, but they are important from a medical standpoint as they cannot be metabolized in mitochondria like the rest of the fatty acids and have to be processed in peroxisomes, another kind of cellular organelles and some people with peroxisomal disorders can have an abnormal accumulation of the very long chain fatty acids.
Classification of fatty acids based on their saturation
Another important aspect of the fatty acids structure is their saturation. Likely, you have heard the terms like “saturated fat”, “omega-3 fatty acids” or “trans-fat” and here I can explain what all these terms mean. The key thing to know about fatty acids is that most of the carbon atoms in a fatty acid carbon chain are linked by single bonds, but some of them might be linked by double bonds. If all the carbons are connected by single bonds, such fatty acid is called “saturated”, which refers to the fact that all carbon atoms are saturated with hydrogen ions. If a fatty acid is not saturated, it must have more covalent bonds between some carbons and thus it forms double bonds between them.
Since these double bonds can form at a variety of places, we have many ways of categorizing of unsaturated fatty acids. First of all, if there is only one double bond in a carbon chain, we would call such a fatty acid “monounsaturated”, if there is more than one, such fatty acid will be called “polyunsaturated”.
The second thing we must keep in consideration is the location of the bonds – the last carbon atom in a fatty acid carbon chain is “numbered” as omega, the last letter of Greek alphabet and we can count carbons from the end using the terms ω-1, ω-2, ω-3 (sounds familiar, eh?) and so on (See Fig.4). If the last double bond (when counting from the end) is located at the 3rd carbon atom, we will call such a fatty acid ω-3 fatty acid, if it’s the 6th or the 9th carbon, we will call them ω-6 or ω-9 fatty acids, respectively.
To make things more complicated, the fatty acid molecules can twist differently around these double bonds. Since it’s a double bond, it solidifies the molecule conformation and it can exist in either cis-conformation or trans-conformation. The prefixes cis- and trans- refer to the location of the remaining hydrogen atoms on the carbons forming the double bond. I know there is a lot of biochemistry here, but I felt that I simply couldn’t omit this information. Just to illustrate it, I added an image of cis- and trans- conformations of oleic acid in Fig.5.
As you can see, the conformation of the molecule changes its shape significantly, but most importantly, it has a very important role for metabolism as certain enzymes can only process cis- or trans- variants of the molecule.
Now, when we know the key concepts of cis- and trans-, saturated, mono- and polyunsaturated fatty acids, and their nomenclature, we can finally talk about their properties and health effects.
The health effects of various fatty acids
The saturated fats are generally vilified in scientific literature to the point that it will be a blasphemy for me to say that they are not all that bad. And that’s exactly what I am planning to do – challenge “the common wisdom”, which turned out to be neither common nor wisdom. Sorry for such harsh words, but there is enough evidence nowadays that shows that the problem with dietary fat lies far beyond the specific type of fats consumed2.
First of all, it was shown that the saturated fat is not associated with cardiovascular mortality and this is a scientific fact4. The monounsaturated fats were never considered a big issue, whereas polyunsaturated fats were almost considered to be some sort of an antidote to saturated fat, which was also proven not to be true. In addition to that some of these polyunsaturated fats are trans-fats and it just shows how oversimplistic our understanding of their role and effects is (since we “know” that trans-fats are not supposed to be good for us). The truth is somewhere in the middle and is much more nuanced – effectively, the evidence shows that saturated fat is not as bad as it thought to be, rather neutral as a matter of fact. The unsaturated fats are generally fine except for trans-fats. Interestingly enough, the research shows that naturally occurring trans fats are not bad for our health whereas industrially produced ones are4. These are some partially hydrogenated oils that were chemically modified to increase their stability and shelf-life.
The last thing that must be mentioned is the ω-3 vs. ω-6 fatty acids debate. There is a notion that ω-3 fatty acids are very good for us whereas the ω-6 fatty acids are just terrible. I will agree with the statement that ω-3 fatty acids are very good for us – we use them in medicine to improve cognitive functioning and I prescribe them to my patients as there is solid evidence of their beneficial health effects. If we look at the evidence on ω-6 fatty acids we will see that, first of all, one of the 2 essential fatty acids, namely, linoleic acid, is an ω-6 fatty acid, so effectively we cannot live without ω-6 fatty acids to begin with. Also, the notion of ω-6 fatty acids being pro-inflammatory stems from the idea that some pro-inflammatory chemicals in our body are produced from them, but there is no sufficient evidence to show that the consumption of ω-6 fatty acids increases inflammation, they are just more readily available and that’s it. In fact, current research shows that ω-6 fatty acids are anti-inflammatory and thus should be considered rather beneficial than detrimental to our health5.
So, let’s summarize: the idea that fats are bad for us is overly simplistic and essentially wrong – some fats are essential and we cannot live without them, the ω-3 fatty acids are very beneficial to our health as well as the ω-6 fatty acids. The only “bad” type of fats with proven detrimental health effects are the industrial trans-fats and these are some that we should avoid. The rest of the fats are fine and as long as we don’t exceed our caloric intake limit, there is nothing to be concerned about.
Well, we’ve covered a lot today and I also see that there will be much more to talk about in the future. I hope that this blog will give you a good general idea of the variety of fats that we consume as well as their effects on our health. I will continue working on various aspects of nutrition and health, so in order not to miss anything, please subscribe to my YouTube channel and to my website. And of course, you can always ask questions, make your comments and suggestions either on YouTube, or here on my website.
1. Williams BA, Grant LJ, Gidley MJ, Mikkelsen D. Gut Fermentation of Dietary Fibres: Physico-Chemistry of Plant Cell Walls and Implications for Health. Int J Mol Sci. 2017;18(10).
2. Forouhi NG, Krauss RM, Taubes G, Willett W. Dietary fat and cardiometabolic health: evidence, controversies, and consensus for guidance. BMJ. 2018;361:k2139.
3. Dehghan M, Mente A, Zhang X, et al. Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. Lancet. 2017;390(10107):2050-2062.
4. Tardy A-L, Morio B, Chardigny J-M, Malpuech-Brugère C. Ruminant and industrial sources of trans-fat and cardiovascular and diabetic diseases. Nutrition Research Reviews. 2011;24(1):111-117.
5. Harris WS, Mozaffarian D, Rimm E, et al. Omega-6 fatty acids and risk for cardiovascular disease: a science advisory from the American Heart Association Nutrition Subcommittee of the Council on Nutrition, Physical Activity, and Metabolism; Council on Cardiovascular Nursing; and Council on Epidemiology and Prevention. Circulation. 2009;119(6):902-907.