Intro to Lipids
Do you know that fats/lipids are the same? Lipids are just a more fancy word. And we eat fat and we also store fat. But what is fat really, how is it processed and what’s the difference between saturated and unsaturated fatty acids? And why are omega-3 fatty acids are called that way? You are probably curious to know which foods contribute the most to your total fat intake.
Some foods are rich in saturated fatty acids, such as cheese, coconut oil, palm oil, fatty meats; whereas, other foods are rich in unsaturated fatty acids: most vegetable oils, avocado, nuts, fatty fish, and many other foods.
Chylomicrons are special particles that transport the fat throughout the bloodstream. I mentioned chylomicrons but there are many other particles, called lipoproteins, that carry fat in our blood, there is VLDL, LDL and there is HDL.
Now besides fat in our diet, there is also fat stored in our body, the so-called bodyfat. What you may not know is that we constantly make and break down fat, and you may also not know that when you lose weight, the individual fat cells shrink in size.
Chemistry of lipids
Now dietary lipids consist of triglycerides, phospholipids and sterols. Triglycerides are quantitatively the most important. They take up approximately 95% of our daily fat or lipid consumption and consist of three fatty acids linked via a glycerol molecule. Triglycerides are also referred to as triacylglycerols. It’s the same molecule.
Triglycerides are fats if they are solid at room temperature and they're oils if liquid at room temperature. Moreover, the vegetable oils such as sunflower oil, olive oil or peanut oil, they entirely consist of 100% triglycerides or fat.
Now, what is a fatty acid?
Chemically, a fatty acid is a chain of hydrogenated carbon atoms connected to a Carboxyl Group. And the fatty acids that are part of the triglyceride molecule differ in three major properties:
- First of all, there is the chain length, which can range from 3 to 24 carbon atoms or even more. And most of the fatty acids in our diet have 16 or 18 carbon atoms.
- The second property is the degree of unsaturation, which means the number of double bonds. Unsaturated fatty acids have at least one double bond. Now mono-unsaturated fatty acids have one double bond, whereas polyunsaturated fatty acids have at least two double bonds.
- And the third property is the point of unsaturation, which describes where the double bonds are located in the fatty acid molecule.
Now in this context, it’s important to understand the nomenclature of the fatty acids and that there are two different nomenclature systems. There is the “n” or “omega” designation and there is the “delta” designation. And both systems denote the carbon chain length, followed by the number of double bonds. Now concerning the position of the double bonds, the “n” or “omega” designation counts from the methylene and only indicates the position of the first double bond.
So, for instance, n-6 or omega-6 means that the first double bond starts after 6 carbons,
counting from the methylene and the next double bond will be positioned three carbons down. There is the delta designation and that counts from the carboxylate and indicates the position of all double bonds.
Now, what are the main fatty acids in our diet?
Oleic and palmitic acid is the most abundant fatty acids in our food supply. And together they account for approximately 2/3 of our fatty acid intake. The fatty acids are often abbreviated, so,
- palmitic acid is c16:0
- stearic acid is c18:0
- oleic acid c18:1
- linoleic acid c18:2, and finally,
- there is linolenic acid, c18:3
These are the five common fatty acids in our diet!
Lipid Content and Composition of Food
Lipid content of foods
Many foods contain substantial amounts of fat. Cooking oils such as sunflower, peanut and olive oil are 100% fat in the form of triglyceride. Butter is about 80% fat, such as mayonnaise. Lean cuts of meat such as steak contain little fat, whereas the fat content of bacon and sausage can easily reach 40%. Fruits and vegetables generally contain little to no fat. Cholesterol is only found in foods of animal origin. The highest cholesterol content is found in shrimp and egg yolk.
Fatty Acid composition
The fatty acid composition of a particular oil varies depending on the variety of the crop, growing conditions etc.
Lard and butter are rich in saturated fatty acids, whereas vegetable oils contain mostly unsaturated fatty acids, the exception being coconut oil (and cocoa butter, not shown). What is remarkable is that coconut oil is very rich in medium-chain fatty acids of chain length 8 to 12. Soybean oil is unique in that it is relatively rich in linolenic acid (C18:3) when compared to other common vegetable oils such as sunflower oil and peanut oil. Fish oil provides large quantities of fatty acids of chain length 20 to 22. Typical fatty acids present in fish oil are eicosapentaenoic acid (C20:5) and docosahexaenoic acid (C22:6). The oil that contains very high amounts of linolenic acid (approx. 50%) is flaxseed oil.
Lipid Digestion and Absorption
Before dietary fat can be taken up into our bodies, it needs to be digested. So here, I will discuss fat digestion or triglyceride digestion.
Digestion of dietary fat involves the breakdown of the triglyceride molecule into two fatty acid molecules and one mono-glyceride molecule. And a mono-glyceride molecule is glycerol attached to one fatty acid. Most of the dietary fat digestion takes place through the activity of an enzyme called pancreatic lipase, that is produced by the pancreas, and the remainder of fat digestion is accounted for by an enzyme produced by the stomach called gastric lipase.
The salivary glands also produce a lipase that is called lingual lipase. It is not believed to be quantitatively important for fat digestion but it’s believed to be important in oral fat detection or fat taste. And funnily it is the same enzyme as gastric lipase!
Fat digestion relies on the presence of a set of compounds that we call bile acids and these bile acids play an important role in fat digestion by functioning as emulsifiers. What it means is that they disperse the dietary fat into smaller fat droplets and thereby help the pancreatic lipase to gain easier access to the fat. These bile acids are produced in the liver from cholesterol and they’re stored together with cholesterol and phospholipids in the gallbladder.
When dietary fat enters into the small intestine, it causes the secretion of a hormone called cholecystokinin or CCK. And this hormone triggers the gallbladder to contract and release its content into the duodenum. The main bile acid that is released is called cholic acid and this cholic acid is normally attached to amino acids. The amino acids taurine or glycine to form taurocholate and glycocholate. These bile acids we call conjugated bile acids.
The fatty acids and mono-glycerides that are released upon digestion of dietary triglycerides from a special structure called micelle and bile acids are an important part of these micelles. And they facilitate their formation and thereby assist with fat absorption or fatty acid absorption. These fatty acids and the mono-glycerides that are part of the micelles are taken up by the enterocyte, by the intestinal cell. Within the enterocyte, they are reconverted into triglycerides via a process that we call re-esterification.
These triglycerides that are formed in the intestinal cell are packaged into specialised particles that we call chylomicrons. These chylomicrons carry the dietary fat throughout the body and are released into the lymphatic circulation into small lymphatic vessels called lacteals. Thus, they ultimately reach the blood circulation at the site of the subclavian vein.
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