Merck Manual

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Overview of Fats
Overview of Fats
Overview of Fats

    Fats are an essential part of a healthy diet. They contribute to the taste and texture of foods, like the smoothness of guacamole and the flakiness of a croissant. Fats are also a major source of energy, a critical component of cells and tissues, help absorb essential vitamins, and can be converted into other molecules like prostaglandins which help cells communicate with one another.

    Fats have a three carbon backbone called glycerol, as well as fatty acid chains. The fatty acid chain is basically a string of carbon and hydrogen atoms. When an “OH” from the glycerol molecule binds to an “H” from the fatty acid, an “H20” - a water molecule - gets released, and the two molecules link up. If this happens once, the result is a monoglyceride, if it happens twice it’s a diglyceride, and three times makes a triglyceride.

    Now, there are various types of fatty acid chains, and one way to categorize them is by their length, in other words, how many carbons they have. Short chain fatty acids have 2 to 5 carbons, medium chain fatty acids have 6 to 12 carbons, and long chain fatty acids have more than 13 carbons.

    Fatty acid chains are also categorized by the bonds connecting the carbons in the chain. A single bond is just one bond between the carbon atoms, and when a fatty acid chain has only single bonds, it’s called a saturated fatty acid - because it has as many hydrogen atoms as possible - it’s saturated with them. Triglycerides with saturated fatty acids are nice and straight so they pack together really well, and as a result they’re usually solid at room temperature. And the longer the saturated fatty acid chain, the more likely it will be solid at room temperature.

    Carbons can also have double bonds between them, and when a fatty acid has one or more double bonds, it’s called an unsaturated fatty acid because it’s not saturated with hydrogen atoms - for every double bond there are two fewer hydrogen atoms. A double bond causes a kink in the molecule so the triglycerides don’t pack together as nicely as saturated fats. As a result, unsaturated fats are usually liquid at room temperature.

    Unsaturated fatty acids can be further classified, according to the number of their double bonds. Monounsaturated Fatty acids are fatty acids with only a single double bond. Polyunsaturated fatty acids have more than one double bond. Also, they can be classified according to their location as well, since these hydrogens can get kinda crazy, we’ll take them away for now. So, we also call the methyl end the omega end, and and then we count the number of carbons until the first double bond. Since this one’s three, it would be an omega-3 fatty acid. If the double bond is 6 carbons from the end, it’s omega-6, and if it’s 9 carbons from the end, it’s called omega-9.

    Now, to make things even easier, I’m just going to show the bonds. Alright, so omega 3’s are usually polyunsaturated fatty acids, and include alpha-linolenic acid, or ALA eicosapentaenoic acid, or EPA, docosahexaenoic acid, or DHA, and EPA and DHA are marine sources of omega-3’s. They’re produced by microalgae, and end up in the tissues of fish like anchovies, mackerel, salmon, and sardines. ALA is found in plants like flaxseed, walnuts, and canola and soybean oils. Our bodies can convert ALA into EPA and DHA, but it’s an inefficient process that yields only small quantities, and that’s why dietary recommendations include foods that have EPA and DHA.

    Omega-6 polyunsaturated fatty acids are also usually polyunsaturated, and include and linoleic acid and arachidonic. Linoleic acid is found in oils like safflower, corn, and soybean oils. Arachidonic acid is found in animal sources like fish, meat, and eggs. Our bodies can convert linoleic acid into arachidonic acid, but once again the process is inefficient. Because ALA and linoleic acid can only be obtained in the diet, they are considered essential fatty acids. Omega-9 fatty acids are typically monounsaturated fatty acids, and an example would be Oleic acid, these can be made by the human body. Foods like canola and olive oil, as well as almonds contain omega-9s.

    Now, looking at the double bond of this unsaturated fatty acid, like most unsaturated fats, it’s got a cis configuration. In a cis configuration, the two functional groups are on the same side of the double-bonded carbons. Now when this happens, the fatty acid chain naturally bends. A molecule that bends doesn’t pack tightly together, so it’s a lot more fluid - think about cooking oils, which are liquid at room temperature.

    Some fats are in a trans configuration. In a trans configuration, the functional groups are on opposite sides of the double-bonded carbons. This keeps the chain more straight and easier to pack. Trans fats result from a process called partial hydrogenation.

    In just hydrogenation, hydrogens are added to cis-fats to get rid all the double bonds double bond, so let’s say you’ve got this triglyceride, it has a total of two double bonds, so we’d add 4 hydrogens, 2 for each double bond. That turns the unsaturated fatty acids with cis double bonds into saturated fatty acids! Partial hydrogenation, on the other hand, refers to adding hydrogens to most but not all double bonds. Let’s say now we just add 2 hydrogens. When this happens, some double bonds can be turned into single bonds by the hydrogens, but then reform, and what you end up with is an unsaturated fat but some that have trans double bonds.

    Partial hydrogenation is a process that occurs naturally in the digestive tract of some animals like cows and pigs, which is why trans fats can be found naturally in meat and dairy products. Partial hydrogenation is also sometimes used in the food industry, in the processing of liquid oils to make them more solid, which can lead to the creation of Trans fats. Partially hydrogenated oils have been largely removed from foods in North America and Europe because trans fats have been associated with coronary heart disease.

    Although some foods may have more of one type of fat than another, the truth is that all foods are made up of a blend of fatty acids. Now when you eat a food like peanut butter, which has about 75% of its calories from fat, the body goes through a set of steps to digest and absorb the fatty acids. First of all, triglycerides are hydrophobic. So they form large globules of fat - like what you see when you pour oil into water. Enzymes called lipases in the saliva, stomach, and secreted by the pancreas can break down triglycerides into free fatty acids and monoglycerides. But working on the surface of a globule is inefficient, so to speed things up, bile salts produced by the liver, break the large fat droplet into smaller droplets which increases the surface area for the lipases to work.

    Once the triglycerides are broken down into monoglycerides and free fatty acids, these self-assemble into mixed micelles which have a hydrophobic interior and hydrophilic or water loving exterior. The micelles glide through the watery environment of the intestinal lumen, and reach the enterocytes in the intestinal wall. When they get to the enterocytes, the micelles release the fatty acids and monoglycerides, which diffuse into the enterocyte. Inside the enterocyte, the fatty acids and monoglycerides reassemble into triglycerides, and these get packed into a larger structure called a chylomicron.

    The chylomicron has lipids and proteins - so it’s a lipoprotein. It has an outer membrane with phospholipids and proteins, and a hydrophobic core that contains triglycerides, cholesterol, and fat-soluble vitamins A, D, E and K. The chylomicron then leaves the enterocyte. It’s too large to get into the endothelial cells, so instead it enters a nearby lymphatic capillary called a lacteal.

    From there, the chylomicron floats in the lymph and flows into the thoracic duct, and then gets dumped into the blood - essentially bypassing the portal vein. Once in the blood, the chylomicron releases fatty acids and monoglycerides in peripheral tissues like muscle which use them for energy, and adipose tissue which can store them. After delivering the triglycerides, the chylomicron shrinks in size and eventually gets engulfed by the liver.

    Fats play a very important role throughout the body. They have a great number of health benefits and those benefits can vary by the type of fat we eat. For example, polyunsaturated fats are precursors for hormone-like molecules called prostaglandins that stimulate endothelial cells that line blood vessels to release nitric oxide. Nitric oxide is a vasodilator, so that decreases resistance to blood flow and, in turn, lowers blood pressure.

    Polyunsaturated fatty acids also help reduce the total and LDL cholesterol - and that’s linked to lower rates of cardiovascular diseases like heart attacks and strokes. Long-chain omega-3 fatty acids like DHA and EPA both help to lower plasma triglyceride levels which also protects against cardiovascular disease. DHA is important in the development of eyes and brains of young infants.

    The impact of saturated fat on cardiovascular health is more complex. Generally speaking, it’s recommended to keep consumption of saturated fat low, but just like with unsaturated fats there are different types of saturated fats. Evidence suggests that different types of saturated fatty acids may have different effects on our cardiovascular health. Evidence also suggests that the health impact of reducing or replacing saturated fat in the diet depends on the nutrient that replaces it. For example, replacing saturated fat with polyunsaturated fat has been shown to benefit cardiovascular health, whereas replacing saturated fat with refined carbohydrate has not. Based on this, the National Academies of Medicine recommends consuming 20-35% of our daily calories as fats - for a 2000 daily calorie intake, that’s between 400 and 700 calories from fat, or about 44 to 78 grams. And it’s not just about the amount of fat we eat, the type also matters. The World Health Organization and the US Dietary Guidelines recommend that less than 10% of daily calories come from saturated fats and that trans fat consumption should be kept as low as possible.

    All right, as a quick recap: Fats are an essential part of our diet and health. Fatty acids are either saturated or unsaturated, and foods with fat always contain a mixture of fatty acid types. Healthy diets that emphasize mono and polyunsaturated fats over saturated, and trans fats are associated with lower cardiovascular disease risk. Improvements to your diet and health can be achieved by focusing more on the type of fat we eat and less on the amount.

Video credit: Osmosis (https://osmosis.org/)