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, and a critical component of cells and tissues. And they also help absorb essential vitamins and can be converted into other molecules like prostaglandins, which help cells communicate with each other. Fats have a 3-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 group from the glycerol molecule binds to a hydrogen from the fatty acid, an H2O, or 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 13 or more 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, or 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 though. 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 2 fewer hydrogen atoms. Also, a double bond causes a kink in the molecule so that unsaturated fats 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 unsaturated fatty acids with just 1 double bond. Polyunsaturated fatty acids have 2 or more double bonds.

    Also, they can be classified according to their location as well. Since all these hydrogens can get kind of crazy looking, we'll just take them away for now. So another name for the methyl end is the omega end, and then we can count the number of carbons until the first double bond. Since this one is 3, it would be an omega-3 fatty acid. If the double bond is 6 carbons from the end, it's an omega-6. And if it's 9 carbons from the end, it's called omega-9.

    Now, to make things even easier when looking at these molecules, I'm just going to show the bonds. Alright, so omega-3s are usually polyunsaturated fatty acids and include alpha-linolenic acid or ALA, eicosapentaenoic acid or EPA, and docosahexaenoic acid or DHA. EPA and DHA are marine sources of omega-3s. 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 flax seed, 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 fatty acids are also usually polyunsaturated and include linoleic acid and arachidonic acid. 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, and 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 2 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 does not 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, though. In a trans configuration, the functional groups are on opposite sides of the double-bonded carbons, and this keeps the chain straighter and easier to pack.

    Trans fats result from a process called partial hydrogenation. In just plain old hydrogenation, hydrogens are added to cis fats to get rid of all of the double bonds. So let's say that you've got this triglyceride that has a total of 2 double bonds. So in this case 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. So let's say now that we add just 2 hydrogens. When this happens, some double bonds can be turned into single bonds by the hydrogens, but then they might reform. And what you end up with is an unsaturated fat. But some of those fats have trans double bonds. Partial hydrogenation is a process that happens 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 actually 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 might have more of one type of fat than another, the truth is that all foods are made up of a blend of fatty acids. 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 in water. Enzymes, called lipases, in the saliva and 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 a 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 has triglycerides, cholesterol, and fat soluble vitamins A, D, E and K. The chylomicron then leaves the enterocyte, but 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, that chylomicron releases fatty acids and monoglycerides in peripheral tissues like muscle, which use them for energy, as well as adipose tissue, which can store them. After delivering the triglycerides, the chylomicron shrinks in size and eventually gets engulfed by the liver.

    Now, fats play a super important role throughout the body. They have a huge 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 can also help reduce the total and LDL cholesterol, and that's linked to lower rates of cardiovascular diseases like heart attacks and stroke. Long chain omega-3 fatty acids, like DHA and EPA, can both help to lower plasma triglyceride levels, which also protects against cardiovascular disease. Finally, DHA is important in the development of eyes and brains of young infants.

    The impact of saturated fat on cardiovascular health is a little 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 might 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 to 35% of our daily calories as fat for a 2000 daily calorie intake. That comes out to between 400 and 700 calories from fat, or about 44 to 78 grams. And it's not just about the amount of fat that 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.

    Alright, 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 emphasize mono- and polyunsaturated fats over saturated fats, and trans fats are associated with an increased risk of cardiovascular disease. Improvements to our diet and health can be made by focusing more on the type of fat we eat and less on the amount.

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