Here, we review the different classifications of dietary fat, their chemical structures and physical properties, and their nutritional profiles (according to the latest research). As part of this discussion, we will explore trans fat and what makes it so different from other fats, including an explanation of the hydrogenation process by which trans fat is artificially created. 

Classifications and Molecular Distinctions

Dietary fat can be broadly divided into two categories: animal-based and plant-based. Within those categories, fat is classified as either saturated or unsaturated. Unsaturated fats are further categorized as monounsaturated, polyunsaturated, or trans fats.^1,2 

The terms “saturated” and “unsaturated” refer to the chemical structure of the fat molecule. A fat molecule is composed of glycerol and fatty acids. The glycerol molecule is an alcohol containing three carbon atoms, five hydrogen atoms, and three hydroxyl groups (each hydroxyl group contains 1 oxygen atom and 1 hydrogen atom). Fatty acids are composed of long chains of carbon and hydrogen atoms, termed hydrocarbons, that end with a carboxyl group (the carboxyl group is what makes it an acid). To make a fat molecule, fatty acids bond to each of the three carbon atoms in the glycerol molecule by way of a reaction between the hydroxyl groups in the glycerol and carboxyl groups in the fatty acids. This results in a fat molecule with three fatty acid tails. Within those fatty acid tails, there can be single bonds or double bonds between the hydrogen and carbon atoms. Single bonds increase the number hydrogen atoms within the tails, “saturating” them with hydrogen. This is considered a saturated fat. When the tails contain double bonds, they are considered “unsaturated” because they contain fewer hydrogen atoms. If there is only one double bond within the fatty acid chain, it is known as a monounsaturated fat, and if there is more than one double bond within the fatty acid chain, it is known as polyunsaturated fat. The double bonds within a fatty acid chain can exist in either a cis or trans configuration. Cis means the two hydrogen atoms associated with the bond are on the same side of the chain, whereas trans means the two hydrogen atoms are on opposite sides of the chain.^1–3 

In saturated fats, the fatty acid tails are straight; this allows the molecules to lie tightly packed against each other, resulting in fat that remains solid at room temperature. In cis-unsaturated fat, the fatty acid tails are bent due to the cis double bond. This prevents the molecules from packing tightly against each other. As a result, these types of fats tend to remain liquid at room temperature. Trans unsaturated fats are a bit of an anomaly, in that they rarely exist in nature; they are, for the most part, made through an industrial process call hydrogenation. We’ll get to them a bit later in the article. All fats provide the same amount of energy: nine calories per gram. Dietary fats can be a blend of saturated, monounsaturated, and polyunsaturated fatty acids in different proportions.^1–3

Sources, Physical Characteristics, and Impact on Health

Saturated fat. Saturated fat is primarily sourced from animal-based foods, such as meat and dairy. However, some saturated fat comes from plants, (e.g., coconut oil, palm oil). Saturated fat typically remains solid at room temperature, but melting points vary. For example, coconut oil will melt into liquid at 77°F, but palm oil stays solid until 95°F. Animal fat, on the other hand, can usually withstand heat up to 108°F before melting.⁴ 

Health impact. Historically, we’ve been told that consuming too much saturated fat increases the risk of cardiovascular diseases. More recently, however, conflicting data have been emerging, causing contention among the medical community, including physicians, nutrition experts, and members of medical societies and United States (US) governmental entities responsible for creating dietary recommendations for the American people.⁵ Over the last decade or so, several meta-analyses have been conducted examining the association between saturated fat intake and risk of cardiovascular disease and other comorbidities—some reporting no association between saturated fat intake and all-cause mortality, chronic heart disease/cardiovascular disease mortality, total chronic heart disease, ischemic stroke, or Type 2 diabetes, and others reporting that there is indeed an association between saturated fat intake and incidence of cardiovascular disease.^6,7 This conflicting information is causing a bit of confusion among American consumers. Should saturated fat be avoided altogether? Should we limit its consumption? How much is too much? The jury is still out. The American Heart Association recommends that no more than 5 to 6 percent of your daily calories should come from saturated fat. For a 2,000-calorie-a-day diet, that is around 120 calories.⁸ The US Department of Agriculture (USDA) Dietary Guidelines for Americans, 2020 to 2025, recommends that up to nine percent (<10%) of daily calories can come from saturated fat (for a 2,000-cal/day diet, this would be 180 calories or less).⁹ A reasonable inference might be that 5 to less than 10 percent of daily calories coming from saturated fat is a healthy range; however, consulting with a licensed nutritionist, dietitian, or physician to determine how much is right for you would probably be prudent.

Monounsaturated fat. Monounsaturated fat comes from plant-based sources—olive oil, canola oil, avocados, nuts, and seeds all contain monounsaturated fats. This type of fat typically remains liquid at room temperature.⁴

Health impact. While there is a lot of debate regarding whether saturated fat contributes cardiovascular disease and other comorbidities, the research regarding the health benefits of monosaturated fat is pretty solid. Monounsaturated fat is considered heart-healthy and has been shown to reduce low-density lipoprotein (LDL) cholesterol levels while maintaining or increasing HDL (high-density lipoprotein) cholesterol levels.^10,11 Research has also shown that diets high in monounsaturated fat can help with losing weight, compared to low-fat, high-carbohydrate diets, and with reducing blood pressure and inflammation. It is important to note that these benefits were seen when monounsaturated fat was included in calorie-controlled, heart-healthy diets in which calories from monounsaturated fat replaced calories from saturated fat and/or carbohydrates. Monounsaturated fat also contains nutrients that are essential to producing and maintaining cells, such as vitamin E.^12–18

Polyunsaturated fat. Polyunsaturated fat can be found in both plant- and animal-based food sources, including soybean, corn, and sunflower oils and fatty fish (e.g., salmon, mackerel). This type of fat is liquid at room temperature. The two main classes of polyunsaturated fat are omega-3 fatty acids and omega-6 fatty acids.

Health impact. Omega-3 and omega-6 fatty acids, which are essential for the functioning of the body’s cells, cannot be made by the body and thus must be obtained through diet.¹⁹ Omega-3 fatty acid, which can be found in both animal- and plant-based foods, is known for reducing inflammation, supporting heart health, and promoting brain function.^20,21 Omega-6 fatty acid is primarily found in plant-based foods. While omega-6 is considered an essential fat, some research indicates that high dietary intake of omega-6 fatty acid contributes to inflammation and obesity; however, other studies indicate that these data are inconclusive. Overall, research suggests that maintaining a balanced ratio of omega-3 and omega-6 fatty acids in the diet provides anti-inflammatory benefits.^22–27

Trans fatty acid. There are two types of trans fats: naturally occurring ruminant trans fatty acid and industrially produced trans fatty acid. Ruminant trans fatty acid forms naturally when bacteria in the stomach of ruminant animals, such as cattle, sheep, and goats, digest grass. A very small amount of ruminant trans fatty acid is present in fats derived from these animals. Industrially produced trans fatty acid is created through a chemical process called hydrogenation, which is commonly employed in the food industry to produce fats with desirable properties for various applications, such as improving the texture and shelf life of food products.^28,29 

Hydrogenation is a chemical process used to convert liquid (unsaturated) vegetable oils into semi-solid or solid fats (saturated). As discussed previously, unsaturated fats contain double bonds of carbon atoms within the carbon-hydrogen fatty chain. During the hydrogenation process, hydrogen is added to a vegetable oil that has been heated to a very high temperature, along with a metal catalyst (typically nickel). This causes a chemical reaction in which the double bonds of carbon atoms are broken and each carbon atom becomes single-bonded to an individual hydrogen atom. The hydrogenation process can be either partial or complete. In partial hydrogenation, some but not all of the double bonds are broken with hydrogen atoms. In complete hydrogenation, all of the double bonds are broken with hydrogen atoms. The degree of hydrogenation is controlled by the amount of hydrogen added, the reaction temperature and time, and the type of catalyst used during the process. A side reaction of this process changes the geometric pattern of the fatty acid chain, in that instead of the hydrogen atoms being on the same side of the chain, as seen in cis-unsaturated fats, they are on opposite sides of the chain. It is this molecular structure that makes trans fats so unhealthy.^28,29

Health impact. The enzymes the human body uses to breakdown and utilize unsaturated fats are adapted to the cis configuration. When industrially produced trans fat enters the digestive process, the fat-metabolizing enzymes our bodies have used for thousands of years to process dietary fats are not able to properly process trans fat, due to its unnatural molecular structure, which makes it easier to trans fat to build up in the body. As a result, a diet high in trans fat can result in a number of health problems, including heart disease, stroke, and Type 2 diabetes, to name a few.^30–32

In Conclusion

Balancing the intake of different types of dietary fats is essential for overall health. It’s important to focus on sources of healthy fats, such as those found in fish, nuts, seeds, and plants, while minimizing the consumption of trans fats, which may be present in highly processed foods. Consult with a qualified healthcare professional to discuss how to incorporate a healthy balance of fat into your diet.

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