With the expanding popularity of ketogenic and low carb diets leading people to embrace high-fat foods they may have avoided for many years, it’s useful to have a refresher on different types of fats, where they’re found, and their effects in the body.

Fat is not a homogeneous entity. Talking about “fat” as if it’s a single molecule is as misguided as talking about “protein” or “carbs,” when there are striking differences in animal and plant proteins, and dramatic differences in the metabolic effects of low-glycemic, high-fiber carbohydrates compared to refined sugars.

When we talk about “fats,” we’re really talking about fatty acids. When three fatty acids are joined to a glycerol molecule, a triglyceride is formed. All fats and oils consist of multiple types of fatty acids. (“Fat” is generally used to refer to fats that are solid at room temperature, like butter, lard, and tallow, while “oil” refers to those that are liquid. There are exceptions, such as when coconut oil solidifies at cooler temperatures in non-tropical climates.) To keep things simple, here we’ll use “fat” to refer to both fats and oils.

There are no fats or whole foods of either animal or plant origin that are entirely saturated or entirely unsaturated. Even lard, the mere mention of which might have you fear your arteries are clogging if you lived through the 1980s, is mostly monounsaturated fat. And of the monounsaturated fatty acids in lard, the one that predominates is 18-carbon oleic acid, the very same one responsible for the health halo conferred upon olive oil. Olive oil is nearly 14% saturated fatty acids, so anyone with lingering fears about the dangers of saturated fat might wish to steer clear of that artery-clogging Mediterranean diet!

Joking aside, in order to support patients using higher fat diets, it’s important to understand the nuances regarding dietary fat. So, before going any further, let’s differentiate between different types of fatty acids. All fatty acids have a similar chemical structure: they are chains of carbon atoms with attached hydrogen atoms. Within each category of fatty acid, there are multiple subtypes—that is, there are several different individual fatty acids that saturated, monounsaturated, and polyunsaturated, so even these three designations don’t tell us everything. Individual fatty acids of the same class—stearic acid and butyric acid, for example, which are both saturated, or arachidonic acid and gamma-linolenic acid, which are both polyunsaturated, may have different biochemical properties in the body.

Saturated: Saturated fatty acids have no double bonds between any of the carbon atoms. Every available binding site is occupied by a hydrogen atom—hence, the molecule is “saturated” with hydrogen. Double bonds cause kinks or bends in the fatty acid chain. With no double bonds, saturated fatty acids are straight lines, which allows them to pack together closely. For this reason, they are typically solid at room temperature. Examples of fats that contain a large proportion of saturated fatty acids include beef and lamb tallow, butter, lard, dairy fat, coconut oil and palm oil.

Medium chain fatty acids: These special fats are a subcategory of saturated fat and are commonly called medium chain triglycerides, or MCTs. They are abundant in coconut oil and palm kernel oil, the latter of which is primarily used in manufacturing candy and confections. The use of coconut as a source of MCTs underscores that saturated fat is not exclusively found in red meat or other fatty animal foods. The most highly saturated fat most people consume—especially on ketogenic diets—doesn’t come from animal foods at all, but from coconut oil, which is over 80% saturated, with some sources stating over 90% saturated. MCTs are metabolized differently than other fats and are more readily converted into ketones. For this reason, MCTs are being investigated for their potential to provide fuel to neurons in Alzheimer’s disease to compensate for impaired brain glucose use.

Monounsaturated: Monounsaturated fats have one double bond in the carbon chain (“mono” meaning one). Owing to the double bond, the fatty acids have a slight bend, which means they’re not able to pack together as tightly as saturated fatty acids. For this reason, monounsaturated fats are liquid at room temperature but tend to solidify when cold. (This accounts for why olive oil solidifies in the refrigerator, but be aware that the accuracy of the “fridge test” for verifying the quality of olive oil has been questioned.) Foods that contain predominantly monounsaturated fatty acids include olives and olive oil, avocados and their oil, canola oil, pork, poultry (chicken, duck, goose), almonds, hazelnuts, and cashews.

Polyunsaturated: The carbon chains in polyunsaturated fats have two or more double bonds (“poly” meaning many). With multiple double bonds, polyunsaturated fatty acids (PUFA) don’t pack together well, so oils high in PUFA remain liquid even when refrigerated. All foods contain at least some PUFA, but the biggest sources in the modern American diet are industrial seed and vegetable oils, such as corn, soybean, cottonseed, and safflower oils. Most nuts and seeds are higher in monounsaturated fats than in PUFA, but they still contain substantial amounts of PUFA, as does chicken fat.

Omega-6 & Omega-3: These are polyunsaturated fatty acids that are both “essential fatty acids.” A nutrient is considered “essential” when the human body does not synthesize it, and it must therefore be obtained from foods or supplements. Once provided with the required raw materials, the body can convert various omega-6 (n-6) and omega-3 (n-3) fats into others, but the “parent” fatty acids here—linoleic acid in the case of n-6, and alpha-linolenic acid in the case of n-3—must come from exogenous sources. The “omega” part of fatty acid nomenclature has to do with where the first double bond occurs in the fatty acid molecule. When it’s on the third carbon atom from the methyl end, it’s omega-3; when it’s on the sixth carbon atom, it’s omega-6. (There are also omega-7 and omega-9 fats, but they’re rarely called by these names. Omega-7 includes palmitoleic acid, found in macadamia nuts; oleic acid is an omega-9.)

Omega-6 fatty acids (especially linoleic acid) predominate in industrial seed and vegetable oils, with very small amounts found in animal foods. (Poultry fat and pork fat typically contain more n-6 than ruminant animal fat, such as beef, lamb, or bison.) Omega-3s are found in nuts and seeds (as alpha-linolenic acid), and in fatty cold-water fish, such as salmon, sardines, and mackerel (as EPA and DHA).

Double bonds make fatty acids chemically unstable. The more double bonds, the more susceptible a fat is to oxidation and rancidity upon exposure to heat, light, or air. This is why it’s best to keep fish oil and flaxseed oil refrigerated, and why no one uses these oils for cooking, aside from it being cost prohibitive! It also accounts for why nuts and seeds turn rancid more quickly when exposed to oxidizing elements and may stay fresh longer if refrigerated. Saturated fats, with no double bonds, are highly stable and not easily oxidized, which is why grandmothers of yesteryear could keep a metal coffee can filled with bacon grease on the counter for ages and it wouldn’t go bad.

Trans fatty acids: In most naturally occurring unsaturated fats, the double bonds are configured such that the hydrogen atoms are on the same side of the molecule (called cis configuration). The term “trans” fat denotes when the hydrogen atoms are on opposite sides of the molecule. This seemingly small difference has profound implications, because in the cis form, the molecule is bent, as we would expect in any unsaturated fatty acid. In the trans form, however, the molecule straightens out and adopts some of the properties of a saturated fat. (See here for an illustration.) In fact, partial hydrogenation—the process of adding hydrogen atoms to unsaturated fats—was seen as a boon to food manufacturers because trans fats increase shelf life and have a mouthfeel more like saturated fats, which make them appealing in processed foods. In the age of demonization of saturated fat—an era we increasingly look back upon with regret—industrially produced trans fats were seen as a perfect substitution for those terrible “artery-clogging” saturated fats. Unfortunately, trans fats came with their own unintended consequences (including increased risk for coronary heart disease and all-cause mortality), and in 2015, the US Food & Drug Administration (FDA) determined that partially hydrogenated oils are no longer generally recognized as safe, although food manufacturers have until January 2020 to completely remove them from their products.

To complicate matters further, there are naturally occurring trans fats—mostly in the fat and dairy fat of ruminant animals—that appear to have very different biochemical effects in the body compared to the industrially produced trans fats that come from hydrogenation of vegetable oils. Quite to the contrary of the industrial trans fats, the ruminant trans fats are not associated with increased risk for heart disease, and are inversely associated with risk for type 2 diabetes. Conjugated linoleic acid (CLA), a natural trans fat, has been shown to be beneficial for reducing inflammation, facilitating fat loss even independently of diet and exercise, and possibly inhibiting growth of various human cancer cell lines, including breast and colon.

Numerous studies and reviews have determined that intake of dairy fat, with its rich complement of saturated and natural trans fatty acids, is not associated with increased risk for CVD and may even be inversely associated with CVD and stroke risk. To be fair, findings have been somewhat mixed, which underscores the difficulty of teasing out associations, or lack thereof, between individual dietary elements rather than the whole foods they occur in, which may contain factors we’re not even aware of yet. Nevertheless, on the whole, intake of dairy fat appears to be inversely associated with risk for T2 diabetes and CVD.

It should be emphasized again that all foods contain combinations of multiple different fatty acids. We tend to think automatically of butter, cheese, bacon, and red meat when we think about saturated fat, but olive oil, poultry, seafood, avocado, and nuts & seeds also contain saturated fat, although it’s not the predominant type of fat in these foods. Similarly, avocado, nuts & seeds, olive oil, and seafood probably come to mind first when we hear “unsaturated fat,” but beef, pork, lamb, and other animal fats also contain mono- and polyunsaturated fats, including omega-3s.

Numerous meta analyses have concluded that, despite decades of popular belief to the contrary, science does not support a link between saturated fat intake and cardiovascular disease. Such analyses have been published in journals no less than the American Journal of Clinical Nutrition, so they’re not exactly sensationalist headlines on clickbait websites. Instead, increasing evidence suggests that excessive consumption of PUFA (the majority of which is in the form of vegetable oil) may be responsible for some of the deleterious effects that were once erroneously attributed to saturated fats. As one author stated, “the lack of any clear evidence that saturated fats are promoting any of the conditions that can be attributed to PUFA makes one wonder how saturated fats got such a bad reputation in the health literature.”

And since “saturated fat” is a heterogeneous category, there’s at least one particular type—stearic acid—that’s been shown repeatedly to reduce the total cholesterol:HDL ratio, lower LDL and ApoB, and to be non-thrombogenic. The Cleveland Clinic even endorsed dark chocolate as being good for the heart, owing in part to its stearic acid content.

If this seems overwhelming, you’re not alone. You don’t have to have it all mastered. Just have a general appreciation for the degree of nuance called for when talking about “fat,” or even “saturated” and “unsaturated” fats. It’s not as straightforward as we might like, but black and white thinking (this food good, that one bad) has gotten us into nutritional trouble in the past, which is still taking decades to undo. Individual patients and public health as a whole deserve better.