Fat has long been a topic of debate in the nutrition world. The low-fat craze of the 80s and 90s created a generation fearful of fat but also caused rates of diabetes and obesity to rise drastically [1, 2]. While we’ve moved away from the low-fat diet as mainstream health advice, today there is much debate about saturated fat and its impact on metabolic health outcomes, such as chronic disease, insulin resistance, and weight.
So what to do? How do you know what type of fat is good for you, and how much you should be getting in your diet? Will fat make you gain weight? What about saturated and trans fats? In this article, we’ll bring clarity to your questions about fat and help you make informed choices about dietary fat to best serve your metabolic health.
What are the functions of fat in the body?
Fat is essential to several processes in your body. Some of the ways fat is used to support metabolic health include:
- Provides the body with energy. With 9kcal per gram, fat is a rich source of energy for the body and is more energy-dense than protein or carbohydrates.
- Energy storage. Extra energy can be stored in fat cells, and there’s research to support the idea that fat cell signaling can help to maintain energy balance in the body by releasing hormones to influence food intake and insulin action .
- Supports cell structure and function. Dietary fat helps to create and maintain the membranes of your cells, ensuring permeability for molecules to move in and out of the cells as appropriate, which is crucial for metabolic processes [4, 5].
- Provides insulation for body temperature regulation and protection for organs .
- Blunts glucose spikes. Fat has little effect on blood sugar on its own, and when paired with carbs, fat can help slow the absorption of glucose into the bloodstream, blunting a potential glucose spike.
- Vitamin absorption. Vitamins A, E, D, and K are fat-soluble vitamins, meaning that in order to be absorbed and utilized, fat must be available in the stomach.
Fat is important for maintaining all types of processes in the body, and getting adequate amounts of dietary fat is key to your metabolic health. However, different types of fat may affect health differently.
What are the types of dietary fat?
A lot of the confusion about fat is around the different types of fat and how they impact health. There are three types of dietary fat: saturated fats, unsaturated fats, and trans fats. It’s worth noting that most foods high in fat have a mix of both saturated and unsaturated fat, they just may have much more of one than the other.
Saturated fats are found in animal products like red meat, butter, eggs, pork, coconut oil, palm oil and full-fat dairy.
Saturated fat is one of the most hotly debated topics in the nutrition world. For some time, saturated fat was referred to as the “bad fat,” as a large body of research had linked it to poor health outcomes, like type 2 diabetes, heart disease, high cholesterol, and inflammation [7, 8, 9].
However, the view of saturated fat began to shift when a few compelling studies concluded that saturated fat was not associated, or only weakly associated, with an increased risk of cardiovascular disease [10, 11]. These studies led journalists, influencers, and proponents of the high-fat/low-carb diet to declare that saturated fat was no longer a threat to health and that limiting saturated fat will not limit disease risk.
However, a lot of nuance is lost here when we take a binary approach, stating that saturated fat is either good or bad. It is worth noting that the studies declaring saturated fat has no association with CVD risk had several methodological problems and major limitations, including the quality of the studies they evaluated, the variables they controlled for, and the issue of the single nutrient hypothesis. It’s also important to consider what replacing saturated fat with another type of fat — like mono- or polyunsaturated fat — does to disease risk, which these studies did not address.
Several studies have shown that when saturated fat is replaced by polyunsaturated or monounsaturated fat, the risk of cardiovascular disease decreases [12, 13, 14]. One study in particular concluded that for every 5% of calories saturated fat was replaced with the same amount of calories from polyunsaturated fat, the risk of CVD decreased by 10% .
This brings us to the “single nutrient hypothesis,” which refers to the isolation of nutrients for the purpose of research and attempting to establish cause and effect relationships. However, people do not make food choices in a vacuum. In other words, foods contain multiple macronutrients, and it’s nearly impossible to eat one single nutrient, absent of any other vitamins, minerals, or macros, at one time. What we are lacking in this discussion is a clear understanding of how fat, as a component of a dietary pattern, affects human health.
Thankfully, there is research to support the idea that healthy diets that are high in fat can support cardiac health. For example, research supports that a Mediterranean dietary pattern, which includes fat in the form of nuts, seeds, olive oil, fish, and meat, can reduce the risk of cardiac disease . There are many trials that include dietary patterns with saturated fat but are highest in monounsaturated and polyunsaturated fats showing a cardioprotective effect (14, 15, 16).
Another study looking at 13-year outcomes of 26,000 people and the incidence of cancer found that cancer was higher in people with low vegetable intake and high red meat intake, but in the group with high vegetable intake and high red meat intake, cancer incidences were low .
This suggests that the diet and what you eat most of the time alongside your saturated fat sources matters. In other words, it’s important to look at the whole diet rather than just a single nutrient when discussing health outcomes.
So what’s the takeaway when it comes to saturated fat?
- Saturated fat likely isn’t as harmful as we originally thought. But that doesn’t mean we should be eating only saturated fat all the time. If you eat animal products, you don’t need to go out of your way to add saturated fat to your diet.
- Studies consistently show that replacing saturated fat with poly- and monounsaturated fats reduces several markers and risk factors of cardiometabolic health
- It is important that we look at our diets on the whole, rather than single nutrients. Aim to eat a diverse diet that is high in fiber via colorful vegetables, whole grains, berries, nuts, and seeds.
- If you do eat red meat, look for lean cuts that are minimally processed.
Unsaturated fats are found in foods like plant oils, nuts, avocados, and seeds. Both monounsaturated and polyunsaturated fats fall into this category, and are commonly discussed types of unsaturated fats. Both are named because of the differences in their chemical structure, differentiated by the number of double bonds they contain .
Both mono- and polyunsaturated fats have been shown to have favorable effects on health outcomes when they replace saturated fats in the diet, and replacing saturated fats with polyunsaturated fats may be even more impactful for preserving cardiovascular and metabolic health [19, 20, 21].
Omega-3 and omega-6 fatty acids are two types of polyunsaturated fats that are essential to human health. This means that they cannot be synthesized in the body via other metabolites, but rather need to be eaten in sufficient amounts to meet your body’s needs. Omega-3 fatty acids are found in fatty fish, like mackerel, salmon, and tuna, and have been seen to be associated with a lower incidence of cardiac disease [22, 23]. This equates to about 2 servings per week of fish to hit Omega-3 targets for health.
Omega-6 fatty acids have gotten a bad rap, with claims that high omega-6 consumption may be harmful to health and cause inflammation. However, the research does not support this claim. High-quality evidence from randomized controlled trials does not support the notion that high omega-6 consumption causes heart disease, nor does it increase inflammation [24, 25, 26]. However, it is true that most people eating a Westernized diet do not get enough omega-3s, so instead of trying to cut omega-6 out of your diet, aim to improve the amount of omega-3 you get to better balance your intake of both types of essential fat.
Trans fats are widely accepted to be unhealthy fats. They can occur naturally or can be created artificially. Naturally occurring trans fats can be found in trace amounts in animal products, but are not a cause for concern in such small quantities. Artificial, or man-made trans fats, however, are highly correlated with increased LDL-cholesterol, lowering healthy cholesterol, impaired glucose tolerance, development of type 2 diabetes, and heart disease [27, 28, 29].
Man-made trans fats occur when hydrogen is added to vegetable oil, creating a solid. These solids are “partially hydrogenated,” and historically, we have seen partially hydrogenated oils used in products like margarine, pastries, potato chips, fried foods, frozen pizzas, and other highly processed snack foods.
Since 2020, trans fats have been banned from use in food manufacturing in the United States, and since 2021, the European Union has limited the use of trans fats in foods to less than 2g per 100g fat [30, 31]. However, not all countries have banned or limited trans fats, and there may still be traces of trans fats in highly processed foods.
The takeaway here is that the banning of trans fats has greatly limited trans fat intake, and we don’t need to worry too much about trans fats in our diet. However, a diet high in processed red meat and highly processed snack foods may contain some trans fats, and should be avoided.
Does eating fat lead to weight gain?
When we talk about body fat, there are two types we often refer to:
- Subcutaneous fat, or the fat under our skin. This fat is found all over our bodies and helps insulate and protect us. But it can accumulate in excess in the upper arms, legs, and bellies, with excess subcutaneous belly fat being especially harmful to health .
- Visceral fat or the fat that surrounds our organs. This fat is important and protective to our internal organs, but, like subcutaneous fat, in excess, it can lead to health complications, like insulin resistance, high cholesterol, metabolic syndrome, type 2 diabetes, and heart disease [33, 34].
Weight gain can increase both of these types of fat in the body, sometimes to an unhealthy level. As we’ve mentioned, dietary fat is higher in calories per gram than both protein and carbohydrates. It is a rich source of energy, but because of its high caloric content, many people tend to think that dietary fat leads to weight gain.
Calories we eat are certainly part of the equation when it comes to weight gain or loss, however, satiety, body composition, and source of calories are also key components of maintaining a healthy weight.
Foods that are highly processed often are high in fat and sugar, which targets our brain chemistry and triggers us to eat more. These foods, like chips, candy, and pastries, are not filling, so even though they’re high in calories, we don’t feel satisfied. Shortly after eating them, we feel hungry again. When diets are high in highly-processed high-fat foods, this is where we can see unwanted weight gain.
However, foods high in healthy fats like nuts, seeds, avocado, oils, and eggs are highly sating, meaning that they keep you feeling full and satisfied . When these fats are paired with lean protein and fiber, you’ll feel fuller for longer, and are less likely to eat excess calories.
The takeaway here is that fat itself does not cause weight gain, but rather, poor sources of fat from highly processed foods can cause weight gain over time. Highly processed foods should be avoided to avoid unwanted weight gain, improve satiety, and protect metabolic health.
Takeaways: how does fat affect metabolic health?
As a crucial dietary macronutrient, fat affects your health in a number of ways. Fats can and should be incorporated into your diet, and there’s many ways to do that healthfully. Here are the takeaways from what we’ve discussed:
- Dietary fat has the potential to both support or harm our metabolic health. Eating a diet high in fiber, complex carbs, and healthy fats can protect heart health, decrease the risk of chronic conditions, and help to maintain a healthy weight.
- We know that consuming highly processed foods, man-made trans fats, and highly processed animal products may increase obesity and the risk of chronic conditions. Obesity is highly correlated with the development of insulin resistance, metabolic syndrome, type 2 diabetes, and inflammation.
- Body fat is essential to maintaining health, but excess body fat can lead to poor metabolic health, even if you’re not obese. For example, a body composition that is low in lean muscle mass and high in fat mass, this may make you susceptible to poor metabolic health.
- A diverse diet high in fiber, lean protein, and poly- and monounsaturated fats, as well as maintaining a body composition that is high in lean muscle and low in fat mass can help you achieve your best metabolic health.