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What is dietary fat?

A photo of healthy fats, including oily fish, avocados and olive oil
(Image credit: Getty Images)

Dietary fat is one of the three macronutrients that make up the bulk of our diets. Alongside protein and carbohydrates, they are vitally important for our health and the proper function of our bodies. 

Fat has long been demonised as the cause of many health problems, and the source of the obesity epidemic in America. Whilst there may be an element of truth to this, dietary fat comes in many different forms, some of which can be beneficial for our health and some of which can be detrimental. Check our article on the unsaturated fats vs saturated fats to understand the major differences between these groups. With clearer dietary guidelines, we can hopefully develop a more rounded view of dietary fat and the role it plays in our bodies. 

Dietary fat also plays a key role in low-carb, fat-heavy diets such as the keto diet. So what is dietary fat and why do we need it? Read on to discover more. 

Why do we need fat in our diets?

All fats – saturated or unsaturated – provide nine calories per gram, but the quality, health benefits and risks vary hugely between types of dietary fat. 

Laura Clark (opens in new tab), registered dietician and nutrition consultant, told Live Science: “Fat provides us with essential fatty acids which we cannot make in the body. It also enables us to store the fat soluble vitamins A, D, E and K.”

Dietary fat also helps keep hair and skin healthy, insulates the body, protects organs and fills fat cells. Essential fatty acids in particular contribute to brain development, blood clotting and inflammatory control.

Types of dietary fat

Saturated fats

Saturated fats are mainly found in animal sources such as meat or dairy, although some plant sources such as coconut oil can also contain saturated fats. A saturated fat is made up of chains of carbon atoms that are all bonded with hydrogen.

“Saturated fats are solid at room temperature,” said Ximena Jimenez, a Miami-based nutritionist and national spokesperson for the Academy of Nutrition and Dietetics. She cited lard as an example. Other examples include: 

  • Butter
  • Ghee
  • Coconut oil
  • Cured meats
  • Sausages
  • Bacon
  • Cheese

Overconsumption of saturated fats is one of the leading cause of obesity and related conditions in adults, according to a study in the International Journal of Molecular Sciences (opens in new tab).

If you’re looking for a diet low in saturated fats, you can try the Mediterranean diet, which focuses on lean meat and unsaturated fats to promote good health. 

woman making granola with coconut oil

(Image credit: Getty Images)

Trans fats (also called trans fatty acids)

Trans fatty acids are usually caused by heating liquid vegetable oils with hydrogen in a process called hydrogenation. The process of hydrogenation binds extra hydrogen atoms to empty spaces along chains of carbon atoms. Unsaturated fats naturally have spaces along the chain of carbon atoms, instead of being full or ‘saturated’ with hydrogen atoms, and hydrogenation fills these spaces. This turns the liquid unsaturated fat into a stable solid by saturating it with hydrogen.   Fast food restaurants often use them in deep fryers because partially hydrogenated oil does not have to be changed as often as regular oil.

Trans fats include margarine, shortening and olive oil spreads. They can also be found in fast food and baked goods.

Whilst hydrogenation makes fats more stable and less likely to go rancid, trans fats are the worst fats for us. In fact, trans fats are not recommended at all because of the link to heart disease. This is because they raise ‘bad’ LDL cholesterol and lower ‘good’ HDL cholesterol. 

Research in the American Journal of Clinical Nutrition (opens in new tab) supports the phasing out of trans fats over time and returning to fats in their unprocessed form, such as olive oil, in order to reduce inflammation. Inflammation is a known risk factor for the development of cancer and chronic diseases.


Triglycerides are a type of fat found in the blood which our bodies use for energy or store for use later. When we eat, our bodies convert calories that they don’t need right away into triglycerides, which they then store in fat cells to be released for energy between meals.

Poor diet and inactivity have a knock-on impact on blood triglyceride levels, with a study in the American Journal of Cardiology indicating that high blood triglyceride levels are a biomarker of an increased risk of cardiovascular disease. To keep blood triglyceride levels low, you should aim to get at least 30 minutes of cardiovascular exercise a day and try to eat a balanced diet. 

The Mayo Clinic provides the following guidelines for healthy triglyceride levels:

  • Normal: Less than 150 milligrams per deciliter (mg/dL), or less than 1.7 millimoles per liter (mmol/L)
  • Borderline high: 150 to 199 mg/dL (1.8 to 2.2 mmol/L)
  • High: 200 to 499 mg/dL (2.3 to 5.6 mmol/L)
  • Very high: 500 mg/dL or above (5.7 mmol/L or above)

A blood test can reveal your triglyceride levels.

Monounsaturated fats

Monounsaturated fatty acids are generally liquid at room temperature and contain unsaturated double-bonded carbon chains. These fats can promote ‘good’ HDL cholesterol levels in your body and reduce levels of ‘bad’ LDL. It is important to strike a balance between HDL and LDL to maintain good health and prevent the accumulation of LDL in veins and arteries. 

Examples include:

  • Olive oil
  • Olives
  • Peanut oil and canola oil
  • Avocados
  • Pumpkin seeds
  • Pecan nuts 

A study in Nutrients (opens in new tab) found that consumption of monounsaturated fatty acids leads to positive cardiometabolic outcomes. The promotion of HDL cholesterol may also lead to a reduction of inflammation in the body, making it potentially cardioprotective, according to a study in the Annals of the New York Academy of Sciences (opens in new tab).

examples of monounsaturated fat

(Image credit: Getty Images)

Polyunsaturated fats

Polyunsaturated fatty acids contain more than one double carbon bond in their molecular chain, with empty spaces for hydrogen along the chain making them unsaturated. Polyunsaturated fats include omega-3 and omega-6 fatty acids, which are found in a variety of animal and plant-based sources. Our bodies can’t make omega-3 or omega-6, so it is important to consume enough of both fatty acids from dietary sources. 

Polyunsaturated fats are found mostly in plant food sources, such as soybeans and soybean oil, sunflower oil, sunflower seeds, walnuts and flaxseeds. They’re also present in fatty fish like salmon, tuna, herring, mackerel and trout. 

Both omega-3 and omega-6 need to be consumed in balance with one another, according to a study in Biomedicine & Pharmacotherapy (opens in new tab), as high levels of omega-6 can contribute to the development or worsening of cardiovascular diseases, cancer and inflammatory diseases. 

Clark adds: “Omega-3 supplementation is recommended if you don't consume oily fish once or twice a week. It's the oily fish in our diets that provide what we call long chain fatty acids which are really important for our cognitive function as 40% of our brain cell walls are made up of them. ” 

How much fat should you eat?

For those aged two or older, the USDA guidance (opens in new tab) for 2020-2025 recommends that less than than 10% of your daily calories should come from saturated fat. Unfortunately an estimated 70% of Americans are consuming more than this, which has led to an increase in health problems such as cardiovascular disease and obesity.  

Clark tells us: “Public health guidance suggests we have no more than 35% of our energy from fat, which equates to 95g for the average man and 70g for the average woman. We also have more specific guidance around saturated fat as this is the type of fat which links to raised cholesterol levels and heart disease. We should have no more than 10% of our energy coming from saturated fat which equates to 30g for men and 20g for women.”

Yet eliminating fat too much can have serious health consequences, says Jennifer Fitzgibbon, a registered oncology dietitian at Stony Brook Hospital Cancer Center (opens in new tab) in New York. "Mental health deficits like depression and vitamin deficiencies can occur,” she says. “The vitamins A, D, E and K are fat soluble, meaning the body stores them in fatty tissue and the liver. The intestines need dietary fat to properly absorb these nutrients. These vitamins are also necessary for the health of your skin, bones and cardiovascular system, among other organs and systems.”

The average adult should get 20-35% of their calories from healthy fat sources. An adult eating a 2,000-calorie-a-day diet could eat 44-78g of fat in a day. Some healthy fat sources include olive oil, avocadoes, salmon, tuna, walnuts, flax seeds and sunflower seeds.

Dietary guidelines for fat

Clark advises that while reducing saturated fat in our diets can help you to lose weight, you must consider your wider diet as a whole. “Replacing fat with wholegrains and more fruit and vegetables is favourable for health,” she said. “It's important that fat is not displaced by higher intakes of refined carbohydrate and sugar. Much of the controversy surrounding fat has to do with what the fat is replaced with as this too will have an impact on our health risk.

“The balance of fats in the diet is key, with the emphasis placed on unsaturated fats coming from nuts, seeds, avocado's and plant oils. These are associated with reducing LDL cholesterol and raising HDL cholesterol, which is the type that protects us.”

Additional resources

Lou Mudge
Lou Mudge

Lou Mudge is a health writer based in Bath, United Kingdom for Future PLC. She holds an undergraduate degree in creative writing from Bath Spa University, and her work has appeared in Live Science, Tom's Guide, Fit & Well, Coach, T3, and Tech Radar, among others. She regularly writes about health and fitness-related topics such as air quality, gut health, diet and nutrition and the impacts these things have on our lives. 

She has worked for the University of Bath on a chemistry research project and produced a short book in collaboration with the department of education at Bath Spa University. 

With contributions from