What is Fat?

See also: Body Mass Index (BMI)

Fat is an essential part of our diet and nutrition, and we cannot live without it.

Our bodies require fat to function and help prevent disease. We cannot make the fats that we need without consuming them as part of our diet. Dietary fats also make food tasty: they often improve the texture, flavour and smell of food. However, many of us avoid dietary fat, especially saturated fat, because scientific studies done many years ago found links to serious health issues including obesity, high blood pressure and heart disease. Indeed, government guidelines have consistently suggested avoiding saturated fat for this very reason.

There are many problems with this approach. First, many of these studies are flawed, and often overlook the impact of other unhealthy behaviours. Second, foods that contain saturated fats often contain many other forms of fat too—many of which have been declared ‘good’. Third, foods with saturated fat removed are generally more highly processed. The fat has often been replaced by sugar, which is far more likely to cause health problems than the fat it replaces. This page explains more about the different types of fats, and the arguments for and against them. It also explains the origins of some of the myths about fats.

What is Fat?

‘Fat’ is an umbrella term for anything that is made up of ‘blocks’ of three fatty acids joined together—hence the scientific term triglycerides.

There are many different types of fat, some natural and some created by humans.

The types of fat generally differ in how many fatty acids are connected together (short- or long-chain fatty acids), and in the type of bonds that are between the fatty acids (see box).

The Chemistry of Triglycerides

Fatty acids—like any other carbon-based molecules—can make different types of bonds between the carbon atoms within the molecules.

Every carbon atom has the capacity to make four bonds with other atoms. Sometimes atoms will bond with other carbon atoms, and sometimes they bond with other substances, such as hydrogen or oxygen. Here we are largely concerned with the bonds between carbon atoms.

  • Some pairs of carbon atoms have a single bond between them.
  • Some pairs of carbon atoms have a double bond between them.

The nature of the bonds between carbon atoms affects the properties of the fat.

  • When all the bonds between carbon atoms are single bonds, the fat is said to be ‘saturated’. These fats tend to be solid at room temperature (like butter or lard).
  • Where a fat contains one or more double bond, it is unsaturated. It has the capacity to bond with more molecules of something else. These fats are more likely to be liquid at room temperature (oils).
  • When a fat has several unsaturated bonds, it is polyunsaturated.

Note: This is a very simplified and generalised description of carbon chemistry. In reality, the situation is much more complicated.

How Our Bodies Use Fat

Like protein, but not carbohydrates, fat is essential to human life. We all need fat in our diets:

  • Fat is a concentrated source of energy and can be stored by the body. A gram of fat contains nine calories, much more than a gram of protein or carbohydrate, which both contain four calories. The body can draw on its fat reserves during lean times, converting fat into glucose to provide energy to run various biochemical processes within the body.

  • Fat provides a cushion to help protect our vital organs. Without fat, our organs would be more prone to damage. Fat also acts as an insulator, helping us to maintain the correct body temperature.

  • Fat enables our bodies to store vitamins A, D, E and K, which are all fat soluble and vital to good health. (More on Vitamins)

  • Fat contains essential fatty acids. These are, as their name suggests, essential to good health. In particular, they seem to be necessary for the heart and immune system. The human body cannot make its own essential fatty acids, and therefore has to get them from consumption of fat.

  • Some fatty acids, such as omega 3 acids, may provide other health benefits such as supporting the cognitive processes of the brain.

Guidelines, Recommendations—and the Evidence on Fat

Government guidelines on fat

In the UK, the Department of Health suggests that no more than 35% of total calories should come from fat. In the US, recommended fat intake is 30% of total calorie intake.

However, are these based on scientific evidence? The answer is probably not.

To understand the position of governments and health bodies on fats, it is necessary to have a little look at history.

Back in the 1960s, some studies found a link between blood cholesterol and health. A leap of faith was then made that the cholesterol in our blood was associated with the cholesterol in our diet—and therefore that eating foods containing cholesterol was bad for you.

This is completely untrue. More recent—and more robust—studies have shown that dietary cholesterol has nothing to do with blood cholesterol. Our livers make all the cholesterol that we need—and we do need it (see box).


Cholesterol is a type of fat found in the blood. Nearly all the cholesterol in the body is produced by the liver. It is not affected by what we eat, although cholesterol is found in plenty of the foods that we consume.

Cholesterol is vital in the body. It seems to play a role in maintaining the structure of cell membranes—and therefore holding cells together. It also has a role in transporting fats around the body.

Cholesterol is carried around the body in the bloodstream combined with proteins. These combined molecules are called lipoproteins. There are two main types of lipoprotein that are used to measure cholesterol levels in the blood: low-density lipoprotein (LDL) and high density lipoprotein HDL). Low density lipoprotein (LDL) is often considered ‘bad’ whereas high density lipoprotein (HDL) is considered ‘good’.

This would suggest that you should reduce the levels of LDL in your blood, and aim to increase the levels of HDL.

However, it is not clear whether this is an accurate picture. Indeed, one study found that low levels of one LDL, LDL-C, were actually associated with higher levels of mortality from all causes, as well as increased risk of coronary heart disease and cancer. All that we can really say with any certainty is that dietary fat is not linked to cholesterol.

By the 1980s, therefore, ‘dietary cholesterol is bad’ had been replaced as an idea by ‘all dietary fat is bad’. This seems to have been based on the idea that fat contains more calories per gram than carbohydrates. At that stage, there was a general idea that more calories = bad, and therefore fat must be worse for you than carbohydrates. Someone somewhere also advanced the idea that dietary fat could stick to your arteries, making you more likely to have a heart attack. This too does not seem to be true.

By the early 2000s, it was clear that reducing total dietary fat had absolutely no effect on health.

However, scientists then shifted to the idea that certain types of fat—and particularly saturated fat—was bad for you.

The problem is that this is just not true.

Large-scale studies have found no evidence that reducing fat intake—either total fat or saturated fat—can improve heart health or reduce mortality. Indeed, one major study in Spain actually found the opposite: the group that ate more fat was actually healthier.

However, other studies have found a mixed effect. Some people were better on a high-fat diet, and others on a low-fat diet. This points to several important messages. First, the levels of fat, protein or carbohydrate in our diets probably matters less than eating a wide variety of foods, especially plenty of fresh fruit and vegetables. Second, we are far more individual than dietary guidelines suggest. There really is no one size fits all when it comes to diet.

The other factor that may differ is the effect of eating more processed foods, which often contain processed fats (see box).

The Effect of Processing

Both saturated and unsaturated fats are found naturally. Generally but not invariably, animal fats tend to be saturated, and vegetable fats tend to be unsaturated.

See our page: Cooking Fats and Oils for information about various types of cooking fats.

However, fats may also be processed to make them ‘better’ for use in processed food.

Hydrogenated fat is vegetable fat that has been treated with extra hydrogen. This changes the chemical makeup of the fat, making it solid at room temperature—which is often more desirable for food manufacturers. They were therefore extensively used in low-fat spreads and margarines for many years. However, these hydrogenated or trans-fats definitely do increase the risk of coronary heart disease and other conditions. Over the last couple of decades, therefore, pressure has grown on food manufacturers to stop using these products.

Unfortunately, they have often been replaced by other manufactured or processed fats. We have not generally gone back to more natural products such as butter or pressed olive oil.

The problem here is that we genuinely do not know what effect these new products will have on our bodies and health.

The Missing Link: Our Individuality

There is no question that certain forms of fats, particularly highly processed versions such as trans-fats, are bad for our health.

However, the evidence on other forms of fats is very mixed—or is it?

Studies have often found that on average neither high fat nor low fat diets had an effect on weight loss. However, if you dig deeper into the data, you find that some people in each group showed significant weight loss during the study—and others did not.

Scientists are now coming to the conclusion that the reason for these conflicting results in dietary studies is simply our individuality—and particularly our gut bacteria or microbiome. The bacteria in our gut help us to digest and manage food, and everyone has a slightly different gut microbiome. This means that we digest different foods differently.

Some people’s gut microbiomes and metabolisms are better at handling high-fat diets. Other people are better on low-fat diets.

This goes a long way to explaining why, on average, neither high fat nor low fat diets help with weight loss.

There is no ‘average’ person—and we should stop trying to develop dietary guidelines to fit everyone.

Of course it follows that eating too much of anything is not going to be good for you—and that includes fats. However, it is no good trying to demonise whole food groups such as fats. We certainly need fats in our diet—but we also generally need to eat more fruit and vegetables, and probably slightly less meat, and almost certainly less sugar and ultra-processed foods. We also need to avoid over-simplified health messages about food, and take them with a pinch of salt.