What is Fat?

See also: Body Mass Index (BMI)

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

Our bodies require small amounts of 'good fat' to function and help prevent disease.  However, a lot of modern diets contain far more fat than the body needs.  Too much fat, especially too much of the wrong type of fat, can cause serious health complaints including obesity, higher blood pressure and cholesterol levels, which in turn lead to a greater risk of heart disease.

Dietary fats make food tasty they often improve the texture of food as well as flavour and smell - they make food more appealing. 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. In reality most Western diets derive at least 40% (and sometimes a lot more) of their energy from fats.

Fat is Good!

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 – 1 gram of fat contains 9 calories, much more than a gram of protein or carbohydrate which both contain 4 calories per gram.  The body can pull on its fat reserves during lean times for energy, converting fat into glucose. 
  • Fat provides a cushion to help protect our vital organs – without fat our organs would be more prone to damage.  Furthermore, fat acts as an insulator, helping us to maintain the correct body temperature.
  • Fat enables our bodies to process vitamins A, D, E and K, which are all fat soluble and vital to good health. (More on Vitamins)
  • Like amino acids in protein, fat contains essential fatty acids (EFA’s).  These EFA’s are, as their name suggests, essential to good health and likely to help the heart and immune system.  The human body cannot make its own (synthesize) these EFA’s and therefore has to get them from consumption of fat.
  • Some fatty acids – like omega 3 – may provide other health benefits such as complimenting the cognitive processes of the brain.
  • Fat makes food taste better.  Hot buttered crumpets, double cream on trifle, gravy made from dripping!

Although we need fat we only need small quantities of the right kinds of fat to stay healthy.  We all know that too much fat and consuming the wrong kind of fat can be detrimental to our health. (See 'Fat is Bad' below).

Body Mass Index (BMI)

The term obese has become more commonplace in recent years.  In the UK it is estimated that one in four adults is clinically obese – having a body mass index (BMI) of 30 or more.  Obesity is, in most cases, caused by inappropriate diets, consuming too much fat. 

To calculate your BMI:

  • Divide your weight in kilograms by the square of your height in metres.
  • Divide your weight in pounds by the square of your height in inches, and then multiply by 703.

The answer you get is your BMI:

  • If your BMI is less than 18.5 then you are underweight, you may need to gain weight.
  • If your BMI is between 18.5 and 24.9 you are an ideal weight.
  • If your BMI is between 25 and 29.9 you are classed as overweight and should take measures to lose weight.
  • If your BMI is over 30 then you are classed as obese.  You should lose weight by changing your diet and/or increasing exercise.

BMI is not accurate in all cases – for example, people with athletic builds may have a high BMI – muscle is heavier than fat which can skew the results.

See our page: Body Mass Index (BMI) for more information including about Body Mass Index, icluding a BMI calculator and a downloadable BMI chart.

Do you struggle with numeracy skills and mathematics? See our Numeracy Skills section for some easy-to-follow help.

Fat is Bad

  • Due to its high calorific value (1 gram of fat = 9 calories) it is easy to consume too many calories when eating fatty foods.  Unused calories can be stored by the body as fat and will cause weight gain.
  • Our bodies store fat for lean times and have evolved to cope with seasonal availability of food – storing fat when food is plentiful and burning it off when food is scarce.  In the modern world, and for most people, food is plentiful all year round – our bodies store fat but never burn it off, as fat accumulates we become overweight. See our page Dieting for Weight Loss for more information on maintaining a healthy body weight.
  • Fat can cushion and protect our internal organs; however too much cushioning means more bulk and weight which in turn increases the workload of the heart and other organs.
  • Your body (the liver) produces cholesterol which is vital to a healthy body and a building block for other essential chemicals that the body produces.  Cholesterol is a waxy substance that, in low levels, flows freely around your body in the blood.  Higher levels of cholesterol mean a higher risk of developing coronary heart disease.  See below for more on cholesterol.
  • Some fats are worse than others.  Saturated fats are worse for you than unsaturated fats – this is to do with their chemical structure and how the body processes them.  Trans or hydrogenated fats – which are almost exclusively manufactured (although do occur naturally in small quantities in meat and dairy produce) and are used in many processed foods are particularly bad and are linked to an increased risk of high cholesterol levels and coronary heart disease.

Types of Fat

Saturated and Unsaturated

The two main types of fat are saturated and unsaturated. Unsaturated fats are generally considered better for us than saturated fats.

The reason that unsaturated fats are better is down to the molecular structure of fat.  Saturated fat molecules form regular shapes that clump together easily; unsaturated fat molecules however, form irregular shapes that cannot clump together so easily.  Saturated fat is therefore more likely to stick to the sides of arteries and allow other saturated fat molecules to build up; this can gradually clog the arteries leading to higher blood pressure and making it more difficult for the heart to pump oxygen rich blood around the body. 

Fats are not soluble in water (or blood) and unless this problem is addressed – usually through change in diet and increased exercise - it can lead to serious health problems such as coronary heart disease.

Generally (although not exclusively) saturated fats come from animal sources (meat, dairy, eggs etc.) and are usually solid at room temperature.  Unsaturated fats come from vegetable sources (sunflower oil, olive oil, soya oil), oily fish (salmon, trout, mackerel etc.) and soft margarines.

Vegetable sources do contain saturated fats but usually in low amounts; take oats for example, which contain almost 9% fat, made up of the three main types, saturated, monounsaturated and polyunsaturated.

Monounsaturated and Polyunsaturated are the two main types of unsaturated fat – they are unsaturated as they are missing one (mono) or more (poly) hydrogen atoms in their chemical makeup – this is what gives them irregular shapes.

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

Hydrogenated or Trans Fat

Hydrogenated fat is manufactured fat used in processed foods.  It contains some qualities desirable to food manufacturers, but is perhaps the worst of all fats when it comes to health. 

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.  Technically unsaturated fat, hydrogenated or trans-fat increases the risk of coronary heart disease by raising levels of LDL cholesterol and lowering levels of "good" HDL cholesterol in the blood.  This is the most important fat to avoid.


Cholesterol is a type of fat found in the blood.  Nearly all the cholesterol in the body is produced by the liver, very little is found in foods although seafood, liver, kidney and eggs do contain some cholesterol.  Cholesterol is vital in the body, not only does it play a role in how all cells work but it is also a ‘building block’ for other essential chemicals that the body produces.

Cholesterol is carried around the body in the bloodstream combined with proteins, these are called lipoproteins.  There are two main types of lipoprotein that are used to measure cholesterol levels in the blood.  LDL – low-density lipoprotein and HDL – high density lipoprotein.  Low density lipoprotein (LDL) is often called ‘bad’ cholesterol whereas high density lipoprotein (HDL) is considered ‘good’ cholesterol.  HDL is ‘good’ as it can remove extra bad cholesterol from the bloodstream.

Blood cholesterol is measured by looking at the total LDL, HDL and other fats in the blood.

People with high cholesterol levels are more likely to develop health problems – the risks are increased further for people who also smoke, have high blood pressure, are physically inactive and unfit, are overweight or obese or suffer from diabetes.

A common cause of high cholesterol levels in modern society is the consumption of too much saturated fat.

Healthy Fat Tips

Read the labels of foods you buy.  Try to reduce the amount of trans fats, hydrogenated fats and saturated fats in your diet – always favour foods with unsaturated fats. 

If you are overweight then you should attempt to reduce your total fat intake – try to replace fatty foods with fresh fruit and vegetables.  Increase your consumption of oily fish – omega 3 fats are known to provide many health benefits and most people do not consume enough - salmon, trout, fresh tuna and mackerel are all good.

See our page: Cooking Fats and Oils for a comprehensive guide - which oils and fats should you buy?