What Is Fat?

Fatty Meat showing subcutaneous fat and intramuscular fat

Subcutaneous fat and intramuscular fat

You are desperate to get rid of it, but do you know what it is, or why you have it? Fat is of course totally natural and without it we would struggle to survive for more than a few days without food. It provides us with energy, protection and warmth.

However, it is more than just a way for the body to store energy, it also helps to control hormones, which are essentially signals to switch bodily functions on and off – fat is an endocrine organ. For example, women who lose too much fat suffer from reduced fertility due to less estrogen being produced. Excess fat tends to lead to many health problems.

3 Types of Body Fat


There are 3 main types of fat that we are concerned with: subcutaneous fat which forms below our skin (like the thick fat found on the edge of steaks as seen in the photo), visceral fat which covers the internal organs, and intramuscular fat which is within the muscles (as shown in the main joint of beef in the photo).

People who store more intramuscular fat sometimes mistake this for being fitter and stronger – in reality they are still unfit and overweight, they just hide it better! What’s more, intramuscular fat is linked to increased risk of diabetes. On the plus side, intramuscular fat can be used as an energy store when exercising.

Why fat is stored in different places is not something we will cover here. Let’s focus on how fat is formed and how it is broken down again.

Some Human Biology

In human biology fat is called adipose tissue. Fat is a compound which consists of triglycerides (triacylglycerols, TAGs) which are formed when a molecule of glycerol combines with 3 fatty acids.

Glycerols come from the break down of fats in the gut during digestion. It is an energy source, along with glucose, amino acids and fatty acids. While glycerols are derived from dietary fats, the human body can also convert glucose into glycerols. These glycerols are then combined with fatty acids to produce TAGs.

This is the vital stage of fat formation – we have excess fatty acids, glycerol and glucose in the body, or more accurately, circulating in the blood stream, after eating too much food.

When glucose levels are high the body reacts by producing the hormone insulin. Insulin provides a means for glucose to be removed from the blood by transporting it into the muscle cells to store it as glycogen (the muscles use glycogen as energy when we exercise), and into the liver where it is converted to glycerol. Once converted to glycerol it can then combine with fatty acids to be stored as fat / adipose tissue. Insulin also reduces breakdown of fat into fatty acids and glycerol, so even if you exercise a lot, a diet high in sugar may not result in fat loss. This is why a low GI diet aids weight loss.

Fat In A Nutshell

So to recap, when we eat food, it is broken down into glycerols, fatty acids, glucose, amino acids. When we eat too much, blood sugar levels remain high and this triggers the conversion of glucose into glycerols (in the liver) which are stored as fat – and we get fatter.

These fats are accumulated all over the body – under the skin, around the internal organs and within the muscle tissue. Where we store fat first is determined largely by genetics – some people store more visceral fat, others more subcutaneous fat on the hips, others more fat on the chest. What unites everybody is the simple fact that eating too much leads to more fat – genetics can only control where it is stored, not if it is stored.

How To Breakdown Fat

The key to losing weight is to reverse this process; to cause the body to need to breakdown the TAGs in the adipose tissue to release its energy. The only way this will ever happen is if the body energy supplies are used up – no glucose, amino acids, fatty acids in the blood or glycogen in the muscles.

In the same way that the body triggers the production of fat when blood sugar levels are high, it also triggers the breakdown of fat when blood sugar levels are low. So lowering blood sugar levels can cause fat loss. The process is: low blood sugar levels triggers a release of the hormone glucagon, which removes TAGs from fat storage to break them down into the glycerols and fatty acids. The glycerols are then converted back into glucose by the liver, by a process called gluconeogenesis.

This happens quicker when the glucose supplies stored in muscles (as glycogen) have also been depleted – which is why exercise aids faster weight loss.

So, to breakdown fat efficiently, you need to eat less food (run a calorie deficit) and exercise to “burn up” your glycogen reserves. This leaves your body with one option – breakdown your fat for energy.

Note – this is a simplified explanation of the biological and chemical processes at play in the human body. However, the principals are central to gaining and losing weight.

More like this in the Weight Loss section

  4 comments for “What Is Fat?

  1. Nick M.
    April 11, 2013 at 7:23 pm

    Hey, I thought that fat, is in our fat, was a combination of sugar and fat, as in glycerol is a type of sugar, and fatty acids are fat. But you say it is all “fat”. And, is our fat the same as animal fat? Like what you get on beef and bacon?

  2. MotleyHealth
    April 11, 2013 at 10:52 pm

    Glycerols are fats, but the liver can convert glycerol into glucose, and back again. So glycerols are stored as fat and then converted into glucose when fat breaks down – the glycerols are processed by the liver to produce glucose which can then be used for energy.

    This diagram sort of explains it:

    (Image created by Unused0026 at en.wikipedia)

  3. Andy Eppink
    September 19, 2013 at 4:08 am

    Why does fat weigh less than the food which gave rise to it? I.e. 3600kcal ~ 1lb body fat but 3600 kcal worth of say green beans at 25.71kcal/100g would be ~ 14kg ~ 31lb. If you figure this for an energy dense food, say macadamia nuts at 718 kcal/100g, the results are much closer to 1lb. Water content? Something else?

  4. MotleyHealth
    September 19, 2013 at 8:11 am

    Andy, we break the food down into many things and not all the food is digested. There is water, but also roughage, and lots of other waste. Some of the food is used elsewhere in the body, stored in the glycogen reserves etc. Much of it is defecated later.

    And if you eat only as much as you need, none of the food is converted to fat. Fat does not make you fat – too much food makes you fat.

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