How long could you live off body fat alone? © Getty Images

How long could you live off body fat alone?

In this extract from Fat: The Secret Organ, obesity researchers Mariette Boon and Liesbeth van Rossum explain the purpose of fat in the body and how long it can keep us going without food.

When we talk about the fuel ‘fat’, which is suspended in our blood and which can be absorbed and burned by organs, what we’re referring to are fatty acids. These are long ‘tails’, or chains, that are usually made up of 16 to eighteen carbon atoms.

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Just like glucose, fatty acids are cleverly packaged so that large numbers of them can be stored without taking up too much space. You can compare it to a zip file on your computer. This is why fatty acids are stored in bunches of three in the form of what are known as ‘triglycerides’.

Thousands of tightly packed triglycerides are stored in a single fat cell. This is a huge amount of fuel, a real goldmine. As soon as you haven’t eaten for a few hours, or if you’ve been physically active for an extended period of time (for example, exercising or doing housework), this fuel will be tapped into.

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At this point, fatty acids will be ‘clipped’ from the triglycerides, released into the blood and then sent to the organs that need fuel. So, in this way, fuel (in the form of fatty acids) is literally ‘sent’ from your fat to your other organs.

Our fat tides us over during times of hunger. But how long can this last? In other words, how long can we live off our fat? Chemistry research studies have shown that burning 1 gram of fat yields 9.4 kilocalories.

This is more than twice as much as the 4.1 kilocalories you get when you burn 1 gram of glucose. This is why our body stores fuel not only as glycogen but preferably in the form of fat. Wouldn’t you rather put petrol in your car that lets you drive 700 kilometres instead of just 200?

If we wanted to store the same number of calories as glycogen, we would have to lug around many more kilos. And our fat reserves weigh quite a bit as it is. A healthy adult weighing 70 kilos has around 14 kilos of body fat. Since burning 1 gram of fat yields 9.4 kilocalories, this is equal to a total of 131,600 stored kilocalories – an enormous amount!

A woman of average build, who exercises moderately intensively every day, burns about 2,000 kilocalories a day, and a man around 2,500 kilocalories. This means that you can live off your fat reserves for an average of 66 days (for a woman) or 53 days (for a man), as long as you don’t become more active.

In practice, you would actually be able to manage for even longer, because we have a third energy supply, one we would rather not tap into unless our life depends on it.

This is our protein reserve. A healthy adult who weighs 70 kilos is carrying around nearly 10 kilos of protein, half of which can be broken down into tiny pieces known as amino acids, which are then used as fuel.

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Your body would rather not touch this energy supply, because the body’s own proteins are not intended to be used as a source of energy. These proteins are an important constituent of muscle, including the heart muscle and respiratory muscles, and also play a role in defending the body against pathogens.

This becomes clear in patients with the disease anorexia nervosa, who eat very little or nothing at all for an extended period of time. Along with having very little body fat, a person with anorexia also has very little muscle, because the body has broken this down to get enough energy.

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Moreover, even the most innocuous viruses, bacteria and funguses can make someone with extreme anorexia sick because their immune system can no longer fight them.

Fat: The Secret Organ by Mariette Boon and Liesbeth van Rossum is out now (£14.99, Quercus).

Fat: The Secret Organ by Mariette Boon and Liesbeth van Rossum is out now (£14.99, Quercus)