How long does it take for your body to use fat as energy

Asked by: Joyce Burley, Hull

If you step on the scales as soon as you have eaten a jam doughnut for lunch, you will weigh an extra 70g, because this is the mass of the doughnut itself.

But 22g of this is water, which you’ll lose over the next few hours in your breath and urine. You’ll lose another 2g sometime tomorrow, when you poop out the small amount of dietary fibre. The other 46g is digested and if your body doesn’t immediately need it to grow or for energy, it is stored as fat.

A 2012 study at Oxford University found that the fat in your food ends up on your waistline in less than four hours. Carbohydrate and protein take a little longer, because they need to be converted into fat in the liver first and it takes nine calories of protein or carbohydrate to make 1g of fat.

Altogether, that doughnut has about 225 calories. Around 100 of those come from the 11g of fat in the doughnut, which will be on your waistline by teatime. Then the 125 calories of carbs and protein will be converted into another 14g of body fat sometime tomorrow, unless you hit the gym after work and burn it off again.

Read more:

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Authors

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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.

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.

Read more about fat:

• Radical ideas: Diets make you fat
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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!

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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.

Read more about nutrition:

• Calories and nutrients: the complex calculations we do every day
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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.

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).

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