How the body uses food for fuel

You may have learnt that the body uses glucose for energy; a fact that is correct but only part of the story. The body converts certain components of food into its own energy currency; a molecule known as ATP. The glucose released from broken down carbohydrate (which remember includes all forms of dietary sugar and starch) is one component used to make ATP. The important thing to take away here is that protein and fats can also be used in the complex pathways that result in ATP production; not just carbohydrate.


Carbohydrates are broken down on their journey from the mouth and absorbed, as glucose, into the bloodstream via the small intestine. From here the glucose is taken to the liver, where as we saw earlier it is stored as glycogen with the excess being shunted to the fat cells for storage. As long as you provide your body with a source of dietary carbohydrate it will use it as the primary source of fuel; which means that fat stays where it is.


Dietary fats and oils are also broken down on their journey through the gastro-intestinal tract and eventually arrive at fat cells to be stored as triglycerides. A triglyceride is just a convenient unit of three saturated fatty acids. As well as vital components of many of the body’s processes, these fatty acids can be mobilised through the ATP pathway to provide energy. In the absence of dietary carbohydrate, the glycogen stores in the liver will be depleted and the body will use fat as its energy source; a very efficient one at that.


Protein is the least preferred source of energy for the body but can be converted to ATP if necessary. Proteins are broken down into their component amino acids and sent throughout the body to be used where they are needed. When mobilised for energy production the amino acids are converted to sugars by the removal of nitrogen and then enter the ATP pathway. Protein is essential in the diet for building and repair, as well as playing certain roles in appetite control, but ideally should not be consumed as a primary source of energy.

The message is quite simple; by reducing carbohydrate intake and increasing fat intake you ensure that your body uses fat for fuel, as well as providing the essential fatty acids needed for optimal functioning. Protein intake should be carefully balanced to provide just enough for your body’s building and repair needs but not consumed as a primary energy source.

Why increase fat intake?

If carbohydrate intake is to be reduced and protein intake not to exceed requirements then why not just eat less fat too? Won’t eating extra fat just replenish my fat stores? The answer to this brings us back to overall calorie intake. As we pointed out at the beginning, the common view that a reduction in overall calories is the only way to lose weight is not entirely correct. Whilst an excess of calories vs expenditure (amongst other factors) will lead to weight gain, there is more to the science of nutrition and weight loss than calories alone.

A body without enough calories is under stress and various feedback mechanisms will kick in that make you hungry, conserve energy and store fat. Calorie restriction or excessive cardio is not a healthy or sustainable route to weight loss; when you stop exercising or start eating the weight will return. Less calories for a sedentary lifestyle is not the answer either; the body needs movement, and plenty of it, for optimum health.

With a reduced carbohydrate intake and an optimum protein ratio of around 20% of total food intake that leaves a calorie deficit that needs to be filled with fat. Not surprisingly, most individuals are fairly fat phobic as we have all been indoctrinated over the years with the low-fat mantra. But if you do not make up your calorie deficit with fat then your weight loss will stall. Your hunger may lead you back to excess carbohydrate intake and back to square one.

Why fats are essential for health

As well as being the preferred energy source for the body, fats are essential components of cell membranes and vital molecules. Without healthy membranes cells simply cannot support the functions necessary to life, including communication of regulating hormones; which as we have seen are vital to weight control and optimal health.

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