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The Fat of the Matter – Fat Metabolism and Fuel Physiology

This is an editorial that I put together for this year’s Athletics South Africa yearbook. The article looks at the basics of energy provision for our muscles during exercise and the theory and research behind the concept of using fat as a fuel source.

The Fat of the Matter

Exercise Metabolism and Nutrition

Any topic on nutrition is never far off turning to fat; The good, the bad and the ugly. A few years ago it was all evil but fat is making a comeback thanks to quality research and some popular punting. Let us take a closer look at what fat means to athletes so you can make up your own mind and understand the role that fat plays in exercise metabolism.

Exercising muscles require energy, released from breaking down our energy molecule called adenosine triphosphate (ATP). ATP is mainly produced via one of 3 pathways and is dependent, amongst other things, on the intensity of exercise.

1. Phosphocreatine – The Usain Bolt of energetics

Usain Bolt winsThe first few seconds of exercise needs energy, and quickly. We use a little free ATP immediately before breaking down phosphocreatine to produce ATP. Phosphocreatine is stored in our muscles and we only have a limited supply, enough for 10-15 seconds of exercise.  This won’t get us very far before we need to summon a more reliable source of energy. At high intensities we call on carbohydrate, at lower intensities we use more fat.

2. Carbohydrate –  The Mo Farah

Mo Farah

Once phosphocreatine stores are depleted, the next quickest and easiest energy store to break down is carbohydrate, the endurance athlete’s best friend. It is the fuel of choice for high intensity exercise. We start by using glucose circulating in our blood before tapping into our reserves of carbohydrate, stored in our liver and muscles as glycogen. We usually have enough for about 60 minutes of high intensity exercise but an efficient diet can improve this to almost 90 minutes. After this runs dry we must rely on providing blood glucose through food containing carbohydrate to sustain high intensity exercise.



3. Fat – Lisa (or Homer), Two Oceans veteran

homer LSD

Fat is not an efficient fuel to use, we consume more oxygen breaking it down and producing ATP. As such we prefer to use it at lower exercise intensities when the demand for energy is less and oxygen is available. This allows us to keep our glucose and glycogen stores for when the going gets tough and exercise intensity increases. The range at which we burn fat varies greatly between individuals but is generally around 65 to 75 % of your maximum heart rate.

Why the interest in fat then?

Despite the fact that fat requires more work to metabolise it does provide more than double the energy per gram than any carbohydrate will. We also have vast reserves when compared to the small amount of stored glycogen and blood glucose. In theory if we could be more efficient at using fat and use it at higher intensities then we can spare glycogen and possibly see performance benefits. Training within the fat burning range can also benefit weight loss, assist recovery and help prevent overtraining.

So can we improve how we use fat? Research suggests that we can in the following ways:

  1. Train more. Athletes commonly utilize fat better and for longer during exercise when compared to non-athletes. Exercise leads to adaptations such as an increased number of mitochondria in cells (these are the power houses that metabolise fat and produce energy), a better network of blood vessels and more carrier molecules to transport fat.
  2. Train in a fasted state. This means avoiding calorie containing food or drink for 6-8 hours before and during a training session. This enhances the exercise related adaptations mentioned above. Studies show a similar effect when training in the morning following a low carbohydrate dinner the night before. Include this in your training program a few times per week but not if you are planning on measuring your performance (e.g. a timed run), you will still need carbohydrate for this!
  3. Eat sufficient, but not too much fat. Fat should contribute around 30-40% of your dietary intake. The actual amount will vary however, according to your sport and individual goals. Interestingly fat loading, or significantly increasing fat intake before an event, has shown to enhance fat use during exercise but researchers have failed to find a benefit to performance. In fact the perception of effort during exercise may be higher after fat loading and excess fat intake can impair your body’s ability to use glycogen. This hindrance may prevent athletes from having that “next gear” when they need it.  The answer then seems to be eating enough good quality fat but not too much.

This quality element is an important one. Little evidence exists on the effect fat quality has on performance but we do know more about how this impacts general health and possibly recovery too. Knowing which fats to include more of and which to avoid can help.

Omega 3 fats are leading the Good Guy race. Omega 3’s are a type of polyunsaturated fat found in oily fish (herring, mackerel, sardines, pilchards and salmon), nuts and seeds and grass fed beef or dairy.  They can benefit health and assist recovery by reducing inflammation. They may also assist mental function, keeping you sharper. More research is needed to determine how much we actually need but increasing your dietary intake should be a focus. Eat oily fish twice a week, use a handful of nuts or seeds as snacks 2-3 times per week and choose grass fed dairy and meat where possible. Other polyunsaturated fats (vegetable oils, omega 6’s) should be limited by avoiding processed foods and high-fat, ready-made meals. These can worsen inflammation, delay recovery and impair heart health.


The remainder of the fat content in your diet should be predominantly heart healthy monounsaturated fats from olives, avocado, nuts, seeds, canola and their respective oils. Add these in small amounts to meals daily.


The belief of the evil of saturated fats is waning and animal products can be included in moderation. Vegetable sources of saturated fat such as coconut oil can provide a good option for cooking or baking too and appear to be neutral fat in terms of health and inflammation issues.

Don’t ignore the benefit of fats, use them wisely and train accordingly. Contact a specialist in the area for any further advice or to tailor your nutrition approach.


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