It's All About Fat And Sugar

A very important overriding concept of Ironman racing and training is balancing the carbohydrate and fats used to fuel performance. This is the main goal of all long training sessions. If the body is not using its fuel sources optimally, then race performance will not be the best. In order to understand and hopefully better direct training it is important to understand how fuels (substrates) are stored, utilised and regenerated.

Protein is used, but only as a minor fuel source during Ironman racing. There is not much we can do in training to influence the amount of energy protein supplies, so I will treat its contribution as negligible.

That leaves carbohydrate (CHO) and fat (lipids) as the key sources of substrates. Adenosine Triphosphate (ATP) is where the body obtains its energy to do work. The faster the rate of ATP generation and the greater the amount that is available, then the faster the body can go. Racing a hard Ironman typically involves exercising in the range of 60-70% of VO2max. Remember that the higher the intensity of exercise, the higher the relative contribution of CHO is versus fat. Exercise at this intensity generally requires 2.0-2.5 mmol kg/dm/sec of ATP (don't worry too much about the units, just think of it as between 2.0-2.5 of ATP). The oxidation (breakdown using oxygen) of CHO and fats is the mainstay of energy delivery during Ironman.

Is CHO the best source of energy?

Yes and no is the frustrating answer. Based on maximal oxygen consumption of 3-4 litres/min, it can be calculated that ATP production from CHO will be between 2.0-2.8 mmol kg/dm/sec. Showing that CHO can provide ATP at the correct rate for racing. The problem is that the body does not store enough CHO to maintain a supply throughout the event. Basically this means that while CHO can produce ATP at the correct amount and rate, its supply is limited and will run out well before the end of the race.

So how much CHO does the body store and how long will it last?

CHO is stored as glycogen in muscle and liver. There is also circulating blood glucose from the diet and other sources available. For the average male of 70kg with 15% body fat these stores are as follows:
  • Muscle glycogen 350g
  • Liver glycogen 80g
  • Blood glucose 10g
  • TOTAL 440g

The CHO available from muscle glycogen can fuel exercise at the required intensity for up to 80-100min only. Considering Ironman lasts between 8-17 hours, we have a problem. We can gain a further 45min from the liver glycogen, but at a significantly reduced intensity. Even with carbohydrate loading we can only increase muscle glycogen by up to another 60g to a total of 500g. This again only gains us another 20 minutes. We are still many hours short.

So what about Fat?

The advantage of fat is that no matter how lean the athlete, there is more than enough stores available. In fact, if we could use only fat, we would have enough to last us many days of exercise. The disadvantage of fat, is it provides ATP at a much slower rate than is available through the oxidation of CHO. The maximal rate of ATP production from fats is about 1 mmol kg/dm/s. This falls far short of the 2.0-2.5 mmol kg/dm/s required to maintain exercise intensity throughout the Ironman.

Fats (lipids) are stored as triacylglycerol mainly in white adispose tissue (the blubbery stuff around your gut), but also to a lesser degree in the muscle. The main reason (simplified) that lipids can only generate ATP at such a slow rate, is that the muscles cannot use triacyglycerol directly. As a bit of a misnomer too, the muscle triacylglycerols are believed to play more of a role after exercise than during. In order for the body to use lipids as a fuel source, it must break down the stored triacylglycerol into free fatty acids (FFA) into the blood stream, so these can be taken up by the muscle and then oxidised to produce ATP. This is apparently a lot of work, and requires much larger amounts of oxygen when compared to CHO sources.

Lets look at the energy required just to run the marathon section of the Ironman. People should be familiar with the concept of kilojoules or calories. Both are another measurement of energy, and if you have ever tried losing weight, then you probably have limited your kilojoules or calories. I will speak in terms of kilojoules (kJ). The energy cost of running a marathon is about 12,000 kJ, almost regardless of how fast you cover the distance. Lipids provide 37 kJ per gram of fat and if used as the sole energy source, a total of 320g of lipids is required. CHO on the other hand provide 16 kJ per gram, and again if used as the only energy source, a total of 750g of CHO is required.

Too summarise, carbohydrates provides energy or ATP at a fast rate, but have only a limited supply, whereas fats have an extremely large supply well in excess of what is required, but provide energy at a slow rate.

So what if we combine the CHO and fats during exercise?

In short, we don't really have much choice as this is what the body does naturally and one does not work without the other. It is accepted that muscle glycogen will eventually become depleted during the Ironman. This fact shows the importance of utilising fat and liver glycogen during the Ironman. The maximum rate of ATP production from a combination of fat and liver glycogen is 2.0 mmol kg/dm/s which is just at the lower limit of requirements for the Ironman.

So the obvious question is, where does the extra energy come from?

The simple answer is diet during the race. I will not go into too much detail in this post, as this is a seemingly endless topic. Basically you cannot complete the Ironman without ingesting CHO during the event. There are two main reasons for this. First, because the ingested CHO goes to the liver and then into the blood stream, it helps prevent hypoglycaemia (low blood glucose). The brain needs a certain constant supply of glucose, and without it stops functioning properly. As a protection mechanism, if your blood glucose falls too low, you will fatigue significantly and have to reduce the intensity of exercise. The second reason is the ingested CHO also provides the additional fuel required to fuel your race. Ingested CHO during exercise has been shown to have a sparing effect on muscle glycogen in Type I muscle fibres (the endurance ones). Also when liver glycogen stores have become depleted, ingested CHO can take over this role of energy contribution.

How do we reach the end of the race in the fastest time?

If we limit this to your current level of fitness and ignore other aspects of efficiency and drafting, then the theory is simple. Putting the theory into practice is the hard part.

  1. Race at the correct intensity. 60-70%VO2max for the well trained, and less than 60%VO2max for those not so well trained.
  2. Carbohydrate load to maximise CHO stored at the start of the race.
  3. Ingest CHO during the race to prevent hypoglyaemia and provide further CHO for the muscles.

Looks nice and easy when written as only three points, but these points lead to further questions. How does training affect the mix of CHO and fat used during the race? How do I identify the correct intensity? Do weather conditions affect this mix? What else can I do to use more fat over CHO? How do I carbohydrate load? How much CHO do I ingest during the race? Is the type of CHO important? Does the science match what happens in the real world?

I will endeavour to attempt to find answers to these questions in future posts.

"Science may set limits to knowledge, but should not set limits to imagination." - Bertrand Russell


  1. Excellent post, put about as simply as I have ever seen, thanks for that.

    I have explained until I am blue in the face to people that our bodies just love holding onto those fat stores, that is why they are so hard to lose, so best not to get them in the first place. Damn caveman genes.


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