Using Fat as Fuel in Long Distance Triathlon

Fat burning is a hugely important topic and area of consideration for people completing Iron distance, and even 70.3 distance triathlons.

Fat burning is going to take place naturally anyway – regardless of what you eat or how you fuel yourself. It’s how our bodies typically source fuel when exercising at low intensity.

Coach Rob explains below why this is important in long distance triathlon and how to make the most of it.

For most athletes, higher intensity means your body has a greater reliance on carbohydrate metabolism, and actually shuts off the fat burning metabolism as a source of fuel.

So – training hard and fast all the time means your body won’t make the adaptation to become more efficient at burning fat – i.e. learning to get more of its required energy from fat.

This is why it’s quite common to see very good age-group, and even pro triathletes over the Olympic distance, really struggle to step up to Ironman. Their bodies often simply can’t keep going long enough to perform well over longer distances.

With sustained training in improving your efficiency of fat burning your body can provide an increased amount of energy per minute from your natural fat stores to fuel you over an extended period of time.

This is incredibly important as our bodies can provide a seemingly endless supply (in the context of long distance triathlon) of natural fat. The amount of carbohydrates the human body can metabolise is limited – typically to around 300 calories per hour. An Ironman athlete will burn way more than that per hour, for many hours – so without fat as a fuel source there’d be a significant calorie deficit. It’s this deficit that leads to some of the struggles that we so often see people having in the latter stages of an Ironman race.

So it’s important to ‘train’ your body to burn fat.

If you’re a relative beginner long-distance triathlete and don’t have much of an endurance background, this is one of the biggest things to make sure you get right in the lead up to your race.

That’ll mean dedicating time to train at / below your aerobic threshold. That is – the level at which your body is still mainly using fat as it’s source of fuel. As you spend more time training at this level your body will adapt, providing a greater proportion of your fuel from this fat source. Over time therefore, you’ll gradually be able to go faster while still fulfilling your energy needs from your own natural fat supplies.

A great way to begin to train your body, as counter-intuitive as this may sound for triathlon, is to go for a hike! So – extended periods of time at a relatively low intensity, where you don’t need to rely on gels, sweets etc.

Likewise – long, slow bike rides are also great for this, as long as the pace is kept conversational (i.e. easy). As a general rule of thumb if you can hold a conversation your body is in it’s fat burning zone! The minute you switch to a heavier, more laboured breathing style that’s likely to be when your body is switching to burning carbohydrates rather than fat.

If you use a heart rate monitor the middle of Zone 2 is the perfect place to be. If you use a power meter on your bike, 68-70% of your FTP is around what you should be aiming for. The key though, is for the pace to remain conversational.

Over time your aerobic threshold – i.e. your fat burning threshold – will increase and improve. To give you an idea of what’s possible it’s not uncommon for pro-ironmen to complete their bike leg at ~80% of their FTP while remaining aerobic – i.e. getting most of their energy from fat, even though the intensity is relatively high.

So if you’re stepping up to long distance triathlon – please do make sure you factor this critical element into your preparation and training.


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