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Nutrition and endurance sports can be a little over complicated at times. Magic nutritional strategies/formulas are created trying to offer a solid platform to create your own personal strategy. While there are lots of merits to this approach sometimes they lead to over thinking, over eating and GI distress.

The purpose of this article is to highlight that although during the course of an Ironman race you may burn up to 9000 calories (see below) that you cannot replace everything that you expend during exercise. However, if you replenish appropriately you can keep yourself going all day long if necessary.

source: Asker Jeukendrup

The primary aim during exercise should be to top up or replenish your body with what it can absorb and use. Instead of trying to match the total energy expenditure of the event the focus should be on providing the body with what it can actually absorb.

Using absorption/ utilisation parameters rather than the expenditure parameter gives us realistic amounts to guide our refuelling. When it comes to fuelling, more is not better; smarter is better. Knowing your body’s capacities and how it absorbs nutrition is key to sustained performance, particularly at the tail end of your race.

There are good reasons why you should not aim to replace all the calories you burn during exercise. Your body has already built up stores of nutrients from your daily diet that it calls upon during exercise. This approach may also lead to overconsumption of nutrition, more than what the body can actually deal with during exercise. Over-consumption maladies include a variety of GI and muscle system problems that will cause much distress, impair performance, and probably leave you far behind the wisely-fuelled athletes.

So what can you do? As a good rule of thumb, calorie/ water/electrolyte intake will run approximately one-third of expenditure during endurance exercise. For calories, on average, only 30-40% of what is utilised (“burned”) can be efficiently replenished. In general, fluids are replenished at a rate of only 20-33% of what is spent, and sodium 20-35%. These are guidelines and the only way to find what works is to trial and practice it in training. What works for you may not necessarily work for someone else. This is why developing your own nutrition strategy is so important.

If you err on the “not enough” side in regards to calories, that’s a very easy problem to fix–you simply consume more calories. However, if you over-supply your body with too many calories, that’s a much harder (and longer) problem to resolve (at the very least you’ll have to deal with an upset stomach for quite a while).

The table below highlights what your body will burn or use during exercise compared to what you can absorb. The assimilation rate is based off a range to account for different parameters such as athlete build and weather conditions such as humidity.



Fluids (ml)1000-3000 (30-90 oz) 500-800 (17-28 oz)

Sodium (mg)2000 500-700

Calories 700-900 240-280

It can be personal for different people. Lighter athletes may fall to the lower end of the assimilation rates of fluid intake where as larger athletes towards the top of the range. One thing is for sure is it can all be practiced in training and really should be. Have a think back to your last long ride and see if you can figure out where you were between these ranges.

How did you feel during the session? Were there any issues worth noting from a GI perspective? Were you able to finish the session strong?

These are just a couple of questions you can use to see whether you need to tweak a few things.

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