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Fat vs Carbohydrates – Where are you getting your energy from

Just a quick little post for today.

The table below provides some data estimates on the contribution of fats and carbohydrates to the total energy requirement while cycling at 200w for 2 hours

Fat (grams/min) 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Fat (kj) 888 1332 1776 2220 2664 3108 3552
Carbs (kj) 5512 5068 4624 4180 3736 3292 2848
% carbs 86.1% 79.2% 72.3% 65.3% 58.4% 51.4% 44.5%

Now the key to this table is the top row.   People that have poor fat oxidisation capabilities (either genetically or due to being unfit) need to provide a far higher proportion of their energy through carbohydrates (glycogen).  This is important for several reasons:

  1. carbohydrates only provides 16 kj of energy per gram while fat provides a whopping 37kj per gram, and
  2. our bodies stores of carbohydrate/glycogen are very limited (about 15-20grams per 1kg of lean muscle mass – or roughly 500grams or 8000kj or energy).

As you can see from these numbers, someone with a low fat oxidisation rate will have mostly depleted their glycogen stores within 2-3 hrs (depending on intensity), whereas someone with good fat oxidisation abilities (.5g/min or better) will still have 50-65% of their glycogen stores remaining after the same time.   That’s a huge difference and helps explain why those riders are able to put out big efforts towards the end of the race – remember, high intensity efforts are fuelled by glycogen so if you don’t have much left then your ability to generate your best power will be seriously compromised.

NB: The figures in the table are estimates and assume a metabolic efficiency of 22.5%.  The human body is generally only 20-25% efficient, in other words we burn four to five times the amount of energy that we actually put into the pedals.  Each person’s body works slightly differently and the amount of fat oxidised does increase from base levels up to the FatMax point – which is roughly the middle of the E1 zone. However, the fundamentals of the table still hold – which basically are that if you burn more fat at low to moderate intensities you will have better endurance and you will save your carbohydrate/glycogen stores for the most important part of the race – the finish.

Jason Mahoney Wednesday 30 October 2013 at 08:07 am | | Default, News, Race Reports, Rider Info, Tips, Training Sessions, Linkdump

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