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Understanding INSCYD Fatmax and Carbmax Combustion Rates

Updated: Aug 21, 2020

I coach cyclists of all abilities and one performance metric i'm always interested in is the riders ability to absorb carbs and use fat for fuel. Riders that have done the INSCYD Power Performance Decoder metabolic lactate test, will have a measurement for both FatMax as well as CarbMax. Both are an indication of how efficient you are at burning fat as well as the efficiency to utilise available carbs.

Over the years FTP has become the go to metric to measure both fitness and to model performance. Except there is one major flaw - FTP is only relevant in the presence of an energy substrate that can generate enough ATP (the fuel you muscles use) to keep up with the demand from your muscles. The only energy source that can do this when you are close to or over your FTP, is CARBS! 

As long as you can burn a high enough amount of carbs, for them to be utilized efficiently in your body, your FTP will be of value. BUT as soon as you run out of carbs, FTP becomes technically irrelevant.

Most of us even pedaling at half or 50% of our FTP will utilize in the region of 50 to 60 g of carbs per hour. As you increase the intensity to anywhere between 80 to 90% of FTP the usage can increase to between 160 g to 200 g per hour.  

The body can only absorb between 60 to 90 g of carbs per hour. The 60 to 90 depends on a couple of factors, what kind of carbs you are using, how efficient your body is absorbing carbs, whether your receptors are blunted due to diet, and how damaged or undamaged the mitochondrial in the muscle are. So to get the body used to absorbing 90 g requires some training.


So if you’re burning at race pace (just below FTP) lets say 180 g per hour,  but can only take in maximally 90 g per hour, that means there is a deficit of 90 g. Luckily the body does store carbs and there is anything between 350 to 500 g, stored both in the muscle as well as in the liver. So as the hours tick by the deficit of 90 g is offset by the stored carbs/glycogen in the body. 

So the best case scenario this will give you 4 to 5 hours of energy before you hit the wall. Again that is assuming you can absorb the high end of 90 g rather than the low end of 60. The ONLY solution at this point is to either use fat as a fuel or to slow down the point where carbs in equals carbs out. Again if you've done a INSCYD test, you know exactly where that is.

As long as we can feed the muscles with carbs we can perform at or close to our FTP. Unfortunately when carbs run out the only energy substrate available to us is fat. Fat is a lot less efficient and hence it’s broken down and utilised at a much slower rate than carbs, which means the bottleneck in the muscles becomes how efficiently we can break down and utilise fat - the more efficient the mitochondria is in using fat, the better the muscles respond!

Anything less than a 3 hour race, FTP is very relevant, and the main metric to consider when trying to simulate performance. However as we get to that carb depletion point of 4 to 6 hours, FTP becomes irrelevant, or rather less relevant and fat utilisation becomes the better metric to consider.

Case Study - 

Two athletes both with the same FTP, both the same weight riding on the same course. In the past the assumption would be made that both athletes should finish together because of there FTP. However, if one athlete can utilise fat better, it means firstly they are able to conserve the carbohydrate stores in the muscle, and secondly once those are depleted that the better utilisation allows more energy to be available for the muscles to perform. 

In summary the athlete with a higher fat utilisation, would be the athlete that performs better in a longer distance event. 

Have a look at the example below, these are real numbers looking at two athletes that have a similar watts/KG - 

  • athlete one: 3,8 w/kg, FatMax – 5.3kcal/hr/kg, CarbMax – 2.6 w/kg (that’s 21km/h on a 2% gradient)

  • athlete two: 3,6 w/kg, FatMax – 6.3kcal/hr/kg, CarbMax – 3.1 w/kg (that’s 23km/h on a 2% gradient)

So athlete one has a higher watts per kg, so would be expected to be the better performer. However his fat utilisation is only 5.3kcal,  and the maximum effort he can put out once his body has been depleted (with taking in 60 to 90 grams of carbs per hour) is 2.6 w/kg.

Now look at athlete two, a lower FTP w/kg, but much more efficient in utilising fat (and conserving carbs). Once they run out of carbs, they can keep a pace of 3.1 w/kg.

That’s a massive difference! And by the way athlete two is a woman! 

Annamiek Van Vleuten’s Mitchelton-Scott squad have released a new video that details the Dutch rider’s daunting training regime this winter, which saw her ride with the men’s squad during their camp in Italy.

The 14-minute film, ‘One of the boys’, is the first episode of ‘What it Takes,’ a documentary series made by Mitchelton-Scott focused around Van Vleuten.

Van Vleuten first joined the men’s squad during their now-famous point-point- training camp in January 2019, after being challenged by women’s performance manager Gene Bates.

So who would you bet on winning a 2 hour race and who would you bet on winning a 20 hour race??

So as you can see FatMax becomes a very relevant metric with regards to performance in long distance events.

The next question you should be asking is can FatMax be trained? Fortunately the answer is yes!

  • By knowing where your FatMax is, you can focus on training there and improving your fat utilization efficiency.

  • Fasted rides have to be part of your training (NB: not all rides fasted, the fasted rides need to be periodised into your training).

  • Grow that aerobic engine, the bigger the aerobic engine the more fat is utilised and the less carb dependent you become

  • Stay away from anything that will stimulate and improve your anaerobic engine (they just burn carbs), and last but not least

  • Diet…..reduce those carbs! Or rather train low, race high

I hope this helps you get a better understanding of how important fat utilisation is in long distance events.

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