• Boris Clark

Low carb racing. The math to why it doesn’t work.

Updated: Mar 24, 2020

Over last years Tour de France there was a lot of buzz around ‘ketone esters’. For some reason there seemed to be a lot of controversy around these supplements (perhaps because of their price, around $50 a serve). The reality is, all they are is an extra fuel source, essentially a more readily usable kind of fat (not exactly but let’s keep this simple!).

So, ketone esters may not really be that much of a big deal after all, but the buzz around them did get people talking about low carbohydrate diets and ketogenic diets, and how Tour de France cyclists might be using these strategies during the race.


Let me explain why I can guarantee you that this is not the case.


Jumbo Visma has admitted they use ketone esters with some riders, but they certianly don't use a ketogenic diet


A brief explanation of nutrient metabolism

When we exercise, we burn a combination of fat and carbohydrate (and a small amount of protein). The more power we produce, the more energy we require to fuel this activity, we get our energy from food which we have stored as either fat, glycogen, or in blood as blood glucose.


To get fat from adipose tissue into the muscle cell is quite a long slow process and requires a lot of oxygen. On the other hand, glycogen (stored carbohydrate in the muscle), or blood glucose are much faster to use. We can break down carbohydrate into lactate (or pyruvate technically) creating a small instant release of energy, then use the lactate with oxygen (but less oxygen than fat required) to get even more energy.


As we produce more power, we require energy faster and faster, this means we lean more towards carbohydrate oxidation, and less to fat oxidation. At a certain point (probably around lactate threshold), we burn essentially no fat, and all our energy is derived from carbohydrate sources.


Why does this mean low carb won’t work? The math.

A reasonable size individual on a full blown ketogenic diet, with an outstanding aerobic capacity MIGHT be able to oxidise 2 grams of fat per minute, a more realistic figure would be 1.5-1.6 grams per minute for most ‘fat adapted’ athletes, but lets go with 2 grams per minute just to give the benefit of the doubt. Smaller individuals will likely have lower fat oxidation rates than this (due to lower aerobic capacity and nutrient turnover in absolute terms).


There is 9Kcal of energy in one gram of fat. Therefore, this means an individual oxidising 2 grams of fat per minute has 18 Kcal to play with.


A watt is a joule per second. Therefore, say we want to ride at 300w (a medium pace for most A or B grade cyclists of around 70kg), then this means we need 300 joules per second.

A human on the bike has a mechanical efficiency (how much of the power produced by the body is actually reaching the pedal) of around 23%. Therefore we can take our 18 Kcal * 0.23 = 4.14Kcal per minute. The convert this to joules to be the same units as our power output. The formula to get Kcal to joules is (Kcal * 4.18) * 1000. SO we have (4.14*4.18) * 1000 = 17305 joules. This is our joules available per minute for energy. But remember, a watt is a joule per second. So we divide that 17305 joules by 60 seconds, and we get 288.


So, with our absolutely maximal, pretty crazy high fat oxidation rate of 2g per minute, we can ride at 288 watts. To do that we have almost no carbohydrate metabolism, or any available to metabolise, but let’s suggest we do get a small amount which can bump us up to that 300 watt mark. So what? 300watts might be a solid ironman pace or something like that. But you aren’t going to ride up a mountain pass in the Tour de France at less than 300w, and the last effort you may average 400w for say 30 minutes plus! Not to mention you may need to attack or cover attacks with surges of 5-700w! We have just proven that this sort of power output is impossible to fuel from fat metabolism alone.


With carbohydrate however, we can oxidase pretty much as much as we need to, so we are less limited by fuel availability.


Now, you may say, “but 300w is a lot of power for me, I’d be more than happy with that much”. If 300w is a solid effort for you, then your fat oxidation is not going to be anywhere near 3 grams per minute and you will have the exact same problem we just mathematically described above, but with smaller numbers.


Froome with some sort of rice cake. It's impossible to race, let alone win, the Tour de France without an ultra high carbohydrate intake

The problem with carbohydrate

A ‘typical fit male cyclist’ of say 70kg and <10% bodyfat might have 2000Kcal of glycogen on board if he’s is lucky, but his fat stores are almost infinite (You would have many more problems other not being able to ride your bike fast if you were getting critically low in fat!).


At 300w, a club ‘B grade’ rider might be around lactate threshold, and an ‘A grade’ rider may not be too far away from it, and will therefore be burning mostly carbohydrate. Taking the same numbers above, it doesn’t take long to figure out that if you are riding at around 300w, using around 18Kcal of glycogen a minute, that 2000Kcal of glycogen will only last around 2 hours. We can supplement with up to 90 grams an hour of exogenous carbohydrate in the form of gels, bars, sports drinks etc, but this will only buy us another 720Kcal of carbohydrate, or 40 minutes at this intensity for our ‘B grade’ rider.


What to do

Burning fat only doesn’t provide enough energy, carbohydrate is a limited fuel source which we will easily run low on, so what should be do?


The answer is, train to burn more fat, while maintaining carbohydrate combustion ability.

There are a few key ways of doing this.


1. Raise aerobic capacity.


The closer we are to a lactate equilibrium, the less fat we will burn. By raising aerobic capacity we increase the intensity at which this production and combustion of lactate is equal and stops fat combustion in it’s tracks, and therefore can ride at a higher power output while still burning some fat


2. Lower the glycolytic rate


By reducing the rate of glycolysis, we reduce the amount of broken down carbohydrate (lactate) in blood circulation ready for combustion, therefore the body has to find energy from elsewhere, which will be fat stores. Depending on the rider in question this may not be advisable however, as this adaptation may significantly impair an individuals ability to perform all out maximal efforts such as sprinting or attacks.


3. Train fat combustion through fasted rides or training around ‘fatmax’


Training with lowered carbohydrate availability will increase the amount of fat utilised for fuel. Also avoiding carbohydrate on a ride, or for breakfast will reduce carbohydrate combustion (if we eat carbohydrate, we start to burn more of them), which by default increases fat combustion for a given intensity. Training at ‘fatmax’ (The intensity at which an individual’s fat combustion rate is highest as determined through metabolic testing) is another useful way to increase fat oxidation.


Both Chris Froome and Bradley Wiggins have used low carbohydrate strategies in training, but when they put the hammer down, the effort is carbohydrate fueled.

One final note

Increasing fat oxidation through these methods is all well and good. But don’t neglect sessions with high carbohydrate availability (i.e. eating breakfast and eating a lot of carbohydrate during the training session) as both carbohydrate and fat combustion are really ‘use it or lose it’ adaptations. If you focus solely on fat burning you will have a hard time utilising carbohydrate for quick energy, and will also have reduced transporters in the gut to absorb carbohydrate which can lead to bloating or cramping when you consume carbohydrate while training, while too much focus on making sure you maintain carbohydrate combustion ability will see your fat oxidation suffer, meaning you may run out of energy to soon and ‘hit the wall’.


As always, if you want help with this in your training, send us a message, drop us a comment, and we will be more than happy to help!