A Users Guide to Fueling with Fat and Carbs

A Users Guide to Fueling with Fat and Carbs

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The impact diet has on performance is an important consideration for endurance athletes. We look at the latest recommendations for carbohydrate use, and how to increase fat oxidation.



The body generates energy (ATP) from either carbohydrate or fat. More specifically, the body gets this fuel from:

  1. Ingested Carbohydrate (gels, sports drinks etc)
  2. Intramuscular and liver (hepatic) carbohydrate stores (stored carbohydrate is called glycogen)
  3. Intramuscular and adipose fat stores, i.e. your wobbly bits


These fuel sources each have their own advantages and disadvantages.


A major disadvantage for the ingested and liver carbohydrates; and the fat stores is the rate of energy production. If you had a moderate level of fat burning, the combined energy generation from ingested carbohydrate and fat stores equates to ~480 kilocalories/hour, or about 90-120W on the bike. This isn’t enough for us to get in even one solid hour of training. Which means that we need to bring in the third energy store to generate energy at the rate that we need for a complete training and racing.


For intramuscular carbohydrate stores, the rate at which energy can be produced is much better. Energy can be produced from carbohydrate up to very high levels of intensity. The downside to carbohydrate burning is that the carbohydrate stores in the body are quite limited – somewhere in the range of 1500-3000 kilocalories depending primarily on how well trained you are. Whereas, fat stores can be essentially unlimited, and more than 50 times that amount.


It’s not one or the other but a mix of both that fuels endurance sports. Alan Couzens says “The output for any competitive endurance race…requires some energy to be generated from carbohydrate oxidation, i.e. aerobic glycolysis. This is because fat oxidation is a fundamentally rate limited process, & while enormous in capacity, it is quite limited in power or rate of energy production.


Within the range of specific race intensities (~270-300W) more than half of the example athlete’s energy supply comes from ‘sugar burning’. Again, this is a necessity of the rate of energy (ATP) demand when pedaling a bike at 300W! This is in no way to say that his fat burning isn’t impressive – maxing out at 7kcal/min – in the top 5% of all tests that I’ve done(!) but rather to say that for an Ironman to be competitive, his sugar burning or aerobic glycolytic power must be equally impressive. In this case, topping out at almost 25kcal/min(!) This rate of pure aerobic energy generation is important for all endurance athletes, even those who specialize over longer distances.


This rate is not something that ‘just happens’, it must be trained! In order for GLYColysis to be trained, it requires that the athlete has some glycogen (i.e. sugar) in the system. In the absence of a sufficient amount of carbohydrate intake, this simply won’t be the case & the athlete’s output will be significantly limited.”


This goes some way in explaining why we need both carbohydrates (CHO) and FAT as fuel sources. Let’s start by having a closer look at CHO.




It was discovered that CHO was an important fuel for exercise from the early 1900s. This was reinforced through the 20th century with studies showing that muscle glycogen played a significant role during exercise.


Specifically nutrition during races and high-intensity training sessions. The potential for carbohydrate consumption to improve performance has been extensively researched. A review by of 61 studies on carbohydrate and endurance performance Stellingwerf & Cox (2014) concluded that 82% of these studies demonstrated statistically significant improvements in performance.


There’s no doubt these days that the consumption of carbohydrate before, during and after races is an important element in cyclist’s nutritional strategy. But up until ~2004, it was more the case of just systematically shoving down your throat in any form possible – than a controlled approached. Pasta, bread, bakery stops was the norm and power bars and sports drink the most sophisticated nutritional products. What happened after 2004? There were a series of breakthroughs with regard to feeding during exercise.


Sifting through issues such as absorption-rates and minimum dose and response effects – CHO optimisation during exercise at this point started to be refined. CHO oxidation (oxidation, being to combine or become combined chemically with oxygen. As in, when during aerobic activity) was discovered to increase in a linear fashion in relation to intensity. Before the last 10 years of breakthroughs, it was thought that ingestion of CHO would only increase performance over 2 hours, but recent studies have shown that there is even a benefit during shorter high intensity exercise.


Alright, it sounds like we are all over CHO. And for good reason – it was and still is the number one fuel source for exercise. So what about fat? It’s rep is darker. Evil fat has been given a lower status in society and even lower in sports. Which is funny because it has been fighting for some limelight over the I’d say the past 5 years or so. I know it’s more like 15 years but I’m talking about the latest increase in popularity.




The greatest chatter you might have heard about fat is maximising fat utilisation. Unlocking the intramuscular and adipose fat stores for fueling your cycling (or any endurance sport for that matter).


One thing I want to clear up before we go any further is that you might think this is mostly to do with burning fat as a means to weight loss. But in reality it greater fat utilisation does not equal weight loss. The two terms are often used to describe the same thing. A big part of losing weight the requirement for negative energy balance (consuming less calories than your body puts expends). Without this , here will probably be no weight loss.


Alright, now that’s out of the way.


Why would you consider increasing the rate of fat oxidation as a fuel source?


One main reason could be to keep your CHO until you really need them. The stochastic nature of racing requires you to be ready for an effort of any length of duration. These stores are finite and once they’re depleted they’re gone. Riders often need to generate their highest power in the final kilometres of an event, after many hours of riding, so training and nutrition must enhance the rider’s efficiency, enabling them to resist fatigue and test the extremes of endurance, maximum aerobic power and anaerobic endurance, all during the same event.


Also, being dependent on carbohydrate as the major energy source during exercise has some obvious limitations which were discussed earlier (limited supply etc.), and therefore adapting the body to utilise more of our body fat stores to fuel exercise makes practical sense.


Fat utilisation is different to CHO as intensity increases. CHO oxidation will increase proportionally with exercise, whereas the rate of fat oxidation will initially increase but will decrease again at high exercise intensities. Meaning CHO will start to approach 100% reliance on CHO at 100% VO2max. But if you could increase “metabolic efficiency” and reduce carbohydrate use at moderate intensities. We could save our CHO stores for when they are really needed.


I don’t know whether I’ve convinced you or not – certainly the jury (and science) is still out on fat burning. The poster study on the idea of “train low” which was also the first modern investigation of the effects of reducing muscle glycogen availability on training adaptation and performance was undertaken by Hansen et al. (2005).


These workers studied seven untrained males who completed a rigorous training program of leg/kneeextensor “kicking” exercise 5 d/wk for 10 wk. Both of the subjects’ legs were trained according to a different schedule, but the total amount of work undertaken by each leg was the same: one leg trained twice a day, every second day (LOW), in which the second training session commenced with low glycogen content, whereas the other leg trained daily (HIGH) under conditions of high glycogen availability.


Muscle biopsies taken from both legs before and after the training regimen revealed that resting muscle glycogen content in both legs was similar pre-intervention but was increased in the leg that trained LOW after 10 wk.


The time to exhaustion during single-leg kicking at 90% of post-training maximal power output was twice as long for LOW as HIGH after training. The results of Hansen et al. (2005) clearly demonstrated that in previously untrained individuals, adaptation is augmented by commencing a portion (50%) of training sessions with low glycogen availability, at least for the first 10 wk of a short-term training intervention.


While the findings (Hansen et al., 2005) were intriguing it was not clear if athletes with a history of endurance training would attain the same benefit as untrained, less fit individuals embarking on a fitness regimen and training with low muscle glycogen availability.


Also, the training load in the study was “clamped” such that both LOW and HIGH legs trained at the same intensity: the LOW leg therefore set the “upper limit” for the workload to be undertaken by the HIGH leg. In a “real world” setting, an athlete would produce greater power outputs or speeds when performing intense endurance-based training when glycogen availability was high. Finally, it was uncertain how improvements in one-legged “kicking” time to exhaustion translated (if at all) to dynamic whole-body cycling.

There was a study that showed the train low idea made it possible to fat metabolism in cyclists.


Training with low muscle glycogen enhances fat metabolism in well-trained cyclists.


The study set out to determine the effects of training with low muscle glycogen on exercise performance, substrate metabolism, and skeletal muscle adaptation.


Fourteen well-trained cyclists were pair-matched and randomly assigned to HIGH- or LOW-glycogen training groups. Subjects performed nine aerobic training (AT; 90 min at 70% VO2max) and nine high-intensity interval training sessions (HIT; 8 × 5-min efforts, 1-min recovery) during a 3-wk period. HIGH trained once daily, alternating between 90 min at 70% VO2max on day 1 and 8 × 5-min efforts, 1-min recovery the following day, whereas LOW trained twice every second day, first performing 70% VO2max  and then, 1 h later, performing 8 × 5-min efforts, 1-min recovery. Pretraining and posttraining measures were a resting muscle biopsy, metabolic measures during steady-state cycling, and a time trial.


Power output during 8 × 5-min efforts, 1-min recovery was 297 ± 8 W in LOW compared with 323 ± 9 W in HIGH (P < 0.05); however, time trial performance improved by ∼10% in both groups (P < 0.05). Fat oxidation during steady-state cycling increased after training in LOW (from 26 ± 2 to 34 ± 2 oxygen per kg per minute). Plasma free fatty acid oxidation was similar before and after training in both groups, but muscle-derived triacylglycerol oxidation increased after training in LOW (from 16 ± 1 to 23 ± 1 μmol·kg−¹·min−¹, P < 0.05).


Training with low muscle glycogen reduced training intensity and, in performance, was no more effective than training with high muscle glycogen. However, fat oxidation was increased after training with low muscle glycogen, which may have been due to the enhanced metabolic adaptations in skeletal muscle.


The studies that are out there kind of do a back and forth on this topic. But anecdotal evidence is high. And if you are willing to make some changes you could run your own experiment to see what happens to you and your cycling.


Like anything in cycling – start with a benchmark (and some testing).


How to measure


Breezing – I’ve mentioned this product before. It is a consumer product that can give you all you need to know. The only thing it lacks though – and it’s kind of a biggie – is it can;t figure out exactly where your fat burning zone is. This is important because you need to know how your body is burning fuel at different durations and intensities to know where you have to train for potential adaptations.


There has been a protocol developed to find your Fatmax – the exercise intensity where fat oxidation is maximal. And where you should train to encourage fat utilisation.


It is basically a step test where you start at 95W and increase workrate by 35W every 3 minutes until exhaustion. During the test a breath-by-breath analysis is performed using an online gas analysis system, which uses indirect calorimetry. A technique that provides accurate estimates of energy expenditure from measures of carbon dioxide production and oxygen consumption during steady-state exercise (and rest).The results are highly individual – so I’m not going to give you a specific range to ride at. But I will have a look at what type of training and diet changes have anecdotally shown an increase in fat utilization over time.


There are some doubts out there about this protocol and the Fatmax results. Those studies are still coming up and there is nothing conclusive from either side yet. Right now though it’s the best that we’ve got. So if you can find a place that does this test with the right equipment there is no reason not to use it for your own purposes.

How do you improve your ability to burn fat?


Training and diet. The impact of training enables you to increase the intensity that fat utilisation occurs. While diet impacts the amount of fat used at those intensities.


Let’s start with nutrition.


1. Nutrition


Quoting Alan Couzens (again!)


“Studies have shown that providing caloric requirements are met that an individual’s ‘burn rate’ will match their ‘intake rate’ with respect to Carbohydrate and Fat after a (quite unpleasant) 3-4 day adaptation period.


Very good ultra-endurance athletes typically metabolize less than 40% of their energy from Carbohydrate. Accordingly, approximately 40% of their nutritional intake comes from Carbohydrate.


Additionally, studies have shown that those athletes who burn more fat at rest also burn more fat at all aerobic exercise intensities.


So, if some is good, is more better? Why not shoot for 100% fat burning? While close to 100% fat burning is physiologically possible, it’s not conducive to the requirements of Carbohydrate replenishment that comes with high volume aerobic training.


Additionally, while possible to alter energy substrates to a large degree via diet, there are a number of hormonal and genetic factors which come into play to affect the level of satiety that diets of different compositions of macronutrients provides. For this reason, a vigilant but steady move towards a more moderate carbohydrate diet is suggested.


The efficacy of the 40/30/30 (40% carbohydrates, 30% protein, and 30% fat) diet by Dr Phil Maffetone…is now a growing body of scientific literature is backing him up. Not only this but cut sugar from your diet.This diet creates the right environment to become a high fat burner. Specifically;


a) The concentration of Free Fatty Acids within the blood

This is THE pre-requisite for fat burning at rest and all exercise intensities. In other words, if your blood is full of glucose as opposed to FFA’s, you will not be providing the muscles with any stimulus to ‘learn’ to use fat as a fuel. High FFA levels (and low-moderate blood glucose levels) are a pre-requisite for fat burning. This has LARGE nutritional implications. If you keep your blood sugar levels perpetually elevated, you will never become a fat burner. Period.


b) The concentration of fat-burning enzymes within the muscle.

While short chain and medium chain FFA’s can diffuse into the mitochondria freely, long chain FFA’s must ‘hitch a ride’ with the enzyme carnitine palmitoyl-transferase in order to make it to the mitochondria. A shortage of this enzyme will mean that even if you have sufficient FFA’s within the blood, the long chain ones will be left by the side of the road with their thumb in the air waiting to hitch a ride. This enzyme is inhibited in the post absorptive state when blood glucose is elevated.


c) Mitochondrial content within the muscle.

Of course, in order for FFA’s to be ‘burned’ and used for fuel we need a sufficient number of ‘engines’ to burn them. In this sense, the number of mitochondria within the muscle can ultimately limit the rates of fat oxidation. This is a function of aerobic fitness, which in turn is a function of the number of contractions performed by each muscle fiber, or put another way, as my buddy Chuckie V is fond of saying, miles make champions.


2. Training


While improving FFA availability by altering your habitual diet is necessary, it is not, in and of itself, sufficient to ensure improved fat oxidation during exercise. After you have liberated the fatty acids so that they are ready to be burned, you still need sufficient power plants to burn them.


At low exercise intensities, fat oxidation is largely influenced by the mitochondrial density within each (low intensity) muscle fiber which, in turn, is mediated simply by the number of contractions each fiber performs.


It should be noted that high intensity exercise (moderately hard and greater) that produces larger amounts of lactate also produces a hostile environment for the key transporters of the long chain fatty acids into the mitochondria.


In summary, a moderate carbohydrate, moderate fat, moderate protein diet coupled with a lot of easy-steady training represents the best method for turning yourself from a sugar-burner into a lean mean fat burning machine.”


Interesting hey. I have seen changes in several athletes. And while the performance aspects aren’t spoken about much. The health and intake benefits are. One final note – this isn’t necessarily suggesting you train low. It is more to do with your diet overall than riding in a glycogen depleted state. Two ideas worth trying – and definitely something we need to keep an eye on because this work is ongoing as Team Sky and BC are currently conducting a study to look at the effect on adaptations when using protein during train low rides to avoid the negative sensations associated with training low.


So while we are getting close to nailing down CHO – FAT has some way to go. Like everything that’s out there though. You will have to try it for yourself to know whether it works for you.




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