Training Science Series #7 - The New Science of Fat Adaptation

ATP (Adenosine triphosphate) is what provides energy to drive many processes inside living cells, such as muscle contractions. What determines your level of endurance is your ability to sustain rapid ATP production to fuel your cells.

Training Science Series #7 - The New Science of Fat Adaptation

ATP (Adenosine triphosphate) is what provides energy to drive many processes inside living cells, such as muscle contractions. What determines your level of endurance is your ability to sustain rapid ATP production to fuel your cells.

This is an article series designed to help further educate my Performance Coaching clients, but also anyone interested in learning how to train more successfully. If you are interested in getting fitter -- irrespective of whether you are a novice or regular athlete -- then please read through this series and learn more about the endurance training process. I also hope to welcome you onboard as your performance coach one day if you need mentoring to reach the summit of your athletic potential!

Understand the Process to Commit 100% to the Process


ATP (Adenosine triphosphate) is what provides energy to drive many processes inside living cells, such as muscle contractions. What determines your level of endurance is your ability to sustain rapid ATP  production to fuel your cells. Ultimately, endurance training is aimed at improving your ability to sustain a high rate of metabolic turnover, which can be done my making the metabolic energy pathways more efficient.

We have metabolic systems designed to resynthesize ATP to constantly fuel the body. More ATP means more muscle power, and the faster ATP is resynthesized the more work an athlete can do per second. As intensity goes up, so does the demand for more ATP, increasing the demands for muscle glycogen to help replenish our ATP stores. When energy is pulled from the muscle glycogen stores, it creates a molecule called pyruvate along with two ATP molecules. While the ATP molecules are used for energy, the fate of the pyruvate molecule is crucial to the endurance performance of an athlete.

Pyruvate can go one of two ways: either re-entering the slow-twitch muscle fiber mitochondria where it can be resynthesized for further energy production, or it reaches a dead-end because its rate of production exceeds the ability of the slow-twitch fibers to uptake it—so it gets converted into lactate. Lactate increases the acidity of cells, forcing a slow-down in glycolysis, thus you must slow your pace. All athletes know this feeling when you hit your maximum capacity, the legs get heavy, the breathing laboured and you have to slow-down a lot.

At lower intensities, the body can utilise its fat stores to sustain near constant ATP fuelling along with recycling any pyruvate that is created from any needed glycolysis. Therefore, an athlete can perform without producing any lactate. The more aerobic capacity you have, the higher threshold you develop for holding off the production of lactate. Furthermore, using lipolysis (burning bodyfat) for fuel, does not produce any pyruvate or lactate, making it the optimal fuel source for endurance efforts.

If you can encourage greater fat adaptation through training, you can use lipolysis to fuel longer (and faster) endurance efforts, thus increasing your overall Lactate Threshold (LT)(explained more later). Briefly, the LT is essentially your endurance limit, and being able to increase the speed you can run at the lactate threshold results in improved endurance performance.

When someone has a history of high-intensity training, they are not training the aerobic metabolism to optimise fat fuelling, but predisposing their metabolism toward sugar burning (glycolysis). Therefore,  they will not be aerobically adapted to sustain higher speeds from more of their fat stores and their aerobic system will have little capacity to recycle all the pyruvate being created from the glycogen fuelling. The outcome is diminished performance because of an increased lactate accumulation experienced much earlier than an aerobically efficient athlete feels.

Becoming a fat adapted athlete is also important from a health reason because glycolysis is a lot more damaging to the body than fat metabolism. Glycolysis produces higher levels of free radicals, called Reactive Oxidative Species (ROS), that can cause problems at the cellular and genetic level. Fat metabolism produces less ROS, thus resulting in less cellular damage over time. This is why utilization training forces an athlete to increase their recovery time following those types of workouts, because the body has more damage to repair. Because of this, fat-adapted athletes bounce back from high-intensity efforts far quicker than those who are not adapted, because of less ROS damage.1 If this point alone, doesn’t convince you to make capacity training your major focus, then nothing will.

But how does that work you might ask? Isn’t a fat adapted athlete still using primarily glycolysis at higher intensities, so they’d be creating the same amount of ROS? The answer is no, they produce less ROS. For years, scientists thought the slow rate of ATP synthesis in the aerobic system wasn’t capable of sustaining higher intensities. One previous assumption was that once an athlete reached the lactate threshold, fat metabolism ceased to contribute any energy to exercise demands. However, newer studies reveal fat usage can be sustained even into maximum intensities, BUT only in highly-trained athletes. Uphill Athlete explain:

Coaches of endurance athletes, across a broad spectrum of sports, have known for many years that well-trained athletes can utilize fats better and at higher (even maximum) intensities than studies indicated was theoretically possible. It was assumed by coaches this this adaptation was an important key to success in endurance sports. But research didn’t support the coaches’ experiences. Several new scientific studies now substantiate the coaches’ beliefs.  For decades, exercise scientists have told us that fat played a significant role in total energy use only at low-to-moderate-exercise intensities and that fat’s contribution was theoretically capped at a paltry amount. This meant that energy demand for high-intensity exercise could be met only with carbohydrates. They were partly correct: this is what happens in the general population and, to a lesser extent, in the recreationally trained athletes who were the subject of their tests.2

In other words, it is only more advanced athletes with a history of capacity training who can begin to use fat-fuelling at very high intensities (speeds). The scientists were never studying these more efficient athletes in those original studies, thus concluding everyone was in the same boat when it came to fat utilization. It is amazing how well the body can adapt itself to the demands placed on it!

The benefit of being able to increase reliance on fat stores when running at higher intensities means lower lactate accumulation and ultimately more running economy at higher speeds. Fat burning produces little to no lactate accumulation, making it the most desirable fuel source.

Ultimately, the more developed your aerobic capacity is, the better your high-intensity utilization training intensities and speeds will be, and the faster you will recover from those sessions. This point should be obvious. Even compared with a light-weight cross-fitter with a similar level of anaerobic fitness who solely trains hill sprints through utilization training, Kilian Jornet, will still win in a hill sprint race – even without a lot of anaerobic training – because his fat burning metabolic pathways are way more efficient. Meaning he doesn’t burn through as much muscle glycogen (sugar), as the other athlete at the same sprint speed, thus he produces less lactate accumulation because of less pyruvate produced, and a larger aerobic vacuum to resynthesize the pyruvate into ATP rather than it turn into lactate. Ultimately, the other athlete will slow down from lactate accumulation before Jornet will EVEN if their anaerobic capacities were evenly matched.

Jornet is highly fat adapted athlete due to the way he trains, which is partly not even deliberate because he often doesn’t feel like eating before he heads outs for his morning training. He has been known to sustain extremely long duration exercise (up to and over 24 hours) on less than 500 calories of fuelling. For some fat-maladapted athletes, that would only supply enough fuel to power through a two-hour workout before they would “bonk.” Kilian won’t bonk until probably over 30 hours or more of fasted training, because he can tap into his fat stores more efficiently. Ultimately, long-duration capacity training enhances fat adaptation, and is boosted further by doing some of your lower-intensity workouts in a “fasted” mildly depleted glycogen state. This forces the muscles to make better use of available fat for fuel.

This was noticeable to me when I raced in the Mont Blanc Marathon in 2015, a net-uphill 42km race with 2,700m of climbing. I remember Jornet explaining how he only needed 2 gels to fuel himself when winning this race. Despite finishing 45 minutes behind Jornet, I remember having to fuel myself with over 12 gels in the same race. I wondered for some time afterward, how he could possibly go so fast in the race while consuming far less calories than I required. Since I had done most of my training geared for short VK races, I was more reliant on muscle glycogen and sugar for fuelling, way more than Jornet was. While I was needing to regularly top up my glycogen stores in this race, Kilian was able to draw upon the efficiency of his aerobic capacity to fuel most of the race from his available fat and muscle glycogen stores. Because the Mont Blanc Marathon is a relatively short duration race, the duration of less than 4 hours is actually just a typical training day in volume for Jornet (although his intensity is much higher in the race), so the demands of the race were will within his endurance capacities and well outside mine.

The body’s fat stores hold hundreds of hours of stored energy (~100,000 calories), but our muscle glycogen stores (~2000 calories) can be depleted in less than a few hours. It is hard to break free from the grip that sports supplement companies have on ensuring you are constantly fuelling with their products during your workouts and races for optimal performance. It is not until an athlete experiences what fat adaptation feels like, before they truly get on board with the idea. That’s not to say you don’t want to be fuelling during races, its just that your requirements for fuelling optimal performance are greatly reduced. Just having to carry less food and water weight while you run should be motivation enough to optimise this capacity.

 Poorly fat adapted athletes typically struggle to go more than an hour without needed to eat something and often start races with large amounts of food and water, further slowing their performance due to the added weight they have to carry. Kilian is renowned for keeping himself lightweight – he doesn’t like to carry food or water unless absolutely necessary allowing him to run freely and faster because of nothing to carry.

You don't need to mess around with extreme diets to become fat adapted and doesn’t mean eating massive amounts of fat either. Instead, it means training in a way that uses your already existing bodyfat stores to fuel more of your endurance performance. There are a handful of ultra endurance athletes (i.e. Zach Bitter) who are performing well with a very low-carb ketogenic diet; however, to balance this argument, Kilian Jornet explains on his blog that he eats a high-carbohydrate vegetarian diet. On 10th June 2020, he writes when asked about his diet, “Mostly carbs, vegetarian. Bread or granola for breakfast, normally a sandwich for lunch, or and carbs and veggies, and salad in the evening ( pasta, potatoes, rice… and veggies). Never count any calorie."3

Extreme ketogenic diets can help you to burn more fat, but the reduction of carbohydrates to replenish muscle glycogen can mean the frequency and intensity of your training will ultimately suffer. While possible to implement with a lot of discipline and patience, ketogenic diets are hard to maintain. Ultimately, both high fat/low carb and high carb/low fat diets are taking extreme approaches, and both can potentially lead to considerable imbalances for optimum performance in ultra-endurance athletics, and possibly long-term health unless they are meticulously executed (which can add more stress to daily life). The studies are minimal and there aren’t any significant numbers of ketogenic world class athletes overwhelming all the other athletes eating normal diets. Until then its best to err on the safe side, a balanced diet is time-tested to work. Jornet shows you don't need to have a ketogenic diet to be a successful fat adapted athlete. As long as you eat sufficient levels of high-quality fats for immune system health, along with liberal amounts of carbohydrate in the diet, then you are good to go.

As to how to do it in training – the goal is to try it out, but not overdo it every time you train. It also depends on the objectives of the workout. An important aspect of training fat metabolism is to train occasionally when the body is depleted in carbs. This can best be achieved first thing in the morning by training before having anything to eat. Various research does suggest occasional long runs in a fasted state will improve glycogen storage and fat utilization, but counters that by saying, a lot of extended training in the fasted state will eventually impair performance and won’t provide benefits to fat utilization. Uphill Athlete suggest:

Initially we recommend doing only basic aerobic (Zone 1) workouts in a fasted state. For most people, this will be one of your shorter aerobic workouts each week. Once you can handle these fasted workouts without feeling a craving for carbs, add a second, longer fasted workout in your week. The process can be extended for many people until nearly all aerobic (Zone 1 and 2) workouts are done fasted.

You will still want to fuel on carbs before and after high-intensity and strength workouts so that you can get the best training effect out of this important training. This is an important point that many people miss. Understand that your performance during these workouts is heavily dependent on the available glycogen stores. If they are not topped up prior to that interval session, your training time will not be as effective as it could be.4

Also important to know, is that when you are training for specific events, you also have a finite number of workouts to which you can gain fitness increases. If all your long-runs are fasted, then you may not always be completing the runs to the best of your ability. By implementing both glycogen depleted and glycogen fuelled runs, you improve fuel efficiency while maintaining consistency in training.

When deciding how much carbs to fuel on long training runs, ultimately, the preferred approach would be as little as possible to prevent significant performance decline. That means for shorter runs you don’t need to eat, but on longer runs, you will need to eat occasionally. Jornet does eat occasionally on some of his training days (foraged berries, an energy bar or an apple), so you should optimally include a mix of fasted and fed long runs in your training plan. However, you can gradually reduce the amount of carbohydrates you ingest during your long-runs as time goes by (you should always take more fuel than you will need on training runs just in case you have a bad day and need more fuel than usual). Over many months you should begin to notice improvements in speed and endurance as fat metabolism becomes more efficient in your physiology.

It is advised that when you start to feel your energy begin to dip, then this is NOT the moment to take in more fuel if you want to teach your body to reach for greater fat adaptation. Only when you start to feel distinctly light headed – and start to suffer a noticeable loss in performance – should you then fuel with 50-100 calories, but do not expect these types of runs to be at your best. Dried fruits (dates) would be a preferred fuel source over faster-burning gels or energy syrups in these fat adaptation training runs.

When you want to practice race pace, develop speed, or do any form of utilization training, you should always eat before and (if needed) during training. For speed the body needs to be well fuelled.

Race day is when you want to fuel your body for maximum performance and that means carbs, carbs. Carbs (energy gels and syrups). You will want to head into a race with a good breakfast and will want to fuel with a constant supply of fuel and water during the race. In the weeks before race day, doing some fuelled runs where you practice your race strategy will also help to acclimatize your stomach to processing simple sugars and fluids. The inclusion of sugar fuelling, on top of a highly efficient fat-burning aerobic base, is how to elevate your performance to the highest level. This process is often called the “train low, compete high” strategy.


Next Article -> Part Eight- Introduction to Threshold Training



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