Nutritional manipulations for training. Part 1 of 6:
What are fat oxidation rates and why do they matter?
Thomas Solomon PhD.
21st Nov 2020.
Off the bat, I will make a disclaimer: My interpretation of the evidence to date is that high carbohydrate availability is essential for maximizing your endurance performance. This is not to say that I am pro-high-carb or anti-low-carb, nor do I make statements that any single approach is “best”. But, I will be bold in saying that, having carbohydrate “available” at opportune moments is key and layering this on top of training that maximises your “fat adaptation” is golden... Right, to find out what all this jargon means and to become well-versed in the underlying evidence that supports my interpretation, stay with me on this series of articles as I plunge you into the world of bioenergetics, fat and carbohydrate oxidation rates, nutritional manipulations for training, and nutritional strategies for performance.
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Reading time ~20-mins (3800-words).
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Humans are a bit like cars. Cars need fuel to make a journey and when fuel levels are low they need to be topped up before a new journey can begin. But, unlike cars, which do not use any fuel when they are parked, our energy expenditure continues rocking even when we sleep.
Our body primarily breaks down fats and carbohydrates to produce chemical energy (ATP). Proteins are also broken down and metabolised but they contribute negligibly to total daily energy expenditure and energy production during exercise (~5% of total, except in extreme conditions, like starvation or disease). In the context of exercise, the chemical energy (ATP) is converted to kinetic energy — chemical energy extracted from food is used to fuel work in the form of accelerating and moving.
Fats are broken down to fatty acids while carbohydrates are broken down to glucose. Of course, we eat food that contains fats and carbohydrates but our body also has a mahoosive store of fatty acids (~10 kg in a 65 kg person with 15% body fat, stored as triglycerides) and a pretty sizeable store of glucose in various tissues (~100 grams in liver glycogen and ~400 grams in muscle glycogen). Plus, muscle has its own store of fat (intramuscular triglyceride, IMTG), and there is an immediately-available but small pool of fatty acids (0.01 grams) and glucose (~ 4 grams) circulating in the blood.
So, we have an abundant supply of “metabolic substrates” from multiple sources that can be used to produce ATP via several energy metabolism pathways, including glycolysis, fatty acid beta-oxidation, the TCA/Krebs cycle, and oxidative phosphorylation.
As a biochemistry student, I was educated in the fundamentals of metabolism with the classic Lubert Stryer biochem textbook. Many moons on, said book still sits on my desk just an arm's reach away. Inside this “bible”, is a table laying out the rates of ATP synthesis from the various metabolic substrates in the body. It is intriguing because, despite the colossal amount of fat stored in the body, its rate of energy production is terrible.
Fatty acid oxidation produces ATP at a rate of 1 mM per kg of muscle tissue per second.
Oxidative glucose oxidation produces ATP at 2 mM/kg/s. (= aerobic; glycolysis plus mitochondrial oxidation)
Nonoxidative glucose oxidation produces ATP at 4 mM/kg/s. (= anaerobic; glycolysis)
Phosphocreatine breakdown produces ATP at 9 mM/kg/s.
These rates of energy production are generally inversely related to the size of the stores of each substrate — the amount of stored fat is colossal and lasts several weeks, while the amount of stored phosphocreatine is tiny and lasts just a few seconds — explaining why we cannot sprint a marathon.
Back in the Oasis-infested era of the late ‘90s, as a hormonal nerdy 19-year-old student I was fascinated by two things: there’s a heck of a lot of fat in the body but its rate of energy yield is relatively low, and the frequency that Wonderwall played on the radio. In the years that passed, Liam and Noel Gallagher fortunately piped down but Lubert Stryer stayed in vogue and continues to have a great influence on all our lives.
A crash course in bioenergetics.
The fate of metabolic substrates through energy production processes — or “bioenergetics” — is described by stoichiometric equations. In biochemistry, we call standardised measures of molecules, a “mole”, which is an odd word, so I will use the phrase “measure” from this point forward. The stoichiometric equation for glucose tells us that when 1 measure (1 mole) of glucose (equivalent to ~180 grams) is metabolised, it uses 6 measures of oxygen to produce 6 measures of carbon dioxide and approximately 30 measures of ATP.
C6H12O6 + 6⋅O2 → 6⋅CO2 + 6⋅H2O + 30⋅ATP
While carbohydrates are always broken down to produce glucose, fats are a little trickier since they are broken down into several different fatty acids of varying size, many of which can enter energy-producing pathways. Stoichiometric equations for fatty acids typically use a “model” fatty acid like palmitate to represent the “average” fatty acid metabolised, but recent approaches use a blend of model fatty acids representing the most abundant ones in the body. For simplicity, I will use palmitate as the model.
C16H32O2 + 23⋅O2 → 16⋅CO2 + 16⋅H2O + 106⋅ATP
The stoichiometric equation for palmitate tells us that when 1 measure of fatty acids (equivalent to ~256 grams of palmitate) is metabolised, it uses 23 measures of oxygen to produce 16 measures of carbon dioxide and approximately 106 measures of ATP. Note that if i had used the blend of fatty acids as the model, 1 measure (= ~272 grams) of fatty acids uses 24.4 measures of oxygen, producing 17.3 measures of CO2, meaning that the RER is about 0.71 and the bioenergetics are very similar to palmitate.
Don’t worry if this sounds confusing. There is a purpose to me presenting this complexity and a very simple summary will follow. So, hold on tight as we jump back on the nerd train...
These bioenergetic formulae are derived from the total sum of the free energy change (known as Gibb’s ΔG) at each enzyme step of the metabolic pathways through which glucose and fatty acids must dance along to produce ATP. The calculated free energy change for the hydrolysis of one measure of ATP (into ADP and phosphate) is −30.5 kJ/mol (at standard temperature and pressure). Thanks to one hero of history, Avogadro, we also know that 1 measure of O2 occupies 22.386 Litres of space (at standard temperature and pressure). Note that cellular conditions in the human body have a higher temperature than STP and fluctuate during the day, therefore gas volume and energy yield estimated do vary from those calculated at STP.
Yikes. Why the heck am I telling you that?
Well, let’s take one final look at the stoichiometric equations…
To extract energy from 1 measure of glucose, you need 6 measures of oxygen, which is equal to ~134 Litres (= 6 measures ✕ 22.386 Litres). Since 1 measure of glucose contains 180 grams, we can calculate that 1 gram of glucose uses 0.74 Litres of O2 to produce energy (134 Litres ÷ 180 grams).
To extract energy from 1 measure of fatty acids, you need 515 Litres of oxygen (= 23 measures ✕ 22.386 Litres). Since 1 measure of fatty acids (palmitate) contains 256 grams, we can calculate that 1 gram of fatty acids (palmitate) uses 2.01 Litres of O2 to produce energy (515 Litres ÷ 256 grams)
We know that 3.74 kcals of energy are released per gram of glucose and that 9.75 kcals of energy are released per gram of fatty acids. Therefore, we can calculate that the metabolism of glucose produces 5.05 kcal of energy per Litre of oxygen (= 3.74 kcal/g ÷ 0.74 L/g) while fatty acids produce 4.85 kcal of energy per Litre of oxygen (= 9.75 kcal/g ÷ 2.01 L/g).
In other words, glucose produces more energy (ATP) per litre of oxygen and fatty acids need more oxygen (i.e. a greater VO2 during exercise) than glucose to produce the same amount of energy.
Crikey, Solomon! Chill. Chill!
I understand that mathematics is not everyone’s cup o tea. Fear not, you do not need to remember the complexity of the stoichiometric rules. I have described it so I can highlight five important facts about bioenergetics:
We humans store a lot of fat. Fatty acids produce more energy (ATP) per gram than glucose. Fatty acid metabolism is slower to produce energy than glucose. Fatty acids are a less economical fuel than glucose, requiring more O2 to produce ATP, meaning glucose produces more ATP per litre of oxygen. Bioenergetic processes are hardwired in our biochemistry — we cannot change them — we can only influence the relative proportions of fuels being used.
OK. Now you have a taste of biochemistry, I will start putting things into the context of training by answering the first important question.
What are fat oxidation rates?
Ever since the 1920s, it has been possible to estimate the kcal/min rate of energy expenditure and the gram/minute rates of fat and carbohydrate oxidation at rest and during exercise by measuring the rates of oxygen consumption (VO2) and carbon dioxide production (VCO2). Small tweaks have been made to the calculations over the last century — if interested, I can recommend Keith Frayn’s seminal paper and the logical update from my old office mate and friend, Gareth Wallis — but the tech for estimating such metabolic rates sits in every exercise physiology lab in the world. Yes, these energy expenditure estimates have a small margin of error within and between the various calculations used (as superbly demonstrated in runners by the athlete and scientist, Shalaya Kipp), but the bottom line is that scientists can easily estimate the fat oxidation rate during exercise and it tells us how many grams of fat are being metabolised per minute.
At rest, fat oxidation rates are pretty high in comparison to carbohydrate oxidation rates — your respiratory exchange ratio (RER), which is simply a ratio between VCO2 and VO2, is around 0.8. But these oxidation rates and your RER do not sit at the same level all day long, they fluctuate because of changes in things like body temperature, time since your last meal, the macronutrient content of your last meal, and even specific nutrients (like caffeine). Fat and carbohydrate oxidation rates are also influenced by your level of cardiorespiratory fitness (VO2max), your habitual diet, and your biological sex.
During a workout, the absolute levels of fat and carbohydrate oxidation rates and the ratio between them are influenced by the same factors plus your exercise intensity, exercise duration, and what you eat during the bout. A 2022 modelling analysis by Jeffrey Rothschild and colleagues found that RER decreases (more fat oxidation) with greater exercise duration, dietary fat intake, age, VO2max, and percentage of type I muscle fibres, and increases (less fat oxidation) with greater dietary carbohydrate intake, exercise intensity, carbohydrate intake before and during exercise, and being male. But, the modelling only explained up to 59% of the variation in RER, meaning that we are far from knowing everything! What we do know is that during progressive-intensity exercise, energy expenditure increases (i.e. VO2 increases) but so too does VCO2, pushing the RER value (VCO2 divided by VO2) up to and sometimes exceeding 1. What this means is that as your demand for ATP increases with the increasing workload, there is a natural progression from predominantly using fat as a fuel source to predominantly using carbohydrates. The point at which you switch from predominantly fat to predominantly carbohydrate is known as “the crossover concept”, first coined by George Brooks in 1994. Remember the stoichiometry — fatty acids are slower to produce ATP than glucose and they use more oxygen to produce ATP than glucose — theoretical bioenergetics indicate the logic in your body tapping into the more economical fuel to rapidly produce ATP when energy demand is high and VO2 is pushing towards your max capacity.
Now you may be wondering why are fat oxidation rates so important for an athlete. Well, this is related to a nice chap called “fat max”.
Who is fat max?
Fat Max is the guy who visits gyms far and wide, labelling equipment with “fat burning zones”. Well, perhaps not. In reality, as a noun, “fat max” is the relative exercise intensity at which your maximal fat oxidation rate occurs. In the early 2000s, in the very lab I was then doing my PhD, Michele Venables and Juul Achten embarked on an ambitious series of studies, first to develop a protocol for measuring fat max and then to examine fat max in a large cohort of 300 healthy men and women. They found that, on average, the maximal rate of fat oxidation occurs at 48% of VO2max, equivalent to 62% of HRmax, arising at a lower fraction of VO2max in men than in women. Men also had lower fat oxidation rates in general (when expressed per kg of fat-free mass) and shifted to predominant carbohydrate use at lower fractions of their VO2max than women. Furthermore, a higher physical activity level and a larger VO2max were associated with a greater fat max.
This sounds pretty cool and you might be thinking, “right, I need to train a lot at 48% of my VO2max or 62% of my HRmax to maximise my fat oxidation rates”. Stop having that thought right away! The most poignant observation Michelle and Juul made was the ridiculously mahoosive range of intensities at which maximal fat oxidation rates were recorded: among the 300 folks examined, fat max occurred at anywhere from 25 to 77% of VO2max, or 41 to 91% of HRmax. This variability has been confirmed in a cohort of trained endurance athletes. So, if you only remember one thing about fat max, remember this: the exercise intensity at which maximal fat oxidation occurs is massively variable between people.
Juul also went on to examine the reliability of these measures by repeating the tests twice in 55 young, endurance-trained men, finding large day-to-day variability in fat max (~10% within-subject daily variation). This observation holds true in larger and more heterogeneous cohorts. For example, in 2020, Javier Gonzalez’ lab found that day-to-day variability in peak fat oxidation and fat max had more than a 20% within-subject daily variation. His group also found that this large variability was the same between men vs. women, or fit vs. unfit, or festively-plump vs. lean. So, if you are able to remember one more thing about fat max, remember this: the exercise intensity at which maximal fat oxidation occurs is not only variable between people but is also hugely variable within people, day-to-day.
What all this means is that a fat burning “zone” is impossible to precisely pin down and, because fat oxidation rates are never zero, all intensities of exercise are in fact “fat burning” because fatty acids are always contributing to ATP production when we are alive. Nonetheless, many folks are interested in their fat max. So the next logical question is,
Why is having a high fat max important?
You now know that glucose is more economical than fatty acids. Per gram, glucose requires less oxygen than fatty acids to produce ATP and produces more ATP per litre of oxygen consumed. Glucose also produces ATP at a faster rate than fatty acids. But, you have a limited supply of glucose that, when your engine is revving high, you plough through like a hot knife through butter. Sparing glucose is, therefore, very useful — save glucose for when you need it most, which is, when things get hard and fast!
Because the human body stores many thousands of kcals of energy in fat, you are not in danger of running out of fatty acids (even if you are very lean). So, having a high fat max, i.e. being able to “burn” fat at its highest rate at the highest fraction of your capacity (VO2max) will help spare your precious and finite stores of glucose.
The best endurance athletes typically tend to have the highest rates of fat oxidation during exercise and achieve these high rates of fat oxidation at higher fractions of their VO2max — trained endurance athletes have a high fat max. Cross-sectional studies comparing trained vs. untrained folks and large cohort studies support this sentiment and cohort studies have found that having a higher VO2max is associated with a higher fat oxidation rate during exercise. In addition, multiple studies provide evidence that endurance training causes adaptations that increase maximal fat oxidation rates and/or the intensity at which they are achieved, although (rather surprisingly) most such evidence is derived from training studies in previously untrained and overweight subjects rather than from already endurance trained athletes. Jørn Helge’s lab in Copenhagen even found that higher fat oxidation rates predicted better race day performance in ultra-distance triathlon racers, although fat oxidation only explained 12% of race time performance meaning that many other factors play a massive role.
So, fat oxidation rates and fat max are clearly important and are influenced by training status. The next obvious question…
They found that, on average, an endurance trained athlete has a maximum fat oxidation rate of 0.53 g/min, achieving this at 56% of VO2max. There are also subtle differences between men and women. On average, men have a greater absolute gram per minute fat oxidation rate during exercise than women but when normalised to fat-free mass (i.e. g/min per kgFFM), women generally have a slightly higher fat oxidation rate and typically reach their maximal fat oxidation rate at a higher fraction of VO2max than men. But these sex differences are small and do not correlate with performance since the best male endurance athletes always out perform female endurance athletes, which aligns with the previously-mentioned finding that fat oxidation during exercise only explains 12% of race day performance.
The cross-sectional and cohort studies from which Ed Maunder’s analysis was derived often found a peak at around 1 g/min, which is the “ballpark” max value you might see tossed around. However, this ballpark value is not a useful one because such analyses do not include data in which participants are “adapted” to practices known to increase fat oxidation during exercise (like a high-fat/low-carb diet). In the lab, I have recorded all kinds of values including a couple of fat-munching athletes at around 1.4 g/min during exercise, while published data, particularly from studies in which athletes have consumed a high-fat/low-carb diet for several days — more on that later — often report fat oxidation rates at around 1.5 g/min, with individual cases even as high as 1.7 to 1.8 g/min.
My point here is to remember that there is not a single value that represents maximal fat oxidation nor is there single exercise intensity at which fat oxidation is maximal. Studies will report a mean (average) value around which there is a distribution (or a range) of values (often reported as standard deviation, standard error, or a confidence interval).
George Brooks’ “crossover concept” is often visualised by plotting the rise in the percentage contribution of carbohydrate oxidation and the drop in percentage contribution of fat oxidation to energy expenditure across the range of exercise intensity from low (jogging) to high (nailing it). If we do this for a cohort of folks, we would plot the average contributions of fat and carb oxidation at each intensity and then stick a line of best fit through the fat oxidation dots and another line that best fits the carb oxidation dots. Where the two lines intersect one another is the “crossover” where carbohydrate becomes the predominant fuel being used to produce ATP to power the exercise. At that point, we can estimate the average exercise intensity at which that cross-over occurs. But, remember that around the average value is a range of values representing the whole cohort.
All of that is perhaps a little bewildering, so let’s simplify things…
If we were to compare a group of untrained folks against a group of endurance-trained folks in this way, we would make several important observations:
At rest and during exercise, endurance athletes would derive more energy from fat than from carbohydrate than untrained folks.
As exercise intensity increases, endurance athletes would reach maximal fat oxidation rates at a higher relative percentage of VO2max — they have a higher fat max — than untrained folks.
The “crossover” when carbohydrate begins to contribute to the majority of energy expenditure occurs at a higher relative exercise intensity in trained athletes than in untrained folks.
At maximal exercise intensities, untrained folk are likely to derive 100% of energy from carbohydrate whereas some endurance trained athletes, especially world class guys and gals, might still be able to use some fat at maximal workloads and, therefore, continue to spare glucose.
On top of all of these superior metabolic attributes, athletes will be moving faster than untrained folks at all relative exercise intensities — athletes have a higher “high-end”, can operate at a higher fraction of their high-end for longer, and are more economical.
As you can tell, there are some clear metabolic differences between trained endurance athletes and regular inactive folks. You are also now well-versed in that fat max is highly variable between humans because it is influenced by so many things, including the obvious — exercise duration, exercise intensity, pre-exercise glycogen level — and the less obvious — genetics, sex, ambient temp/humidity/altitude, training history, recent training dose, habitual diet, time since last meal, the type of last meal, caffeine/nicotine intake, etc.
What can you add to your training toolbox?
For now, not too much. I am merely laying the fundamental foundations for important things to come in this series of posts…
But, to summarise thus far:
We humans generally burn more fat than carbs at rest.
Athletes burn more fat at rest and during exercise than untrained folks.
And, being able to burn more fat at higher intensities might be advantageous for exercise performance.
But, there is one (fat) burning question you might be pondering — “Are there training approaches you can use to manipulate your fat oxidation rates?”.
Well, that will be the story for the next installment in the series…
Until that time, keep training smart.
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Buy me a beer. 
Fatty acid oxidation produces ATP at a rate of 1 mM per kg of muscle tissue per second.
We humans store a lot of fat. 
Fatty acids produce more energy (ATP) per gram than glucose. 
Fatty acid metabolism is slower to produce energy than glucose. 
Fatty acids are a less economical fuel than glucose, requiring more O2 to produce ATP, meaning glucose produces more ATP per litre of oxygen. 
Bioenergetic processes are hardwired in our biochemistry — we cannot change them — we can only influence the relative proportions of fuels being used.
At rest and during exercise, endurance athletes would derive more energy from fat than from carbohydrate than untrained folks. 