Increasing fat burning during exercise — acute manipulations of carbohydrate availability.
Thomas Solomon, PhD.
28th Nov 2020.
In the last post of this series, where I described what fat oxidation rates are, I planted a seed and left you on the edge with a key question: “Are there training approaches you can use to manipulate your fat oxidation rates?”. Stay with me on this journey to find out…
Reading time ~17-mins (3500-words)
or listen to Podcast version here.
If you joined me previously to learn about fat oxidation rates, you will know that us humans store a lot of fat and that fatty acids (the breakdown product of fat) produce more ATP per gram than glucose, which is stored in relatively small amounts. It is, therefore, intuitive that you would want to try to tap into your mahoosive fat store for as long as you can when deep into a session. Doing so will spare your precious glucose store for longer, saving it for when it is needed; saving it to fuel high-intensity efforts when VO2 (oxygen consumption) is high. Why? Well, because glucose produces ATP at a far faster rate than fat and glucose is more economical in doing so — it uses less O2 per gram than fat to produce ATP.
Being able to burn more fat at higher intensities might be advantageous for exercise performance. Increasing your rate of maximal fat oxidation and shifting it up to a higher percentage of your maximal aerobic capacity — fat adaptation — is a natural consequence of endurance training. Endurance athletes are, therefore, “fat adapted”. But, there are several training manipulations that increase the reliance on fatty acids as a fuel to produce energy during your sessions and thereby promote adaptations that increase your fat max.
I will start with four “acute” approaches; things you might choose to do occasionally, in and around individual sessions. I emphasise the word might because it is always important to consider the context of your goals before making changes to your training. This context will become clear as you follow my journey through this series of posts...
Note: if you have diabetes, you must work closely with your doctor before considering any of the following since they can increase the risk of hypoglycemia.
Overnight-fasted, long-duration, Easy-/Moderate-effort workout with no feeding during the session.
Because this type of work-out is before breakfast, you start with low liver glycogen and less than optimal but not depleted muscle glycogen. During the workout, both are continually depleted. Depleting liver glycogen forces the liver to use other substrates (like fatty acids and amino acids) to produce glucose so it can help maintain blood glucose levels within a healthy range. Depleting muscle glycogen not only forces muscles to oxidise fatty acids but it also helps to trigger the intramuscular signalling processes that lead to training adaptations. The multitude of data in this area was summarised in a 2018 systematic review and meta-analysis.
Fasted training is very common in endurance athletes. An observational study of 1,950 endurance athletes found that 6 in every 10 athletes sometimes choose train before breakfast for reasons were related to convenience, improving gut comfort, and (you guessed it) increasing fat oxidation. Notably, athletes who avoided fasted training did so to avoid hunger or because they did not feel it helped their training or because their performance was worse. Furthermore, fasted training is a common tool in the training toolbox among the worlds’ finest endurance athletes in East Africa (check out any published data on nutritional habits or read the Sweat Elite blog or Adharanand Finn’s book, Running with the Kenyans).
What are the practical nuances of fasted exercise?
→ Fasted sessions can be challenging to begin but, eventually, you adapt to being able to last for a long time off an overnight fast, feeling fine for the duration of the session.
Delayed post-exercise carbohydrate refeeding.
By delaying post-session carbohydrate intake (and therefore delaying muscle glycogen resynthesis) for a couple of hours, you can theoretically prolong the upregulation of intramuscular signalling that is triggered at low glycogen levels. There is little research in this area but, once again, we can perhaps take heed of what can be observed in the lives of our East African running-brethren — they are often documented to be no rush to eat after sessions, naturally delaying their post-exercise carbohydrate refeeding, even after tough, long-duration and presumably glycogen-depleting sessions.
What are the practical nuances of delayed refuelling?
→ This is practically easy to implement but is often difficult if you are hungry after a session when you just want to eat whatever you can get your hands on. There is also the caveat that delaying refeeding means you will be glycogen depleted for a subsequent session on the same day.
Two-a-day work-outs with no/low between-session carbohydrate intake and no feeding during the sessions.
A hard-effort and/or prolonged glycogen-depleting session in the morning followed by an easy- to moderate-effort session in the afternoon/evening with low-carbohydrate intake in between the two sessions. This approach has shown some remarkable adaptations. A clever 10-week leg-extension training study from Copenhagen had subjects habitually consume a high-carbohydrate diet while training one of their legs for 1-hour on 5-mornings per week and the other leg for 1-hour in the morning and a further hour 2-hours later with no re-feeding in between on 2 to 3 mornings per week. This meant that one of the subject’s legs completed half of their training sessions in a glycogen depleted state. The authors found that the increase in maximal power was the same between legs but that the “two-a-day” leg had more pronounced increases in mitochondrial enzyme activity and more than a 2-fold greater increase in fatigue resistance (exercise capacity at 90% of maximal power).
What are the practical nuances of “training low”?
→ Two-a-days with low-carb intake in between are practically easy to implement but post-session hunger can make it difficult to sustain and you must be well-versed in nutrition to ensure that your total daily caloric intake and daily protein intake is not reduced. The second session can also sometimes feel sluggish and heavy and you might lack the spring in your stride, but you eventually adapt. And, importantly, Two-a-day training should not mean doubling your training load — the best initial approach is often to simply cut your load into two on some days of the week.
A hard evening session followed by an easy or hard session the next morning, with no/low between-session carbohydrate intake and no feeding during the sessions.
The harder evening session depletes liver and muscle glycogen and the morning session is performed in a liver and muscle glycogen-depleted state because no carbohydrate is ingested in between. This is a larger “stressor” than the previously described approaches.
Single bout studies show that a high-intensity running session (6 x 3-min at 90% VO2max, 3-min at 50%) performed in the morning having not eaten carbohydrate since a glycogen-depleting run the evening before, causes a greater increase in intramuscular signalling associated with mitochondrial biogenesis than when carbohydrate is provided to restore muscle glycogen levels. Similar work from James Morton’s lab, using a cycling model, has confirmed these effects of sleeping with reduced carbohydrate availability prior to morning exercise with the addition of enhanced fat oxidation rates during exercise.
What are the practical nuances of “sleeping low”?
→ Overnight low-carb between sessions is practically easy to implement but can feel surprisingly difficult — some athletes find that their sleep pattern is disturbed either because of the late-evening session or the lack of food at dinner/higher-than-normal intake of fat at dinner, or both. Furthermore, the morning session can often feel utterly terrible.
So, those four approaches are the relatively easy ways one can consider if your goal is to acutely increase fat oxidation rates during your sessions that eventually cause adaptations to push your maximal rate of reliance on fat to a higher fraction of your maximal aerobic capacity — increasing your fat max. There is also a fifth option:
Chronic long-term low-carbohydrate/high-fat diet.
With this approach, you consume a habitual diet that meets your energy availability and daily protein intake needs but with a low amount of daily carbohydrate — providing less than 15-20% of total kcals per day, or less than 50 grams total per day to achieve ketosis. Given the ongoing daily restriction of dietary carbohydrate, muscle glycogen levels become chronically depleted while other substrates like amino acids are used to help synthesise liver glycogen and help maintain blood glucose levels. (I will not discuss the evidence here because I will be taking a deep dive into this topic in the next post.)
What are the practical nuances of always “sleeping low” and “training low”?
→ A low-carb diet is the most practically difficult approach to implement since a complete dietary overhaul is often required and you must be exceptionally well-versed in nutrition to ensure that your total daily caloric intake, daily protein intake, and daily micronutrient intakes are not restricted. It is very common to feel terrible during sessions during the first few days after beginning a low-carb diet; for some athletes, the “low-power” feeling never goes away and they continue to feel sluggish.
So, that question once again: “are there training approaches you can use to manipulate your fat oxidation rates?” Yes, indeed there are! And, you might be starting to see a pattern emerging from the fatty clouds...
Approaches that are designed to increase fat oxidation coincidentally reduce carbohydrate availability.
Reducing carbohydrate availability around your workouts decreases your muscle glycogen levels (your muscles’ store of glucose), which are a key determinant of your training adaptations. The strategies I have just described for manipulating fat oxidation can be divided into “train low” or “sleep low” strategies — that is, exercising with a low carbohydrate availability or sleeping with a low carbohydrate availability. These types of approaches are common-place in athletes, either through choice (choosing to adhere to a high-fat diet), or cultural tradition (Kenyan athletes traditionally waking up and lacing up and delaying refeeding), or convenience (not wanting to wake up so early to eat, wait, then train; just walking up and training).
Acutely reducing carbohydrate availability by working-out following an overnight fast or by training twice in a day without refuelling during the between-session period leads to one very important outcome: muscle glycogen depletion. This forces your muscles to use other metabolic substrates — fatty acids derived from the blood and your muscles’ very own fat store, intramuscular triglyceride (IMTG) — to fuel the work. Besides increasing the reliance on fat to fuel your workout, muscle glycogen depletion is also a potent stimulus for activating a protein called PGC1alpha, which is a key molecular switch that triggers mitochondrial adaptations (for a deeper dive, I can recommend an excellent review by my old colleague Andy Philp).
This all sounds absolutely splendid! Perhaps too good to be true. Naturally, that prompts the obvious question…
Is there a downside to low carbohydrate availability?
To help you answer that question, remember what you learned about the bioenergetics of fuel utilisation:
fat is far more abundantly stored in the body than glucose, and
fatty acids produce more energy (~10 kcals per gram) than glucose (~4 kcals/g), but
glucose can produce ATP at a superior rate and does so more economically, using fewer litres of oxygen per gram to produce more ATP per litre of oxygen consumed.
On race day, you will very likely be operating at a high intensity — at a high fraction of your VO2max.
Now remember what you learned about George Brooks’ “crossover concept”:
at lower fractions of your VO2max, you will predominantly be using fat to produce ATP while at higher fractions you will predominantly be using carbohydrate to produce ATP.
Therefore, during easy-effort sessions you will be using the abundantly-stored, oxygen-guzzling, high-energy yield substrate — fatty acids — to power your plodding but during hard-effort sessions, and especially on race day, your muscles will need to utilise the less-abundant, oxygen sparing, rapid energy-producing substrate — glucose — to meet the ATP demands of your fast pace.
Let’s put science aside for a moment. Many athletes and coaches, including myself, will argue that an athlete should aim to maximise their ability to produce a high power output and work rate. After all, as an athlete, you are not training to win a competition of “who has the highest fat oxidation rate?”, you are training to race — training to move as fast as you can for as long as it takes to complete the distance while resisting fatigue until the finish line. Consequently, if one of the training manipulations designed to increase your fat oxidation rates jeopardises your ability to unleash a high work rate during a fast session, then you will be negating the purpose of the session.
During periods of low carbohydrate availability, muscle glycogen levels are low. Besides triggering awesome things like the production of more mitochondria or more mitochondrial enzymes for helping you resist fatigue, muscle glycogen depletion also leaves you with less glucose available to fuel your high end.
Therefore, training to be able to spare glycogen and blood glucose for as long as possible is useful — e.g. sessions with a low carbohydrate availability to become “fat adapted” with a high maximal fat oxidation rate and a high fat max are a good idea — but to be able to move close to your peak aerobic power you will also need to train to avoid reducing your capacity to oxidize glucose on race day — e.g. sessions with a normal or high carbohydrate availability are also a good idea to help you adapt to be able to use carbohydrate when operating at high fractions of your maximal capacity.
Now, this is all rather theoretical and doesn’t disprove that lowering carbohydrate availability impairs performance. I will take a deeper dive into the experimental evidence that reveals the caveats of low carbohydrate availability in the next instalment of this series, but I touch on the theory here because it helps conceptualise the hypothesis that you should:
“Fuel for the work required.”
Trying to complete an arduous high-intensity session with low carbohydrate availability when glycogen depleted will certainly test your mind — try it. Doing so increases your perception of effort (RPE) during the session. This type of “sufferfest” training will perhaps “train” your brain to better handle pain/discomfort — an anecdotal thought echoed in the sentiments of many research subjects who participate in low carbohydrate training studies. But, in my opinion, don’t train like this every time you lace up — train to suffer some of the time; train to be fast most of the time.
When you plan a hard session, it always has a purpose. That is, to produce a high power output. Whatever training manipulations you plan to use to maximise your fat oxidation rates, you must never forget the purpose of hard sessions. Don’t inadvertently blunt your peak power — because then you will not be “training” your body to move fast — and don’t inadvertently cause yourself to detonate early in a key workout — because your overall training load will shrink causing a lesser stimulus and smaller gains.
The “fuel for the work required” paradigm from James Morton’s lab is a strategic way of incorporating both low- and high-carbohydrate availability into your training. This so-called “carbohydrate periodisation” method provides dietary carbohydrate relative to the demands of the upcoming session allowing you to deplete glycogen appropriately during some of your sessions. Later in this series, I will dig into carbohydrate periodisation in more detail, but for a fine overview of the balance between the need to train with both a low- and a high-carbohydrate availability, I can recommend a 2015 review penned by Jon Bartlett, John Hawley, and James Morton.
What have you learnt so far?
Yes, athletes have higher fat oxidation rates and achieve them at a greater exercise intensity than untrained folks. Yes, world-class athletes are extremely efficient with substrate use, using fatty acids and sparing glucose until as late in the game as possible. And, yes, training appears to increase fat oxidation rates and fat max.
We do not know whether the superior metabolic fuel use in world-class athletes is inherently better at baseline, inherently more adaptive to training, or a result of their high (and/or carefully-planned) training load. We currently do not know the best training approach for maximising fat oxidation rates during exercise while maximising performance outcomes. We also do not know whether training with the sole goal of maximising fat oxidation rates leads to the best performance outcomes — in other words, it is unclear if every athlete should be trying to maximise their fat oxidation rate.
Meet Bob and Eddie. Bob’s fat max burns at 80% of his maximum aerobic power while Eddie’s fat max only burns at 70% of his max, but Eddie’s VO2max is 80 mL/kg/min while Bob’s is just 70 mL/kg/min; therefore, both Bob’s and Eddie’s fat max occurs when they are working at 56 mL/kg/min. Bob and Eddie have very similar marathon times.
Over the last 18-years, I have measured substrate oxidation rates at rest and during exercise in hundreds of folks. In similarly performing athletes, I have tested large ranges of values in their economy, VO2max, velocities at VO2max, fractional utilisation, fat max and maximal fat oxidation rates. This is mirrored in the physiological data from the athlete’s screened for the Breaking2 marathon attempt in 2017. My point being that fat max and maximal fat oxidation rates are just a piece of the puzzle for optimising your performance and certainly not factors to hang all your medals on.
Thanks for staying with me on this series through exercise metabolism and performance nutrition. I will pick up again soon by taking a look at chronic fat adaptations using low-carb/high-fat diets.
Until then, keep training smart!
Disclaimer: I occasionally mention brands and products but it is important to know that I am not sponsored by or receiving advertisement royalties from anyone. 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, the A.P. Møller Foundation, and the Augustinus Foundation. 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. 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 scientific information to the masses and helping 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 biochemistry, molecular medicine, and exercise physiology. 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 and regular folk, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, lift, and climb as often as possible. To stay on top of scientific developments, I participate in journal clubs and 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.