It’s our intention at Uphill Athlete to help athletes achieve their endurance and mountain sport goals via sound evidence-based training and nutrition strategies. We have revised and brought up-to-date our original nutrition articles, taking into account the full body of evidence and what is currently better practice.
This article is part one in our updated series, focusing on fat oxidation and its role in supporting endurance performance. In our second article (coming soon), read about the latest findings for fasted training. (In short, Uphill Athlete does not currently recommend fasted training or low-carbohydrate diets, particularly not for female athletes.) And finally, you can read all about low-carbohydrate, high-fat diets in our third article (coming soon).
At every point in their training, we encourage athletes to look after their overall health. We want to share knowledge and provide guidance not only on how to train, but also how to prevent and reduce your risk of injury and illness.
Keep reading to dig into some of the latest research and recommendations on fat adaptation, compiled by high-performance dietitian Rebecca Dent, below.
WHAT IS FAT ADAPTATION?
Fat Oxidation is the actual process by which the body breaks down fats to use them as an energy source during exercise. Fat oxidation is influenced mainly by exercise intensity and duration, but also training status, sex, nutritional intake and environmental conditions (3).
Fat Adaptation is the overall metabolic state achieved through training and dietary strategies, where the body becomes more efficient at using fat as a fuel source.
THE ROLE OF AEROBIC TRAINING IN FAT OXIDATION
WHAT HAPPENS DURING AEROBIC TRAINING?
The ability to move for multiple hours and days, on the trail or in the mountains, requires the majority of an athlete’s training to be spent at low-to-moderate exercise intensity. Think about your own training; you may often slow down in order to stay below your aerobic threshold.
Following a period of consistent aerobic training over time, a cascade of physiological changes take place in the body that allows an athlete to exercise for longer at a given absolute exercise intensity (i.e. you will be able to move faster and go further in your Zone 2 heart rate range).
Training also increases your resistance to fatigue. These changes primarily involve the cardiovascular and musculoskeletal systems and include increased oxygen uptake, improved oxygen delivery, increased capillarisation, increased blood volume, increasing muscle mass and Type 1 slow twitch fibers, and further changes within the muscles themselves including increased myoglobin, increased mitochondrial density, increased aerobic enzymes and transport proteins (1, 2).
Read more: How Aerobic Training Enables Endurance.
It’s within these physiological changes that fat adaptation also occurs. Aerobic training enhances fat oxidation mainly via an increase in mitochondrial capacity. Endurance training increases the body’s ability to oxidize fat and increases the body’s capacity to store and use both fat AND carbohydrate (2).
Reaping the full benefits of aerobic training relies on good health and preventing and reducing the risks of injury and illness.
FUELING AEROBIC TRAINING
As an Uphill Athlete, you need to fuel for your performance. In order to hike, run, climb, or ski over ground for a continuous period of time, your body requires a source of energy. During aerobic exercise, your body will draw upon energy already stored within the body. Fats and carbohydrates are the main fuels used during aerobic exercise. Carbohydrates are stored within the muscle, liver and blood glucose and from fat stored under your skin as adipose tissue, within your muscles as intramuscular triglyceride and circulating within your blood as fatty acids.
To sustain your performance, additional energy is required by an intake of foods and fluids during your activity. Carbohydrate-based fuel sources are the ideal choice, but fat-rich food sources can also contribute to a supply of energy during prolonged hours and days of activity.
Increasing fat oxidation rates via aerobic training benefits endurance performance because your muscles have a limited storage capacity for carbohydrate (4). This limited storage capacity is one of the limiting factors to performance during exercise lasting longer than two hours (5). The fat stored in your body serves as a significant energy source, estimated to be around 50,000-150,000 kcal for individuals with 10-30% body fat. This can potentially fuel a total distance ranging from 800 to 2400 km (6). On the other hand, carbohydrates stored as glycogen in muscles and the liver total approximately 1500-3000 kcal. This limited carbohydrate reserve allows for sustaining about 95 minutes of exercise at marathon pace (7).
Uphill Athletes face additional nutritional challenges during big mountain days due to limited access to food and the limited ability to carry enough to meet daily energy demands. Improving the body’s ability to use fat will help you optimize your nutrition strategy and avoid overloading your bag. Ultra runners often have the option to top up their fuel sources at aid stations, however, they will still be unable to meet the energy demands of a race consuming carbohydrates alone. The increase in fat oxidation will help provide an additional fuel source when running for hours on end.
When moving in the mountains and on the trails, you spend the majority of your time exercising at low to moderate intensities (with the occasional demand to tip into your top gear and work at high intensities such as moving over technical steep ground, post- holing in the snow, moving quickly past overhead hazards, etc).
Regardless of how fit you are, the intensity of exercise is the main influence on the fuel sources used by the body.
Fat oxidation increases from low-to-moderate exercise intensity and then starts to decrease from moderate-to high intensity, with a shift towards an increased reliance on carbohydrate oxidation at moderate-to-high intensities (8).
FAT OXIDATION RATES AND PERFORMANCE
Consistent aerobic training over time increases whole body fat oxidation rates (3). As a result of aerobic training, an increase in fat oxidation rates has been shown to coincide with an increase in endurance performance (9), enhancing an athlete’s ability to sustain prolonged exercise.
Trained athletes have a higher rate of fat oxidation than untrained athletes at a given and absolute exercise intensity. The ability to utilize fats and spare muscle glycogen is often associated with delays in fatigue and an improvement in endurance performance (10).
The goal of increasing fat oxidation rates is to achieve a greater use of fuel from fat at low-to-moderate exercise intensity and spare the limited carbohydrate reserves for higher intensity efforts.
Fat oxidation can be measured by determining maximal fat oxidation rates. This is the exercise intensity at which fat oxidation is at its peak. As explained above, the intensity you’ll be moving at for most of an ultra running event or big mountain day is 45%-65% of maximum oxygen consumption (11) with some need to kick into higher percentages/ higher intensity and also times when you will be moving slower. Maximal rates of fat oxidation have been shown to be reached at intensities between 59% and 64% of maximum oxygen consumption in trained individuals and between 47% and 52% of maximum oxygen consumption in a large sample of the general population (12).
Fat Oxidation Rates of Athletic Populations
| Athlete Type (all sexes) | Fat Oxidation Rate, g/min | VO2 Max |
|---|---|---|
| Untrained (13) | 0.56 | 49% |
| Well-trained endurance, mixed diet (14) | 0.67 | 55% |
| – | 0.52 | 62% |
| – | 0.84 | 63% |
| Well-trained endurance, LCHF diet (14) | 1.15-1.74 | 65% |
| Athlete Type (female) | Fat Oxidation Rate, g/min | VO2 Max |
|---|---|---|
| Untrained, female (15) | 0.32 | 53% |
| Well-trained endurance, female (15) | 0.40 | 56% |
WHAT INFLUENCES FAT OXIDATION?
Exercise intensity
Exercise duration
When training/adventuring for longer than two hours, muscle carbohydrate stores are depleted as are blood glucose levels. This results in an increased reliance on fat as a fuel source, i.e. an increase in fat oxidation. Due to the depletion of stored carbohydrate within exercising muscles, an athlete’s body is unable to maintain a higher work rate and will need to lower their intensity (30-60% VO2 max) in order to use fat as a fuel source (8). The body will still be using some carbohydrate in the form of blood glucose from carbohydrate that is consumed.
Sex
During aerobic endurance exercise, females oxidize more fat as a fuel source compared to males at the same relative intensities (10). These differences are thought to be contributed from women having higher body fat levels, the influence of the hormone estrogen on fat utilization, and a greater number of type 1 muscle fibers (17). Due to these differences, females may naturally possess a metabolic advantage during prolonged endurance activities (18).
Nutrition coaching
Diet
What we eat before, during, and after training has the potential to influence the adaptive responses to endurance training.
Most studies measuring fat oxidation rates have been carried out using a fasting protocol whereby study participants had not eaten any food or had fasted overnight in order to find out the true values of carbohydrate and fat oxidation rates during different exercise intensities. However, this is not a true reflection of actual fat oxidation rates that would take place during a race or event when carbohydrate is consumed.
Fat oxidation rates are reduced when carbohydrate is consumed before, during and after exercise (12).
Eating a diet low in carbohydrates and high in fat (LCHF) has been shown to increase fat oxidation rates during exercise (19). Some research scientists suggest that this enhanced fat oxidation rate can improve performance beyond current sports nutrition recommendations of fueling with carbohydrates (20, 21).
Some studies have shown that fasted training can also increase fat oxidation rates (22).
The influence of fasted training and a LCHF diet on fat oxidation and performance is discussed in detail in articles two and three.
During your long training runs, weighted hikes or long days in the mountains, it’s unlikely that you will be able to consume enough carbohydrates to match the energy expenditure of the activity.
Here are some real life examples of athlete energy expenditure during different training runs:
| Ultra Runners | Weight, kg | Calories Burnt 1 hr Zone 1 Run, kcal | Calories Burnt 3 hr Endurance Run, kcal |
|---|---|---|---|
| Elite female | 63 | 730 | 2087 |
| Elite male | 72 | 893 | 2890 |
| Amateur female | 57 | 511 | 1120 |
| Amateur male | 80 | 771 | 2270 |
| Mountaineers | Weight, kg | Calories Burnt 1 hr Zone 2 Run, kcal | Calories Burnt 3 hr Endurance Hike, kcal |
|---|---|---|---|
| Moderately trained female | 54 | 300 | 907 |
| Moderately trained male | 98 | 800 | 1924 |
As referenced in the table above, ultra runners expended 1120-2890 kcal during a 3-hour endurance run. During these sessions, athletes may only consume up to 60g carbohydrate per hour, which totals 720 kcal. This is less than half of the energy expended.
Typically, endurance and ultra endurance athletes will carry out large training volumes that are going to stress carbohydrate stores towards the end of that prolonged exercise session. This may lead to enhanced fat oxidation rates even when consuming a diet containing carbohydrates (23). Basically, you are unable to eat enough carbohydrates to meet the energy expenditure demands of the training session or big day, so your body will naturally tap into fat as a fuel source and potentially influence fat oxidation rates.
A recent study showed that when endurance athletes ingested carbohydrates before and during training, they had elevated rates of fat oxidation. On average, the group had a maximal fat oxidation rate of 0.8 g/min and greater than 1 g/min was observed in 13 out of 28 participants (23).
This suggests that well-trained endurance and ultra-endurance athletes can achieve relatively high rates of fat oxidation regardless of dietary intake.
At present, it’s not known what the optimal fat oxidation rate is that enhances endurance performance. More research is required to fully understand how fat oxidation rates directly affect endurance performance.
An increase in fat oxidation is thought to preserve muscle carbohydrate stores during prolonged exercise and to serve as an additional fuel source (24). As highlighted, fat oxidation rates are highly variable between individuals and are not directly correlated to a winning performance or increasing the odds of a mountain summit.
Although trained athletes have higher rates of fat oxidation, the question remains: does increasing fat oxidation rates beyond the training effect optimize performance? We try to answer this question in parts two and three in this series.
PRACTICAL IMPLICATIONS FOR THE UPHILL ATHLETE
You may be able to increase fat oxidation rates by eating a LCHF diet or by doing fasted training, but as an Uphill Athlete you will experience far greater benefits to your performance by carrying out consistent aerobic training.
You will also want to ensure you eat enough carbohydrate to support your daily energy and training demands.
Ask yourself whether you are doing everything possible to support your training by:
- Fueling your training with carbohydrate
- Eating to meet your daily energy demands
- Improving your technical ability
- Strength training to build resilience
- Rehabilitating any injuries
- Getting a good night’s sleep
- Balancing your training with rest
- Managing life stressors and working on mental preparedness
There are other training adaptations that will help you store and use a greater amount of carbohydrate, while at the same time increasing your body’s ability to use fat as a fuel source through aerobic training.
When you’re moving for long periods of time, you want your body to be efficient at using fat as a fuel source to help you go further, but also want to be able to use carbohydrates as a fuel source. It’s well established that fueling with carbohydrates during aerobic exercise improves time to fatigue and sustains endurance performance (5).
When you are moving in the mountains or on the trails, despite long periods of exercise spent at lower to moderate intensities, there will also be periods of harder, higher intensity effort which require carbohydrates to fuel these movements.
Metabolic efficiency is the goal
Metabolic flexibility is the ultimate goal. It’s important for your body to be able to tap into and use either or both fuel sources when needed. Increasing your body’s ability to use more fat while moving at lower to moderate intensities preserves carbohydrate for when you really need it: moving at higher intensity, the crux pitch, steep descents, post holing, the hard effort to make that last lift. Supplementing with carbohydrates during your activity will help to sustain your performance.
It would be best not to limit yourself to one fuel source when you have the ability to use multiple fuel sources.
BENEFITS OF IMPROVING FAT OXIDATION
Fat-adapted athletes will enjoy the following benefits:
- Sustained energy levels for long periods of time at low to moderate exercise intensities (Zone 1/2 training)
- Reserved carbohydrate stores for the higher-intensity movements (crux pitch, rocky uphill scramble, steep descent, breaking trail)
- Optimized energy levels through strategic fueling
- Reduced likelihood of bonking when moving at low enough intensity
Below are some common fueling mistakes made in an effort to increase fat oxidation rates:
- Eating a LCHF diet
- Carrying out all training sessions in a fasted state
- Training first thing in the morning before breakfast
- Going without eating during big mountain days
- Carrying out intermittent fasting in addition to carrying out a fasted training session. This can look like skipping breakfast before training, then waiting after your training session until lunch time to eat.
- Avoiding fueling training sessions because they don’t feel hard enough.
KEY NUTRITION RECOMMENDATIONS
Identify your current limits to performance.
- Are you consistently eating enough daily calories to support your training?
- Are you fueling your training sessions?
- Are you training consistently?
- Are you strength training consistently?
- Are you managing your hydration before, during, and after training?
Fuel your training sessions (before, during, and after) with a source of carbohydrate (30-90g per hour depending on duration and intensity).
Consider not only the energy demands of that single training session but also the load of training throughout the week and your training cycle.
The type and amount of carbohydrate will depend on the duration, intensity, your individual training and objective goals, and daily energy demands.
SUMMARY
When moving for hours on end, one of the main limiting factors to your performance is the amount of carbohydrates stored in your body.
Aerobic training consistently over time will enhance fat oxidation, helping you to become more fuel-efficient as you will be able to use both fat and carbohydrates.
It’s important to eat enough carbohydrates to support daily energy and training demands, to fuel your races or other objectives, and to sustain and optimize your performance.
Article reviewed by Director of Coaching Chantelle Robitaille.
SOURCES
-
Farrell C, Turgeon DR. Normal Versus Chronic Adaptations to Aerobic Exercise. [Updated 2023 May 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK572066/
Yeo WK, Carey AL, Burke L, Spriet LL, Hawley JA. Fat adaptation in well-trained athletes: effects on cell metabolism. Appl Physiol Nutr Metab. 2011 Feb;36(1):12-22. doi: 10.1139/H10-089. PMID: 21326374.
Purdom, T., Kravitz, L., Dokladny, K. et al. Understanding the factors that affect maximal fat oxidation. J Int Soc Sports Nutr 15, 3 (2018). https://doi.org/10.1186/s12970-018-0207-1
Burke LM, Hawley JA. Carbohydrate and exercise. Curr Opin Clin Nutr Metab Care. 1999 Nov;2(6):515-20. doi: 10.1097/00075197-199911000-00015. PMID: 10678682.
Cermak NM, van Loon LJ. The use of carbohydrates during exercise as an ergogenic aid. Sports Med. 2013 Nov;43(11):1139-55. doi: 10.1007/s40279-013-0079-0. PMID: 23846824.
Coyle EF. Substrate utilization during exercise in active people. Am J Clin Nutr. 1995 Apr;61(4 Suppl):968S-979S. doi: 10.1093/ajcn/61.4.968S. PMID: 7900696.
Jeukendrup AE, Saris WH, Wagenmakers AJ. Fat metabolism during exercise: a review. Part I: fatty acid mobilization and muscle metabolism. Int J Sports Med. 1998 May;19(4):231-44. doi: 10.1055/s-2007-971911. PMID: 9657362.
Van Loon LJ, Greenhaff PL, Constantin-Teodosiu D, Saris WH, Wagenmakers AJ. The effects of increasing exercise intensity on muscle fuel utilisation in humans. J Physiol. 2001 Oct 1;536(Pt 1):295-304. doi: 10.1111/j.1469-7793.2001.00295.x. PMID: 11579177; PMCID: PMC2278845.
Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc. 2002 Jan;34(1):92-7. doi: 10.1097/00005768-200201000-00015. PMID: 11782653.
Randell RK, Rollo I, Roberts TJ, Dalrymple KJ, Jeukendrup AE, Carter JM. Maximal Fat Oxidation Rates in an Athletic Population. Med Sci Sports Exerc. 2017 Jan;49(1):133-140. doi: 10.1249/MSS.0000000000001084. PMID: 27580144.
Benedikt Gasser. Fat Oxidation While Hiking and Mountaineering – An Analysis from Dom (4545 Meter) – The Highest Mountain in Switzerland. J Phy Fit Treatment & Sports. 2018; 2(5): 555599. DOI: 10.19080/JPFMTS.2018.02.555599
Achten J, Jeukendrup AE. Optimizing fat oxidation through exercise and diet. Nutrition. 2004 Jul-Aug;20(7-8):716-27. doi: 10.1016/j.nut.2004.04.005. PMID: 15212756.
BALCI, Ş.S., 2016, December. SUBSTRATE OXIDATION RATES DURING INCREMENTAL EXERCISE IN TRAINED AND UNTRAINED YOUNG ADULTS. In Conference for Youth Sport in Ljubljana (Vol. 9, p. 10).
Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ, Phinney SD. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016 Mar;65(3):100-10. doi: 10.1016/j.metabol.2015.10.028. Epub 2015 Nov 2. PMID: 26892521.
Stisen AB, Stougaard O, Langfort J, Helge JW, Sahlin K, Madsen K. Maximal fat oxidation rates in endurance trained and untrained women. Eur J Appl Physiol. 2006 Nov;98(5):497-506. doi: 10.1007/s00421-006-0290-x. Epub 2006 Sep 28. PMID: 17006714.
SSE#205 Regulation of Fat Metabolism During Exercise Sports Science Exchange (2020) Vol. 33, No. 205, 1-6 https://www.gssiweb.org/docs/default-source/sse-docs/randell-spriet_sse_205.pdf?sfvrsn=2
Cano, A., Ventura, L., Martinez, G. et al. Analysis of sex-based differences in energy substrate utilization during moderate-intensity aerobic exercise. Eur J Appl Physiol 122, 29–70 (2022). https://doi.org/10.1007/s00421-021-04802-5
Tiller NB, Elliott-Sale KJ, Knechtle B, Wilson PB, Roberts JD, Millet GY. Do Sex Differences in Physiology Confer a Female Advantage in Ultra-Endurance Sport? Sports Med. 2021 May;51(5):895-915. doi: 10.1007/s40279-020-01417-2. Epub 2021 Jan 27. PMID: 33502701.
Burke LM. Ketogenic low-CHO, high-fat diet: the future of elite endurance sport? J Physiol. 2021 Feb;599(3):819-843. doi: 10.1113/JP278928. Epub 2020 Jun 10. PMID: 32358802; PMCID: PMC7891323.
McSwiney FT, Wardrop B, Hyde PN, Lafountain RA, Volek JS, Doyle L. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes. Metabolism. 2018 Apr;81:25-34. doi: 10.1016/j.metabol.2017.10.010. Epub 2017 Nov 3. PMID: 29108901.
Volek, J.S., & Phinney, S.D. (2012). The art and science of low carbohydrate performance. Beyond Obesity LLC.
Rothschild JA, Kilding AE, Plews DJ. What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients. 2020 Nov 12;12(11):3473. doi: 10.3390/nu12113473. PMID: 33198277; PMCID: PMC7696145.
Rauch CE, McCubbin AJ, Gaskell SK, Costa RJS. Feeding Tolerance, Glucose Availability, and Whole-Body Total Carbohydrate and Fat Oxidation in Male Endurance and Ultra-Endurance Runners in Response to Prolonged Exercise, Consuming a Habitual Mixed Macronutrient Diet and Carbohydrate Feeding During Exercise. Front Physiol. 2022 Jan 4;12:773054. doi: 10.3389/fphys.2021.773054. PMID: 35058795; PMCID: PMC8764139.
Yeo WK, Carey AL, Burke L, Spriet LL, Hawley JA. Fat adaptation in well-trained athletes: effects on cell metabolism. Appl Physiol Nutr Metab. 2011 Feb;36(1):12-22. doi: 10.1139/H10-089. PMID: 21326374.
- Achten J, Jeukendrup AE. Maximal fat oxidation during exercise in trained men. Int J Sports Med. 2003 Nov;24(8):603-8. doi: 10.1055/s-2003-43265. PMID: 14598198.