Can burning more fat during exercise make you a better endurance athlete?
Metabolic efficiency is the new hot topic in endurance sports circles. But even if you train your body to use more fat while exercising, will that increase your performance?
Athletes are always looking for an edge. How can I improve my time? What will enhance my training? How can I eat and fuel to perform my best?
Most athletes hop onto a new hot nutrition take without digging into the research, or understanding if the supplement, diet, or fuel will work for their individual needs.
One of the newest trends? Fat adaptation during training.
Let’s dig into the science to see if fat adaptation will make you a faster endurance athlete.
Buckle up - this is a long one!
What is Fat Adaptation in Training?
Fat adaptation is the ability of the body to use more fat as fuel during endurance training.
The theory is that using more fat will help save glycogen stores (our stored form of carbohydrate). And, honestly, using more fat as a fuel source makes sense. Our carbohydrate stores are limited while our fat stores are vast in comparison.
How do you become fat-adapted?
Getting your body to use more fat is accomplished through specific paired training and nutrition protocols, usually implemented for 30-50% of your training sessions.
It involves ‘training low’, a general term used for training with low carbohydrate availability. This could be from low muscle glycogen, low liver glycogen, or low carbohydrate intake before, during, or after exercise. It can also be in the form of a low-carbohydrate diet. Each of these relies on a different protocol that intersects training and nutrition.
Without carbohydrates, the body must use stored fat as an alternate fuel. The more often we make the body use fat as fuel, the more it develops the metabolic fat-burning pathways within muscle cells that make this process easier.
Does fat adaptation work?
Well, that depends on your goal.
Is your goal to burn more fat than normal while exercising? Rely less on endogenous and exogenous carbohydrates (carbs from glycogen stores and from fueling)? Then the answer is Yes!
Research shows that fat metabolism during exercise increased after a surprisingly short amount of time spent ‘training low’- as short as 5 to 7 days. Some studies even found that fat metabolism increased by as much as 2.5 times!
The body is very adaptable.
However, if your goal is to perform better, i.e. have a faster time, then the answer is (so far) a resounding No.
Even though most research shows that more fat is used as fuel when training low, very few studies found this translated into better performance.
In fact, not only did most studies show no performance advantage, but it turns out there are many potential drawbacks to the process of training low, especially if done wrong.
What are the disadvantages of fat adaptation for endurance athletes?
Disadvantage 1: For most people, fat adaptation only works well when exercising below 65% of your VO2max.
To understand the first disadvantage, let’s review how fuels are metabolized during exercise.
The first thing to know is that the body prefers carbohydrates over fat as the primary fuel for exercise. This is because carbohydrates can produce more ATP than fat can from the same volume of oxygen. *ATP is the fuel that carbs and fats break down into for our muscles to use.
At higher intensities, the body needs ATP faster to help maintain the increased muscle contractions, this means it needs more oxygen as well. Above 65% VO2max, fat metabolism becomes inefficient, requiring too much oxygen to make enough fuel quickly. So, the body pushes fuel metabolism toward carbohydrate sources to get a better bang for its oxygen buck.
The body is very economical.
Bottom line: Your race will likely have many higher intensity portions: Uphill, into the wind, sprints to pass or finish, etc. When going into those higher intensities, you’ll need to rely more on carbohydrates, and your body may no longer be adept at using them efficiently, leaving you feeling more fatigued at higher paces.
This leads me to disadvantage number 2…
Disadvantage 2: Once you are fat-adapted, your carbohydrate metabolism is down-regulated.
If your body has made room within the cells to burn more fat, it must reduce the number of pathways available to burn carbs.
Picture this: You go into a race and start feeling fatigued halfway through, so you rip into your stash of gels or other carbohydrate-rich fuels. Well, guess what? Your body can’t handle those fuels as well anymore and you’re left feeling fatigued and possibly with GI distress.
Bottom line: If you don’t use it, you lose it.
Disadvantage 3: Fat adaptation can be lost within a week
And it may take just as long to recoup your ability to use carbohydrates efficiently.
As I mentioned before, the body is very economical and adaptable. Once you stop training low, the body starts adapting back to its preferred fueling pathways within the cells: carbohydrates!
Good news: Completing a carb load after training low doesn’t completely wipe out all fat adaptation. Though slightly impaired, you can still metabolize a relatively high amount of fat as fuel in the first 24-48 hours after introducing more carbs.
Bad news: The problem is that it will take a few days if not more, to restore your ability to fully and efficiently metabolize carbohydrates again.
One study showed that athletes who trained low and then carb-loaded 24 hours before their race could only metabolize between 60 and 75% of the carbohydrates available. This is compared to what they used to metabolize before training low.
Bottom line: If you want to become fat-adapted, you can’t stray from the eating pattern. And if you want to utilize carbohydrates as fuel during a race after training low, your ability to do so will be limited.
Disadvantage 4: Training low may impact your bone health and immune function
While more research is needed, a few studies have found that reduced carbohydrate availability negatively impacts bone health. This may increase the risk for stress fractures and RED-S, especially in athletes who are not eating enough total calories.
Similarly, reduced carbohydrate availability during training may impact your immune function, making you more susceptible to getting sick. This is because carbohydrates play a role in the post-exercise immune response.
Bottom line: Training without enough carbs may impact your overall health.
Do I recommend fat adaptation for endurance athletes?
The reality is that training low may help you burn more fat, but it is very unlikely to help you perform better.
To make fat adaptation work for you, your race would have to be sustained at low intensities, very flat, with minimal sprinting, and in perfect cool and calm weather. Nothing that would make you use more power, and therefore need carbohydrates.
Bottom line: Mountains of research continue to show that carbohydrates are king when it comes to fueling.
Sources
Burke LM, Ross M, Garvican-Lewis L, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017;595(9):2785-2807
Jeukendrup A. Periodized Nutrition for Athletes. Sports Med. 2017.47(1):S51-S63. DOI 10.1007/s40279-017-0694-2
Stellingwerff, T., Morton, J. P., & Burke, L. M. A Framework for Periodized Nutrition for Athletics. International Journal of Sport Nutrition and Exercise Metabolism. 2019.29(2), 141-151. https://doi.org/10.1123/ijsnem.2018-0305
Burke LM, Whitfield J, Heikura IA, et al. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. 2021;599(3):771-790. doi:10.1113/JP280221
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