Endurance athletes spend many hours training in a fasted state. Maybe you do too?

If so, you know that fasted training equals impaired feeling, performance, and significant suffering. Why? Convenience. Preference. Habit. Tradition. You name it. Even the unfortunate belief that it’s the only way to improve endurance training adaptations.

Yes — under the guise of “fat adaptation” the endurance community has equated fasted training directly with improved cellular adaptations specific to exercise.

Let’s get technical and specific real quick: We’re talking gene expression and mitochondrial biogenesis resulting from increased reliance on fat as your body’s preferred energy source during low intensity exercises. Thus, less need for carbohydrates and increased endurance adaptations.

While this all occurs, the question of whether it is as helpful as it sounds is a pertinent one. Does fat adaptation improve performance? And in what events or context?

To answer that, let’s first take a step back. There’s always a trade off when it comes to metabolism. So while training in a fasted state can improve fat oxidation rates, it comes at the cost of carbohydrate oxidation rates.

And we know that carbohydrates are your body's preferred fuel source during high intensity training. Plus your body needs to learn how to efficiently utilize the amount of carbohydrates you need during your race. So training without the carbs that you will be utilizing on race day is like strictly training in Zone 2 and expecting to race at Zone 5. Why wouldn’t you train how you race?

In addition, the belief that there are improved cellular adaptations that can only occur from fat adaptation, aside from improving fat oxidation rates, is, well… simply not the case.

Let’s explore the findings in a recent paper.

A recent study has illuminated the matter further, showing how well-fueled training with carbohydrates results in cellular adaptations that were previously thought only possible with carbohydrate-restricted diets.

That's because your body is in a completely different metabolic state while exercising than while sitting on the couch.

The results? No difference in intramuscular fat content (that’s fat stored in muscles used to fuel performance, somewhat similar to muscle glycogen), and no change in muscle glycogen use or exercise-induced cell signaling when subjects fueled with differing amounts of carbohydrates during exercise (0g/hr, 45g/hr, or 90g/hr).

90g/hr has been the top end of standard carbohydrate supplementation recommendation during exercise. Now, its proven you can implement such nutritional strategies that promote training quality with high carb intake without sacrificing cellular adaptation.

So what does this really mean for you?

Regardless of the type or amount of fuel intake during the training session, exercise reduced the amount of fat in the muscles and did not change the cell signaling involved with building new mitochondria. (Mitochondria, of course, being the powerhouses of the cell, is crucial for energy production and key for performance in endurance sports.)

So if there is no difference, what does it matter?

Carbohydrate intake during exercise improves your ability to perform at higher intensities, allowing you to go faster for longer. That’s just a fact. (And this is dose dependent, meaning the more carbohydrates you take in, the higher your intensity can be.) Additionally, carbohydrate intake reduces fatigue perception, improves fatigue tolerance, and increases exercise capacity.  

The study found that maintenance of glucose levels through carbohydrate feeding and increasing glucose availability without steep spikes improved performance by more than 60% in a time to fatigue trial on the bike.

Note: The above points are largely related to overall glucose availability, limiting steep spikes and drops in glucose levels, and higher glucose intake during the activity. In fact, the reduction of glucose variability, the steep spikes and drops in glucose levels, is what helps your body optimize the use of both fat and glucose in exercise. So, in a sense, fat adaptation is achieved through better glucose control. Optimizing your glucose levels to prepare, perform, and recover to achieve that is the core benefit of continuous glucose visibility.

So eat all of the carbohydrates then?

Not quite. It’s always a little more technical than that.

The body is known to burn what it has available. So the more carbohydrates that were ingested, the more carbohydrates that were supposedly burned. This means that fat was used less, and there was a delay in ramping up the burning of fat.

That being said, your body also has its own limit in terms of maximum oxidative capacity. So, if your aim is to try and achieve 100% fat oxidation, mixing in some fasted training sessions could still be beneficial.

But (and perhaps the most interesting finding) they saw the same rate of muscle glycogen use in the working muscle regardless of intake. That’s right: Even though you intake more carbohydrates, your body still doesn't spare your fuel stores, your muscle glycogen. Which means you run out of your high-intensity fuel stores at the same rate regardless of your nutrition strategy. This key finding highlights the importance of proper pre-exercise glucose loading and sufficient in-exercise glucose intake.

So where does this leave us?

Despite the legends of fasting, there is no need to deprive yourself of energy. In fact, avoiding low glucose events can be crucial for certain performance adaptations. So when you need high output and are seeking peak performance for your training sessions, fuel up and stay fueled. And keep an eye on your glucose levels, it serves as a guide that can inform performance potential.

The takeaway:

You're not sacrificing cellular adaptations when training in a well-fueled state. Training without fuel is not as necessary for cellular adaptations as once thought. If you maintain stable glucose levels through a well-planned carbohydrate feeding schedule (before and during exercise), you will enhance performance while also ensuring key cellular adaptations—some previously considered impossible with a carbohydrate diet.

Bottom line: Well-fueled training not only increases your performance, but also allows for the adaptations you thought you had to suffer through training on a low carb diet to get.


  1. Impey, Samuel G et al. “Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis.” Sports medicine (Auckland, N.Z.) vol. 48,5 (2018): 1031-1048. doi:10.1007/s40279-018-0867-7
  2. Fell, J Marc et al. “Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle.” The Journal of physiology, 10.1113/JP281127. 26 Mar. 2021, doi:10.1113/JP281127
  3. van Loon, Luc J C et al. “Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state.” The Journal of physiology vol. 553,Pt 2 (2003): 611-25. doi:10.1113/jphysiol.2003.052431