Fasting Doesn’t Increase Fat Oxidation or Workout Performance, New Study Shows

By Jason Koop,
Head Coach of CTS Ultrarunning

Intermittent fasting and time restricted eating have gotten a lot of attention lately. It pops up on podcasts, with social media influencers and in an array of apps that will tell you when and for how long your ideal fast should be. I have even written about fasting in this blog (and you can see Chris’ take on it here). Many proponents of using fasting as a tool for athletes focus on the purported benefits stemming from increases in fat oxidation. And certainly, there are situations where taking advantage of an overnight fast followed by a moderate length, low intensity run might be a useful tool in promoting increases in fat oxidation (as I’ve written about here). However, recent research has suggested that not all fasted training scenarios are created equal.

Researchers at the Swansea University in Wales wanted to study how breakfast omission impacted evening workouts. Do to so, they employed eleven highly trained cyclists (VO2max >61 ml/kg/min) to perform two 20-kilometer cycling time trials. In one condition, the cyclists were asked to keep up their normal eating routine of breakfast and then lunch. In the other trial, the cyclists were asked to forgo breakfast and then eat a larger lunch to make up the difference in calories. Each subject performed both trails in a randomized order.

There are a few interesting things to take note of within the study design. First, the cyclists in each trial consumed an equal number of calories by the end of the lunch feeding (~1500 kcals). The breakfast omission group simply consumed their breakfast calories during lunch (see the figure below).

Second, the no-breakfast group essentially created a 14-hour fast, which is similar to a 16:8 time restricted eating protocol that is popular among the fasting crowd. It’s also a scenario that is quite common for a lot of athletes who also fast (skip breakfast, make up for it at lunch, workout in the evening).  Finally, since the study design was a cross-over, we get to see individualized effects.

Results: A rare double negative

During the no-breakfast trial, the cyclists performed ~3% worse.  This is completely unsurprising, as the finding is in line with several other studies that suggest that breakfast (particularly a carbohydrate rich one) improves performance across a range of activities, from short high-intensity bouts, peak power output trials, time trials, and even resistance training (for a review, see this). However, an interesting side point is that the no-breakfast group also showed a decrease in fat oxidation during the trial (and a subsequent increase in carbohydrate oxidation). This is the exact opposite of what the proponents of fasting promote as a benefit. So, now we have a situation where skipping breakfast specifically to promote fat oxidation does the exact opposite in a real world scenario, and causes a decline in performance. You literally get the worst of both worlds. You perform worse and fail to glean any of the purported fat burning benefits from the fast.

In addition to these results, I also found it interesting that 8 out of the 11 cyclists performed worse during the no-breakfast trial, and the ones that performed better did so very marginally (see the right side of the chart below where the dots below the dashed line represent the athletes who were worse after the no-breakfast trial and the dots above the dashed line were better). In many other nutritional intervention studies, we see a wider discrepancy between positive and negative responses, reflecting the highly individualistic nature of nutrition. The fact that the group was so consistent between the trials further reinforces the conclusions and take aways, which are….

Take Home

First off, if you have a high-intensity workout planned for the evening, don’t skip breakfast. This is pretty cut and dry and has been demonstrated across a wide array of scenarios. In fact, if you skip breakfast and think you can ‘make up for it (the calories)’ at lunch, you will still be at a performance deficit. Second, skipping breakfast will not improve your fat oxidation rates. It might even make it worse. The authors theorize that the mechanism of action for this has to do with the timeframe associated with digesting a large quantity of food relative to the timing of the time trial.

Free Cycling Training Assessment Quiz

Take our free 2-minute quiz to discover how effective your training is and get recommendations for how you can improve.

Free Ultrarunning Training Assessment Quiz

Take our free 2-minute quiz to discover how effective your training is and get recommendations for how you can improve.

The bottom line is two-fold:

  • If you want to maximize your evening workout, eat breakfast. A larger lunch does not make up for it
  • If you are looking to increase fat oxidation for your evening workout, skipping breakfast in contraindicated.


Richard S. Metcalfe, Matthew Thomas, Christopher Lamb & Enhad A. Chowdhury (2020) Omission of a carbohydrate-rich breakfast impairs evening endurance exercise performance despite complete dietary compensation at lunch, European Journal of Sport Science, DOI: 10.1080/17461391.2020.1797890

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;12(11):3473. Published 2020 Nov 12. doi:10.3390/nu12113473

Enjoying This Article? Get More Free Running Training Tips

Get our coaches' best training advice, delivered straight to your inbox weekly. 

This field is for validation purposes and should be left unchanged.



FREE Mini-Course: Learn How to Maximize Your Limited Training Time

Learn step-by-step how to overcome limited training time and get faster. Walk away with a personalized plan to increase your performance.

This field is for validation purposes and should be left unchanged.

Comments 15

  1. Workout performance is different from max performance where you feed optimally for output. These should not be optimized with the same diet plan. Also you say “fasting” in your title. But simply shifting a bit of calories between two meals are not fasting, and it barely qualifies as intermittent fasting. No surprise in these results. It’s like saying 1+2=2+1.

  2. Surely this is primarily a study to determine the effect of lunch loading? The influence of skipping the prior breakfast (for a single occasion) seems likely to be minimal…the author suggestion of any performance impact being psychological seems reasonable.
    Jacques question above about riding fasted is more sensible.

  3. Koop’s instagram post about this blog article starts with “News flash – Fasting by skipping breakfast down [sic] not promote increases in fat oxidation and leads to decreases in performance”

    That’s misleading. The athletes fasted for one single period, not as a continued intervention employed to create meaningful adaptations. Then the athletes who had fasted gorged themselves on lunch before their time trial. Maybe they hadn’t pooped yet? That might account for a 3% decline in performance.

    Koop’s claim is analogous to saying “News flash – hard workouts do not promote better running economy and they actually lead to decreases in performance”. The statement I just made could **absolutely** be proven true if I employed a protocol that had athletes perform a grueling workout on the evening before a time trial event. Those athletes would be sore for the time trial, therefore they’d likely show worse running economy, and they’d have worse performances.

    The order of employing such interventions matters, as does the amount of time it takes for the body to undergo relevant adaptations. This post is pure clickbait. Koop is out to discredit the fat-adaptation crowd at any cost, and unfortunately it’s causing him to compromise on the quality of his writing.

    disclaimer: I’m an elite ultra runner who doesn’t eat breakfast before 3-4 runs per week, but I eat lots of carbs and I eat before every hard workout. Both dietary practices are useful. Koop should stick to writing about real-world applications of research, not polarizing one-sided clickbait. No serious athlete would think about skipping breakfast the day of a race, yet that’s what this protocol explored.

    1. I thought the same thing. A Time Trial…. why not a z2 spin on the bike in the morning to check how fat oxidation was effected?

  4. What about if you train first thing in the morning ~430-5am so eating before isnt really ideal since sleep might be more important. In my situation, I have been eating a high carb dinner to have full glycogen stores for the next morning.

    or is it worth it even to just get in a quick gel or something with 100-200 cals in the morning?

  5. Another study designed to get the results wanted to support the belief that fasting is bad.

    Basic physiology is that higher fat oxidation requires lower insulin. So if you measure fat oxidation after eating a large meal (in this case lunch) which will increase insulin (esp. if lots of carbs are eaten) of course fat oxidation will be reduced.

    And about that worse performance. First, three percent is small and likely not significant especially considering all of the variables that affect a persons performance. Second, these subjects were not used to fasting – it takes time to adapt to changes like this.

    So you have your old beliefs about fasting and carbs, but how about just being honest and saying that these are your beliefs not facts.

    The science is weak and biased and changing all the time. A better approach is to see what actually works for athletes with regard to their body composition and performance. Theories come and go, but results persist.

    1. Comments are well said and agree 100%.
      During medical training, we were taught to simply read All journals through the eyes of “the Skeptic”.
      Look for design flaws, look for way to invalidate, look to see How and If they loaded the deck to achieve their desired outcomes (their null hypothesis).

      This one is silly, but at least it gets people talking.
      Ride Safe, fuel intelligently….

  6. The proponents of intermittent fasting stress that if you are performing a cycling workout above Z2 you should ingest carbs early and often. I fast 16 hours per day overnight BUT only on days where I have a planned endurance or recovery ride or no ride at all. When I need to perform intervals or plan on going “hard” (i.e., time trial), I eat a normal breakfast in the morning.

  7. I have been intermittent fasting 16/8 for 13 weeks, and training as usual, (12-14 hours/week) I have dropped 16 lbs, body fat has dropped from 19% to 15%, the quality of workouts have actually improved. So, not too sure about your study. Not a one size fits all.

  8. Curious about your thoughts around this same breakfast fast combined with a low carb/keto diet for calories. Would it promote an increase in fat oxidation due to there being a minimal amount of carbs to burn. I would assume performance would still be worse, but fat oxidation better?

  9. This is all very interesting…..for people who work out in the evening. I actually work out in the morning (~6:00 AM) and on days that I ride (seasonally), I also do so in the morning….and yes, I do so on an empty stomach (If I feel the need for more energy, my gel bottles contain maple syrup….it’s a Québec thing…). What then is the impact on fat oxydation and performance for those of us in the “no breakfast” camp who work out prior to breaking our fast?

  10. Is it possible that the results were skewed due to the training levels of the test subjects? Are there any other comparable studies that focused on lesser trained athletes or nonathletes?

Leave a Reply

Your email address will not be published. Required fields are marked *