pinarello legs closeup

Why Intermittent Fasting Is Worthless for Endurance Athletes


Intermittent fasting is worthless for endurance athletes. There, I said it (donning flak jacket…). You can keep doing it if you want to, just stop doing it because you think it’s going to make you faster.

It’s no better or worse than other forms of caloric restriction

There are three mainstream version of intermittent fasting:

  • 5:2 – Eat normally for five days and restrict energy intake to about 600 calories for two days, although the two days don’t need to be sequential.
  • 16:8 – Consume all your daily calories within an 8-hour window and fast the other 16. Most commonly, this means extending an overnight fast by skipping breakfast.
  • No calories for 1-2 days at a time.

All three can be effective methods of caloric restriction, which will lead to weight loss. However, none of them have been shown to consistently accelerate weight loss or lead to greater weight loss compared to reducing caloric intake every day.

That said, intermittent fasting can be more effective than traditional caloric restriction if it causes a person to focus more on actually eating less. In the short term, almost any caloric restriction strategy will be beneficial if leads people to concentrate and make thoughtful decisions about what they eat. Not only does it lower energy intake, but also people tend to improve the nutritional quality of the foods they eat during these periods of higher focus.

Traditional caloric restriction strategies (“just eat less of what you eat already”) work, but only if people can stick with them. And because reducing portion size is less of a dramatic or attention-grabbing change, it’s sometimes not interesting enough to hold a person’s focus. Long-term compliance is the problem lurking in the background for any dietary strategy aimed a weight control.

Most of you just need to TRAIN

Intermittent fasting is a strategy aimed at tuning your aerobic engine to burn more fat and/or utilize all substrates more efficiently. That’s great, but you’re trying to fine-tune a partially built engine. For athletes who have reached the point of diminishing returns with fundamental aspects of training, optimizing fat oxidation makes more sense. The vast majority of 40- to 60-year old men and women balancing training with work and family priorities, however, have a huge capacity for aerobic development before optimizing fat oxidation will make a substantial difference in performance.

Want to burn more fat? Get MORE FIT.

Your ability to oxidize any fuel for energy increases as you improve your general aerobic fitness and sport-specific fitness. That’s one of the primary adaptations that result from endurance training. Your body adapts to training stress by developing mechanisms to produce more energy more quickly, and that includes developing the mechanisms that convert carbohydrate, fat, and protein into usable energy. Intermittent fasting can help to fine-tune your ability to oxidize fat for fuel, but that improvement pales in comparison to the increased fat oxidation that results from boosting your overall fitness. To put it in financial terms, you’re stepping over dollars to pick up nickels.

There is an in-between step between eating and regular intermittent fasting that can improve performance for some people. The concept of starting select workouts with low carbohydrate availability, called “Train Low”, has been shown to improve endurance performance. Athletes either do two workouts in a day, using the first to deplete glycogen, or they train in the evening and restrict carbohydrate intake through the night. However, Train Low also lowers workout quality, so it has to be balanced with high intensity workout done with high carbohydrate availability. It is also a strategy that is best used when training for generalized fitness, as opposed to during race- or event-focused training.

Weight loss solves a lot of problems

One of the problems with anecdotal stories from athletes who attribute their performance improvements to intermittent fasting is that it’s difficult to discern a causal relationship when the athlete was training and losing weight while also fasting. Many scientific studies suffer from this problem, too. If subjects lose an average of 15 pounds during an 8-week period of intermittent fasting, I’d expect their performance in a time trial or exercise test to exhaustion to improve – no matter the weight loss strategy. Even studies that compare performance improvements following different weight loss strategies typically suffer from small subject pools and issues of individual variability.

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.

Significant weight loss isn’t just beneficial for going uphill. It reduces thermal discomfort and thermal stress, which reduces perceived exertion. It reduces the energy cost of basic daily functions and leads to various hormonal changes. In other words, weight loss leads to a cascade of changes that can improve athletic performance. And the training people did while losing weight helps improve performance, too.

Every workout counts for Time-Crunched Athletes

When you have less time to train, quality takes priority, and high quality training sessions require energy. If you have 3-4 workout opportunities per week, for a total of 6-8 hours per week, and you have fewer than 20 training sessions per month. With schedule conflicts, travel, and work, I find a lot of athletes average around 15 sessions and 25-30 total hours on a good month. My coaches and I have been very successful at improving performance for athletes within this timeframe, but we’ve also realized athletes who want to make significant progress have to be highly protective of that time. A handful of crappy or missed workouts can make a real difference, and a handful of great workouts can, too. Caloric restriction strategies increase the likelihood of low-quality training sessions, and lower power outputs and slower paces during interval workouts.

Eat Less, Just Make Sure It’s Enough

Most people in the US eat more calories than necessary. Athletes fall along the entire spectrum – some eat too much and some are chronically energy deficient. Every athlete has a sweet spot; enough energy from the right combination of nutrients to support high quality workouts and recovery, but without excess energy that leads to gain weight. Finding that sweet spot is a challenge and requires work, but in my view that’s the work you should be doing instead of fasting as a substitution for changing your food choices.

Generally speaking, eating less may be a good long-term strategy. A chronically lower-calorie lifestyle may be associated with longer lifespan and – as long as the food choices are good – reductions in disease risk. This is particularly true for diseases with a strong link to nutrition habits, like Type II diabetes. If fasting fits your long-term lifestyle, it reduces your overall calorie intake, and it makes you feel good, then go for it.

Just, please, stop doing it for performance.

Chris Carmichael
CEO/Head Coach of CTS

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 29

  1. I run 100 miles just about every month, eating low carb training & high carb racing with fewer calories solved most of my gi issues but I could never lose the last bit of weight I’ve carried for years. Throwing in some 16/8 IF daily melted off the extra weight without me cutting calories or nutrition, found it easy and super helpful & I’ve gotten faster.

  2. Pingback: New Year Nutritional Strategies for Runners – Koach Jen

  3. Pingback: How Endurance Athletes Can Decide If Intermittent Fasting Is For You - Chris Carmichael

  4. Why on earth would you, as a coach, offer opinion on this matter rather than the facts. They’re out there. I’ll include the most significant one here for you. It’s proven to lower heart rate for same level of effort, which of course WILL make you faster.
    “Conclusion: A caloric restriction improves athletes’ performance and energy efficiency…”
    Here’s the study:
    Not everyone fasts to lose weight, the health benefits are too numerous to list.

  5. As a retired Olympic athlete still pursuing the best health and fitness possible, the best solution I have found is to do periodic fasting (5 days each quarter) using fasting mimicking diet which is more protective of lean muscle (Dr Longo at USC figured this out). Then, you can get the health benefits of fasting but when training, still get the best quality workouts possible through adequate fueling and recovery. And no, you shouldn’t skip breakfast as an athlete, ever! I tried intermittent, and athletic progress suffered. And many people i know ended up with adrenal fatigue. Just wanted to share my experience..

    1. Wow. This is great to hear. I have been doing an FMD every 3 months (end of year, march break, end of June and September) for the last 6 years. I’m a teacher and this works really well with my yearly cycle.

      I follow Longo and have read parts of his book. Watched so many videos with him.

      Anecdotally, my best recovery from hard training always seems to come just after the fast. I am assuming that autophagy is responsible. After doing this for so many years, my body has become accustomed and can still train pretty hard during the fast. (I’m 57 years old if anyone is wondering and has read this far).

  6. I couldn’t help but comment on this. Part of your argument is a fallacy that people only intermittent fast to lose weight or fine-tune their engine. What if people people intermittent fast because of its efficiencies in everyday life which ironically you mention in your more time to train section. What if you can spend more time training instead of cooking an extra meal a day like breakfast and have a huge post-workout meal instead. Isn’t that a good thing? Or would that go against your narrative that intermittent fasting is worthless?

  7. Pingback: Intermittent Fasting – Is it right for endurance athletes? – Improved Sports Diet

  8. Pingback: Endurance Fueling with My Fat and UCAN – Matthew Kutilek

  9. Hello,

    I find it appalling that such misinformation can be spread without any reference to studies to base your arguments on. That being said, of course it is important to adapt your caloric intake based on the intensity/length of your workouts. It does not discredit the fact that intermittent fasting has been proved to effectively improve endurance performance.

    Here are some studies that have showed some results that contradict this article’s view on intermittent fasting’s benefits on different aspects of endurance training :
    – Triggers a process called “autophagy” that basically replaced old cells with new ones. Digestion prevents autophagy from happening. It has been demonstrated to help with recovery from injury and memory capacity (prevents Alzheimer)
    – Reduces glucose tolerance and improves stress resistance (experimented on nonobese subjects only)
    – Reduced levels of insulin (basically increases insulin sensitivity), reduced fat while keeping bigger proportion of total body composition as muscle mass (experimented on nonobese subjects only)
    – Releases up to 5 times more growth hormone which is known to accelerate tissue repair and, increase muscle growth and fat burning
    – Retain a better growth hormone secretion level while aging
    – Helps modulate a hormone that helps with inflammation caused by intense stress (such as endurance training)
    – Reduces the cholesterol levels and other fat cell ratio found in blood which ultimately helps with inflammation and reduces the risk of cardiovascular diseases

    Needless to say a coach should read about the science behind what they preach or at least try it with an objective mind. I would happily read the articles that helped you forge your arguments if you have time to formulate a response.

    If you are informed on the subject and I am incorrect, I will gladly change my mind. However, as of now, I highly recommend you to stop giving the advice NOT to practice intermittent fasting and abstain yourself from commenting on it until you have read its’ literature.


    1. My thoughts exactly. Fasting has far greater effects than weight reduction, autophagy being the most significant. In fact weight reduction is the least in a long list of benefits. There is life outside of cycling.

    2. The thing is that we are conscious athletes not like regular animals who benefit from fasting sometimes because of life situations. Training to hard on intermittent fasting brings negative consequences. As a professional tennis player and high intensity fitness trainer i´ve lived the negatives of restricting food with conscious hardcore training. Stimulates way more cortisol since we are on alert and depletes/damages adrenals sometimes irreversible. The Keto diet and all that BS i´ve been doing it for years before it became a THING. There´s always a limit and if you pass it you get hurt. My health was badly damage for fasting and training hard. I´m working on reversing the horrible damage to adrenals which got depleted hurting my overall life , losing hair, skin damage, etc. Fasting is great , gets you in great shape, you feel immortal and like superman, but there´s a fine line too. Your body for example gets incredibly good at burning fat 24/7 but then you need to get more fats from your diet as well you need to be careful with electrolites and get more salt. Lots of little details.

    3. Thanks for the references JC. Much appreciated. We interviewed Dr. Mark Mattson of Johns Hopkins, who wrote article for New England Journal of Medicine, on the topic of Intermittent Fasting. I must admit I was very skeptical coming in, but have changed my mind…
      Here is link to the interview —

    4. This reply would be so much more impressive if it included many different sources instead of just one.
      Articles from one institution tends to always lean one way – the way they think is right. It’s best to look at different studies funded by different sources to make an informed decision.

  10. Crickets from the poster lol. Can anyone answer this question for me please . I am an MMA fighter and be JJ competitor and have been having success for the last three weeks on a 16/8 fast. This Saturday will be my first BJJ competition and I will be doing it while fasted . My issue is that I have been going off of the advice that eating after 9 o’clock at night is bad however I’m going to have to eat at 11:00 PM so I am in a fast at 12:45 when I start competing the next day. Doing a hard endurance workout I feel better fasting but with these time restrictions I’m going to have to eat right before or during competition which seems backwards to me any advice

  11. Gentlemen, I must weigh in here because of what I have experienced with a five-day diet that mimic’s fasting. Your article claims that “Caloric restriction strategies increase the likelihood of low-quality training sessions, and lower power outputs and slower paces during interval workouts.” Arnie Fonseca Jr’s experience would seem to contradict the claim.
    My experience is also different from what you claim would be the outcome. I have done a 5-day fasting mimicking diet and experienced fantastic energy (surprised me!), a very clear mind (had not known there ordinarily was brain fog) and just simply felt great. Purpose was not to lose weight, but lost 3 lbs as the diet puts one into ketosis; most of the fat evidently is lost primarily around the waist.
    Based on science, done by Valter Longo, a professor of nutrition at USC (Univ of So Cal). Here is recent information from an email, “We are pleased to share with you that the science of fasting mimicking diets keeps getting supported and validated by the NIH and Patent offices around the world as an evidence-based breakthrough in the field of nutrition and medicine. You have been an early adopter of our fasting mimicking technology and we are happy to share the following news with you. ‘Valter Longo wins $10 million grant for his research on diets: Valter Longo’s research into how fasting and diets can protect against aging and diseases was awarded a $10 million grant from the National Institute on Aging. This is the highest grant ever issued to a research project on nutrition, fasting and aging. The grant is to be given out in $2 million installments over the next five years,’ a USC news release said. Longo is the primary investigator in a project involving researchers from USC and Harvard University. While Longo’s research focuses on how food can affect aging, the program grant applies to a variety of interventions to prevent aging through the regeneration of cells, systems and organs. Longo, who is the director of the USC Longevity Institute, designed a low-calorie diet that mimics the effects of fasting. Part of the grant funding will examine how his diet affects cellular regeneration and aging. We propose to improve these fasting-mimicking regimens, test their effect on mouse health span and test the hypothesis that they promote both cellular protection and stem cell-based regeneration in multiple systems,” Longo told USC News.”
    Valter has a book about the research, “The Longevity Diet: Discover the New Science Behind Stem Cell Activation and Regeneration to Slow Aging, Fight Disease, and Optimize Weight” – Jan 2, 2018, by Valter Longo; available from There is also a documentary film about Valter Longo’s work and fasting in Russia.
    So, ask you to keep an open mind, and read the evidence that is in the book, which is “scientifically verified” and in this case, as compared to many other situations and claims, from a very reputable source (in effect, USC).

    1. This is a brilliant rebuttal to an argument that needs to be challenged. I have also had great results with a one meal a day routine for the past 5 months including losing excess fat, increased stamina and strength. I do most of my trainer workouts and all of my endurance rides in a fasted stage and do not take on calories curing my rides. There is a bit of time to become fat adapted and once that is reached there is no turning back. Chris C. is out of date on his advice!

  12. Not a well researched article Chris. Int. Fasting can be the most useful way to not bunk. First you must become fat adapted, which I have been for almost 5yrs. Have raced as a cat 1 for 3 of those years and just 43 years old. I have ridden 4-6 hrs on nothing but water,electrolytes, and ketone salts. I am health coach and I would never prescribe to my athletes to become carb adapted. I only tell the simply to EST more fat and to intermittent fast so the body will adapt and not bonk cause of carb dependency.

  13. Thanks Kai! Great research find! There is a growing body of good science! As they say there is more than one way to skin a healthy cat!

  14. Another perspective from Dr. Gabe Mirkin

    Overnight Fasting to Increase Speed and Endurance
    A new study from France shows that night-time fasting after intense workouts on alternate days helps athletes exercise longer and faster (Med Sci Sports Exerc, April 2016;48(4):663-72).

    This is an impeccable study in which 21 trained competitive endurance athletes were randomly divided into two groups and all followed the same training program. They all ate the same foods and the same total amount of carbohydrates per day. The only difference between the two groups was the times of day that they ate their carbohydrates.

  15. As a 58 year old Exercise Physiologist and Endurance Athlete having first done Triathlons 35 years ago. I’ve always loved studying the latest dietary fads. A year ago while training for my second 100-mile desert run I was introduced to Fasting. Dismissed it as nuts. But was curious by my training partner who is a prolific Grand Canyon Runner, who shared some personal success and more importantly some health benefits. With my background I did my own extensive homework and discovered some amazing research. I began my own self study. Within a few weeks I was able to lose body fat, measured by inches on my waist, although most would say I didn’t need to. My goal was to maximize my fitness for my 100 and a R3 at the Grand Canyon. The key for me was the ease I was able to lose the body fat. This was because Fasting, 24-48 hours, allowed me to easily get into ketosis and increase my growth hormone, another benefit, which as you know is great for fat burning and building muscle! Fast forward. I’ve cut almost 4 hours off my hundred time, easily completed two R3’s at the Grand Canyon and Just recently easily completed the infamous “Cactus to Clouds to Cactus” trail run in Palm Springs. Which is one of the steepest trails in the world! Fasting isn’t for everyone but my blood tests, energy level, training success are enough evidence for me to continue. I’m also leaner than I’ve been in many years! Plus although I eat very clean. I never count calories! Lastly the amazing benefits of autophagy should not be dismissed! Thanks guys and keep up the great work!

    1. Hi Arnie, I understand from Dr. Longo that aupothogy happens after 4 days or so. If that is true, would you consider a 10 day fast for that benifit?

  16. The bit about intermittent fasting reads like a man in a pub sounding off. No clear basis. I believe the 5/2 diet was invented by Dr Michael Mosley after investigations into the benefits of calorific restriction in longevity and general health. As I understand it the point is that it’s more do-able for most of us than continuous calorific restriction. If you want to restrict calories you need to be rigorous in counting the calories that you eat, and to do that 24/7 is onerous and unlikely to fit into a normal life. But for two days, not so hard. Last year I floated up to 82kgs and decided to go 5/2 to get down to 76. Weight loss was pretty near constant at 700g per week, so it took just over 8 weeks. I see the 5/2 as a practical calorie management technique which will work for anyone, endurance athletes included.

  17. Spot on! I have always believed that I feel and perform my best while training toward a goal. I wish I could have the luxury of being in some form of training year long in my busy life. Reality is, I need to change some priorities.

    You mention Type II diabetes as a health condition that can be staved off by proper nutrition and weight control. Bravo! You also tapped on my shoulder when commenting on the condition of experiencing thermal stress while performing overweight. This may well have been the button-pushing I needed to hear to make adjustments in my life to include more training in my routine.

    Thank you so much for your specificity and timely article. I believe it to have helped stave off what negativity is being built around my life. If I act now, in as few as 6 weeks I could be back on top of my game but today I feel I have slayed a Giant!

    I’m starting today

Leave a Reply

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