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by: Matt Fitzgerald

High-fat diets have become quite the fad within the ultrarunning community. I’m currently training for my first ultramarathon.

So have I switched to a high-fat diet? Uh, no. Everything I’ve learned about eating for endurance performance from my experiences as an athlete, as a sports nutritionist, and as a student of the eating habits of the world’s greatest endurance athletes militates against hopping aboard the high-fat bandwagon.


Here are the 15 reasons I’ve decided to give high-fat diets a pass in my ultrarunning journey:


  1. They’re based on reductionistic theory of “biological plausibility.” Specifically, their supporting theory assumes, wrongly, that the only important thing a runner’s diet can do for him or her is increase the fat-burning capacity of the muscles. No consideration is given to other diet-related benefits, such as immune system support, that might be sacrificed in going all-in for fat burning.


  1. They do not to enhance endurance performance. Increased fat-burning capacity resulting from high-fat eating seldom results in faster race times.


  1. The best athletes, who achieve the best results (London Marathon winners, Tour de France champions, Ironman legends), do not use them.


  1. They do not, as their advocates claim, obviate the need to consume carbs during races to maximize performance. No matter how “fat adapted” you are, you will perform best if you consume carbs at the highest rate you can comfortably tolerate.


  1. They are proven to reduce anaerobic and aerobic capacity.


  1. They reduce training capacity and cause chronic fatigue in many runners. If I had a nickel for every athlete who has come to me complaining that the bottom dropped out from under his or her training after he or she switched to a high-fat diet, I would drive a much better car.


  1. They’re highly disruptive, requiring runners to essentially start over with their training (much like barefoot running).


  1. There are far less drastic means, such as “sleeping low” (i.e. doing an evening workout and then going to bed without eating carbs), to achieve the same ends that high-fat diets seek.


  1. They’re extremely unbalanced nutritionally. Goodbye, fruit and whole grains! It was nice knowing you!


  1. They cause health consequences such as high triglycerides in many runners.


  1. They’re culturally abnormal, monotonous, and difficult to sustain. Good luck ever enjoying another nice dinner out with friends if you’re on a 70% fat diet.


  1. They’re cultish. Like climate-change deniers, many high-fat diet advocates clearly want to believe that high-fat diets are better.


  1. They foster a “magic bullet” mentality that often steers runners away from making more basic dietary improvements that actually would help them.


  1. They appeal to serial dieters and exacerbate this tendency. I’ve never helped an athlete dealing with the consequences of a high-fat diet who had not previously fallen for other extreme diets.

    15. Like other extreme diets, they often serve as a stepping stone toward eating disorders. I’ve seen this all too often as well…


Matt Fitzgerald is an author and runner who is chronicling his journey to become an ultra runner in 2016.


Scott is the founder of Becoming Ultra and spends most of his time with his family and ideas to get people moving!

2 thoughts on “15 Reasons I’m Not On A High Fat Diet”

  1. I’m trying the fat adapted diet and have a marathon this weekend. So far I have accomplished to sustain my muscle with more body fat. My waist size is the same as it was. I like lifting and running and found before the switch in diet my muscles were getting smaller and looked skinny. I’m trying the Maff system for my next race in June and I should see if I’m a believer.

  2. Matt – it’s interesting. I’ve read your book “How Bad Do You Want It” – bravo, outstanding book. Well written and terrific content.

    I’ve also just read “Run” – and while you have some really terrific stuff, sometime you speak out of both sides of your ass. For example: you have a whole section debunking the POSE and Chi Running trainers and philosophies and in the same section talk about how you went from injury prone to changing your cadence and striking with the forefoot/midfoot instead of the heel and all of your injuries went away… hmmmm, sounds like POSE and Chi Running to me…

    In the above article you talk about the “15 reasons…” including the Tour de France winners who don’t use this diet … have you actually asked them? My guess is no (as a former Pro Cyclist, yes they do) – except some use steroids instead but that’s a while other discussion. You talk about cults and enjoying nice dinners… there are many dinners that are quite terrific with a high fat diet…

    Have you actually read the FASTER study? Have you really researched the results that high level athletes are having with various diet lifestyles?

    While, nothing is perfect for everyone that doesn’t mean that just because it doesn’t fit with your ideas that it is not good for anyone. Instead of debunking various “cults” with your opinions, you may want to go back to school and actually study the facts and look at your own results (running form as a case in point) instead of throwing out something just because you don’t like it and didn’t come up with it.

    Other than that.. great stuff, keep it coming! 🙂

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