The Real Problem With FatMax (It's Not About Carbs vs Fat)

If you are focusing on improving your FatMax in 2026, you’re likely wasting time and money. Not only is fat a more expensive substrate to burn than carbohydrate (you get less energy for the same amount of oxygen by oxidising fat), but the Fatmax number you see in your lab report is mostly noise and very little signal.  In today’s episode of That Triathlon Show I’ll explain exactly why that is, but I’ll also give you a tool to evaluate any test or measure that you might (or might not) want to be tracking, from Time Trials to VO2max, HRV and various biomarkers like ferritin and testosterone.  HIGHLIGHTS AND KEY TOPICS:  What is Fatmax and Maximal Rate of Fat Oxidation?  How reliable is Fatmax testing?  How to measure the noise of a test using the Coefficient of Variation (CV) How to calculate the Smallest Detectable Change (SDC) of any test or measure Why the SDC of Fatmax is the equivalent of you having to go from 300 to 384W for your 20-minute power to be able to say that this was real improvement and not just noise (!!)  CVs and CV ranges for common tests and measures used in triathlon, Ironman and other endurance sports, including Time Trials, Time To Exhaustion, VO2max, lactate and ventilatory thresholds, economy and gross efficiency, lactate concentration, Critical Power and W’, HRV, ferritin, testosterone, TSH and more Why carbohydrate is a 7% more efficient energy substrate than fat, and why you should be oxidising carbs in your next Ironman.  DETAILED EPISODE SHOWNOTES:  We have detailed shownotes for all of our episodes. The shownotes are basically the podcast episode in written form, that you can read in 5-10 minutes. They are not transcriptions, but they are also not just surface-level overviews. They provide detailed insights and timestamps for each episode, and are great especially for later review, after you've already listened to an episode.  The shownotes for today's episode can be found at https://scientifictriathlon.com/tts700/ LINKS AND RESOURCES:  Full bibliography in the shownotes: https://scientifictriathlon.com/tts700/ WHAT SHOULD I LISTEN TO NEXT? If you enjoyed this episode, I think you'll love the following episodes, related to sports science and (the third episode listed) fat adaptation and performance.  The replication crisis in sports science with Joe Warne, PhD | EP#468 The Skeptic’s Guide To Sports Science with Nicholas Tiller, PhD | EP#239 High carbohydrate, low carbohydrate, or periodised carbohydrate intake with Louise Burke, PhD | EP#236 You can find our full episode archives here, where you can filter for categories such as Triathlon Training, Racing, Science & Physiology, Swimming, Cycling, Running etc. You can also find separate archives for specific series of episodes I've done, specifically Q&A episodes, TTS Thursday episodes, and Beginner Tips episodes.  LEARN MORE ABOUT SCIENTIFIC TRIATHLON:  The Scientific Triathlon website is the home of That Triathlon Show and everything else that we do Contact us through our contact form or email me directly (note - email/contact form messages get responded to much more quickly than Instagram DMs) Subscribe to our Newsletter Follow us on Instagram Learn more about our coaching, training plans, and training camps. We have something to offer for everybody from beginners to professionals.  HOW CAN I SUPPORT THAT TRIATHLON SHOW (FOR FREE)?  I really appreciate you reading this and considering helping the show! If you love the show and want to support it to help ensure it sticks around, there are a few very simple things you can do, at no cost other than a minute of your time.  Subscribe to the podcast in your podcast app to automatically get all new episodes as they are released. Tell your friends, internet and social media friends, acquaintances and triathlon frenemies about the podcast. Word of mouth is the best way to grow the podcast by far!  Rate and review the podcast (ideally five stars of course!) in your podcast app of choice (Spotify and Apple Podcasts are the biggest and most important ones). Share episodes online and on social media. Share your favourite episodes in your Instagram stories, start a discussion about interesting episodes on forums, reference them in your blog or Substack.  SPONSORS: Precision Fuel & Hydration produce our favourite gels, sports drinks, and electrolyte and carbohydrate products here at That Triathlon Show and Scientific Triathlon. Use the free Fuel & Hydration Planner to get a personalised plan for your carbohydrate, sodium and fluid intake in your next event, and get 15% off your first 2026 order by using the code TTS2026 at checkout. Rouvy is hands down the most complete indoor cycling platform for triathletes. Among their thousands of beautiful bike courses from all around the world, all filmed in stunning quality, they have over 75 IRONMAN and IRONMAN 70.3 race courses plus 20+ Challenge Family courses, so you can pre-ride your race from home. Real gradients, real visuals, and real feel! Head to rouvy.com and use the code TTS to get your first month free on top of a 7-day free trial. Effortless Swimming produce the best swim goggles for triathletes and open water swimmers. Their NanoClear anti-fog lenses give you clear, fog-free vision that lasts and doesn't wear off. Don't let foggy or leaky goggles ruin another swim. Go to shop.effortlessswimming.com and use the code TTS15 to get 15% off your goggles, and get a free two-month Effortless Swimming course membership.LEARN MORE ABOUT SCIENTIFIC TRIATHLON: The Scientific Triathlon website is the home of That Triathlon Show and everything else that we doContact us through our contact form or email me directly (note - email/contact form messages get responded to much more quickly than Instagram DMs)Subscribe to our NewsletterFollow us on InstagramLearn more about our coaching, training plans, and training camps. We have something to offer for everybody from beginners to professionals. HOW CAN I SUPPORT THAT TRIATHLON SHOW (FOR FREE)? I really appreciate you reading this and considering helping the show! If you love the show and want to support it to help ensure it sticks around, there are a few very simple things you can do, at no cost other than a minute of your time. Subscribe to the podcast in your podcast app to automatically get all new episodes as they are released.Tell your friends, internet and social media friends, acquaintances and triathlon frenemies about the podcast. Word of mouth is the best way to grow the podcast by far! Rate and review the podcast (ideally five stars of course!) in your podcast app of choice (Spotify and Apple Podcasts are the biggest and most important ones).Share episodes online and on social media. Share your favourite episodes in your Instagram stories, start a discussion about interesting episodes on forums, reference them in your blog or Substack. 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