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. 

SPONSORS: 

• Precision Fuel & Hydration produce our favourite gels, sports drinks, and electrolyte and carbohydrate prod