New Research:  Low-Carb High-Fat and High-Intensity Performance

New Research: Low-Carb High-Fat and High-Intensity Performance

There's been plenty of heated debate over the years on whether endurance athletes on low-carb fueling regimes - can deliver top-end performance.
A new rigorous research paper has just been published, which presents strong evidence for performance - more on that below.
Before jumping in, lets first answer the question of what 'top-end intensity' is for endurance athletes, and take a look at a few recent performances from low-carb high-fat athletes.

Endurance racing: What is Top-End Intensity?

Racing intensity for most endurance athletes' (even some pros) for a full Ironman is typically in the 77-82% (FTP) range, and upto 85% for 70.3 distances., suggests that full Ironman racing is typically performed at ~70% of Vo2Max, 75% for 70.3 distances, upto 85% for Olympic distances, and likely higher for 5000m track racing.
So the first point is if the race duration is over 4 hours (i.e. Ironman, ultras, and 70.3s etc.) then realize, that your race intensity is mostly supported by the aerobic system, of which fat is the pre-dominant fuel source.


Can Low-Carb Athletes deliver?

This is kind of a tongue-in-cheek question these days, with so many world-class performances from low-carb high-fat athletes.  Here's  a few recent anecdotes from the SFuels stable -


  • Zach Bitter's 100-mile treadmill world record, had him punching out 7.18-minute miles for over 12 hours.  Zach's 2019, 100-mile track world record, had him running sub-6.50-minute miles, for over 11 hours.
  • Dr. Dan Plews, 2018 Kona age-group course record, had him run a 2.50.56 marathon, which translates to a highly consistent sub-7-minute mile pace across the whole run leg. 
  • Matt Kerr's 2021 age-group Ironman World Championship victory in St George Utah, delivered the fastest run leg of all age-groupers, 2.57.57, on a course with 1400ft elevation gain, and temperatures in 80-90F through the bike-run legs.


2023 Research Paper - Findings

In a new paper (Feb, 2023), a group of highly reputable sports-science researchers, challenged the former premise, that athletic performance was superior via a high-carbohydrate diet and fueling regime.  

What was most striking at first glance of the paper, was that the question of 'endurance performance' was tested at a very high intensity (@~85% vo2Max), shorter duration tests including 1mile, and 6 by 800m interval repeats.   Traditional thought would consider such intensity-duration testing, as 'anaerobic' and by nature more dependent on carbohydrate oxidation, and less-so fat oxidation.  But in a rigorous highly controlled testing protocol, and elegant research design, the team produced a set of findings that will certainly challenge traditional thinking.  The study summarized four notable findings - 


  • High-Carb athletes had no superior (high-intensity) performance over LCHF (low-carb high-fat) athletes, 
  • Even at intensities at >85% Vo2Max, LCHF athletes were oxidizing fat at rates of 1.58gr/min (+/- 0.33gr/min).
  • LCHF athletes had higher HDL, LDL, and total cholesterol, 
  • LCHF athletes, had reduced mean-median blood glucose levels.  Conversely, 30% of the High-Carb athletes, had mean-median blood-glucose levels consistent with pre-diabetes. 

    SFuels: Perspective on the Research
    It's great to see such a well-controlled set of factors (diet, training load, body composition, etc.) and high-intensity testing protocol, where the core, pre-dominant factor of difference was diet and fueling. 

    In this study, a 2018 paper on cross-country skiers and our own lab observations of low-carb high-fat athletes, we are seeing more consistent references to high-fat oxidation rates (>1.5gr/min) at high intensities >80% Vo2Max.  This is supportive of the suggestion, that with such metabolic efficiency, the body could provide a glycogen-sparing effect when fat can be predominately used for fuel at these high intensities.

    It is an important note, that aside from the athletic performance outcomes, a number of health markers were also tested, and reference was made to the complications (pre-diabetes) that were associated with high-carb diet athletes.  

    I guess from a balanced perspective, you would like to see more subjects in such a study, and also would like to have the study repeated for women.  Nevertheless, the paper highlighted that prior studies that had less attention to study design controls (diet, fueling and training-load factors) drove different and non-repeatable findings. 

    Most critical to the metabolic and performance outcomes of this study, is the diet protocol used.

       1) Carbohydrate volume, <50gr/day,
       2) % calories from fat per day, 75-80%,
       3) the duration of intervention, 31 days. 

    This is consistent with the guidance given by SFuels and Endure.IQ for an effective transition (~cold-keto phase) to efficient fat oxidation, and metabolic flexibility. 

    As published by Rothschild and team, a review of 125 studies, and over 1100 subjects, revealed that diet (fueling), and training duration are some of the most pre-determinate factors of substrate-fat oxidation efficiency

    In observing professional and age-group athletes' lab data, it's clear that training alone doesn't guarantee fat-oxidation efficiency and subsequent performance in endurance racing. Diet is critical.

    Bottom-line: ensure you pay attention to the guidance (above 3 points in blue) for an effective low-carb high-fat diet transition period - in realizing the intended metabolic shift, and performance outcomes of a low-carb lifestyle.

    To help you with this, download and read our - 

    1) SFuels LIFE Guide - giving you over 40 simple meal, snack, and shake ideas for a low-carb high-fat lifestyle - using SFuels TRANSFORMSFuels RevivialSFuels Keto3SFuels TRAINSFuels LIFE Bars. 

    2) SFuels Fueling Guide - and noted research on the impact of Medium Chain Triglycerides in diet and fueling.

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