Metabolic flexibility describes how well your body matches its fuel, fat or carbohydrate, to what you’re asking of it. Fitter riders burn more fat at a given power and keep more carbohydrate for the hard parts of a race, and aerobic training is what builds that (evidence B). Matching carbohydrate to the work of each session is a sound framework, though it hasn’t been shown to improve performance on its own (evidence C). The research doesn’t support chasing maximum fat burning: in the best controlled trials, keto diets made elite athletes less economical and slower. This is what the evidence says, graded, and how we apply it at reset cc.
The term comes from diabetes research. In 2000, Kelley and Mandarino used it to describe healthy muscle’s ability to burn mostly fat when fasting and switch to glucose when insulin rises, a switch that insulin resistance blunts. In sport the term is used more loosely, for how well fuel use tracks exercise demand. There is no agreed test for it.
In a rider, it shows up in two places. At easy and moderate intensities, a flexible rider gets more of their energy from fat, which spares glycogen. At high intensities, everyone relies mostly on carbohydrate, and a flexible rider can use it well when it counts.
In a 2018 study of 52 men, San-Millán and Brooks compared professional road cyclists with moderately active men and men with metabolic syndrome. The professionals burned fat at a peak of about 0.67 g per minute at about 239 W. The moderately active men peaked at 0.38 g per minute at 132 W, and the men with metabolic syndrome at 0.18 g per minute at 108 W. Professionals also had lower blood lactate at comparable power.
It’s a striking picture, with real limits. The study was cross-sectional, all male, and the groups weren’t matched for age, so it can’t show what causes what. The two non-professional groups also peaked at the first stage of the test, so their real peaks were probably missed. Fitness also explains only part of the difference between people. In a study of 300 adults, fitness, activity level and sex together explained 12% of the variation in peak fat oxidation. Across 1,121 athletes, peak fat oxidation averaged 0.59 g per minute and occurred anywhere from 23% to 89% of VO2max (a study funded by the Gatorade Sports Science Institute, part of PepsiCo).
FatMax is the intensity where you burn the most fat. On average it sits a little below your first lactate or ventilatory threshold, by about 22 W or 11 beats per minute across 42 studies. The gap varies a lot between individuals, so one can’t stand in for the other without testing.
Our zones are anchored to lactate. Most of our riders’ volume sits in zone 1. Zone 2 sits just below the first lactate threshold and counts as a harder session. If your zone 2 is set from a lactate test, it will usually sit close to your FatMax, but don’t call it your FatMax unless you’ve measured it.
Method | What the trials found | Evidence |
|---|---|---|
Building aerobic fitness | Fitter riders burn more fat at a given power, though fitness explains only part of the difference between people | B |
Training fasted | In 20 young men over six weeks, muscle enzymes rose more with fasted training, but fat use during exercise, VO2max and time-trial performance improved equally in both groups | C |
Train low or sleep low | Some sessions with low carbohydrate boost training signals in muscle, but a meta-analysis of nine studies found no overall performance benefit | A for no benefit |
Keto or low-carb, high-fat diets | Fat burning roughly doubles, but in elite race walkers the oxygen cost of race pace rose and the performance gain from training disappeared | B |
Carbohydrate during long rides | 30 to 60 g an hour for 1 to 2.5 hour sessions, and up to 90 g an hour beyond that with glucose and fructose | A |
Two of these deserve more detail.
Fasted training. In Van Proeyen’s six-week trial in active young men who weren’t endurance-trained, the fasted group’s muscle enzymes and the power at their peak fat oxidation rose more than in the group training with carbohydrate. But the share of energy from fat during a two-hour fasted ride rose in both groups, and VO2max (up 9%) and 60 minute time-trial performance (up 8%) improved equally. A meta-analysis of 46 studies found that eating before exercise improved prolonged aerobic performance (evidence B).
Keto diets. A study of keto-adapted ultra-endurance athletes found fat burning of about 1.5 g per minute, roughly double that of athletes on high-carbohydrate diets. It was cross-sectional, didn’t measure performance and was funded by organisations with an interest in low-carbohydrate diets. In Burke’s controlled trials in elite race walkers, who were allocated to diets by preference rather than at random, three weeks of keto adaptation raised the oxygen cost of race pace, and the walkers on high-carbohydrate and periodised diets improved their 10 km times while the keto group didn’t. A short return to carbohydrate restored neither carbohydrate use nor performance (evidence B). In eight cyclists, fat adaptation followed by carbohydrate loading left 100 km time-trial time unchanged but cut 1 km sprint power (evidence C).
What follows is our coaching practice. Where the research is behind it, we say so.
Most masters riders can absorb about two hard sessions a week, three in a big week. Zone 1 fills the days in between and protects the quality of those sessions. Volume is what builds the engine, and a bigger engine burns more fat at any given power.
Hard sessions are fully fuelled, before, during and after. In the off-season, some easy rides are done with lower carbohydrate, and hard sessions stay fully fuelled. This contrast fits the “fuel for the work required” framework from Impey and colleagues (evidence C). We don’t promise that it improves performance on its own, because the trials haven’t shown that.
These are the targets we use:
Easy rides under 2 hours: 30 to 60 g an hour
Easy rides over 2 hours: 60 to 90 g an hour
Hard rides under 2 hours: about 70 to 90 g an hour
Hard rides over 2 hours: 90 to 120 g an hour
These are coaching targets for fuelled rides, outside planned low-carbohydrate rides, and some sit above consensus guidance, which tops out at 90 g an hour. Trained riders absorb and burn more carbohydrate at 120 g an hour than at 90, but trials haven’t yet shown a performance benefit above 90 g an hour. Lighter riders burn less of what they take in (evidence C), so they should build up gradually.
Training with low carbohydrate availability can raise protein needs (evidence C). Easy low-carbohydrate rides should still sit inside a day with enough protein.
Blood lactate profiling is a practical way to locate your thresholds, as long as the protocol, the pre-test diet and the threshold method stay the same between tests. Estimates of fat and carbohydrate burning from breath gases are reasonable at steady state, but they become unreliable above the second threshold, so treat ramp-test readings at high intensity with caution (evidence B).
Chasing maximum fat burning. It’s a marker of fitness, and there’s no trial showing that raising it improves race performance.
Going keto for endurance racing. The best trials in elite athletes show worse economy and no performance benefit.
Training fasted every day. It adds stress and has not beaten fuelled training for performance.
Under-fuelling hard sessions. The work you can do in your key sessions drives adaptation. Fuel them.
12-week plan / start any Monday
This is how I sequence the off-season for the athletes I coach, written so you can ride it on your own. It takes you from the end of the off-season through the preseason, with every session laid out.
Riding hours per week, Standard tier. Strength adds about 2 h.
On the bike
In the gym
Two weeks of technique, then single-leg main lifts at 4 to 6 reps.
Every main lift is single-leg, with no barbell and no load across your back.
Pick your tier
All three ride the same key sessions.
Choose it ifYou ride 8 to 10 hours a week in season.
What you get
You need a power meter or smart trainer, and a gym with heavy dumbbells or kettlebells. A home version of the strength program is included.
It’s how well your body matches its fuel, fat or carbohydrate, to the demand of the moment. The term comes from diabetes research, and in sport it’s used for how well fuel use tracks exercise intensity. There’s no agreed test for it.
Not on its own, as far as the evidence goes. In a six-week trial, fasted training raised muscle enzymes more, but time-trial performance improved equally with fuelled training.
The best controlled trials say no for racing. Keto adaptation raises fat burning, but in elite race walkers it made them less economical and wiped out the performance gain from training, and a short return to carbohydrate didn’t reverse it.
Consensus guidance is 30 to 60 g an hour for 1 to 2.5 hour sessions and up to 90 g an hour for longer ones. We use up to 120 g an hour for long, hard rides, built up gradually in training.
They’re close on average, with FatMax a little below the first threshold, but they can differ a lot in an individual rider. Only a test tells you where yours is.
Coaching and membership · 2027 season
Every reset cc rider gets training, strength and a nutrition plan matched to each day's work, written week by week for their schedule.
Team membership · 2027 season
Three levels, from one team camp to the whole season on the road to Worlds. Every level includes:
One-to-one coaching
Your own reset cc coach, and a plan built around your week, your data and the races you care about.
Kelley DE, Mandarino LJ (2000). Fuel selection in human skeletal muscle in insulin resistance: a reexamination. Diabetes.
Goodpaster BH, Sparks LM (2017). Metabolic flexibility in health and disease. Cell Metab.
San-Millán I, Brooks GA (2018). Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Med. Evidence C.
Venables MC et al. (2005). Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol. Evidence B.
Randell RK et al. (2017). Maximal fat oxidation rates in an athletic population. Med Sci Sports Exerc. Funded by the Gatorade Sports Science Institute (PepsiCo). Evidence B.
Hogan C et al. (2026). The agreement between Fatmax and the gas-exchange and blood lactate thresholds: a systematic review and meta-analysis. Med Sci Sports Exerc. Evidence B.
Van Proeyen K et al. (2011). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol. Evidence C.
Aird TP et al. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: a systematic review and meta-analysis. Scand J Med Sci Sports. Evidence B.
Gejl KD, Nybo L (2021). Performance effects of periodized carbohydrate restriction in endurance trained athletes: a systematic review and meta-analysis. J Int Soc Sports Nutr. Evidence A.
Impey SG et al. (2018). Fuel for the work required: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Med. Evidence C.
Volek JS et al. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. Evidence C; funded by Quest Nutrition and the Atkins Foundation.
Burke LM et al. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. Evidence B.
Burke LM et al. (2021). Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. Evidence B.
Havemann L et al. (2006). Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance. J Appl Physiol. Evidence C.
Thomas DT et al. (2016). American College of Sports Medicine joint position statement: nutrition and athletic performance. Med Sci Sports Exerc. Evidence A.
Podlogar T et al. (2022). Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g/h versus 90 g/h. Eur J Appl Physiol. Evidence C.
Gillen JB et al. (2019). Low-carbohydrate training increases protein requirements of endurance athletes. Med Sci Sports Exerc. Evidence C.
Ijaz A et al. (2025). Exogenous glucose oxidation during exercise is positively related to body size. Int J Sport Nutr Exerc Metab. Evidence C.
Jeukendrup AE, Wallis GA (2005). Measurement of substrate oxidation during exercise by means of gas exchange measurements. Int J Sports Med. Evidence B.
Zack Morris is the founder of reset cc. Silver medalist, M35-39 time trial, 2026 UCI Gran Fondo World Championships. National champion, elite and masters. Built and ran Team EF Coaching for five years.
