The idea of a fat-burning zone sounds temptingly simple: there is an intensity at which the body oxidises the most fat per minute — find it and run there. That intensity does exist, it is called FATmax, and it can be measured. The problem starts at the next step, when people try to assign the same one to everybody.

The largest sample we have

In 2017, Medicine & Science in Sports & Exercise published a study that remains the benchmark for scale to this day. 1,121 athletes — 933 men and 188 women — from different sports and different levels. All of them did an incremental treadmill test in a fasted state (at least five hours without food), and fat oxidation was determined by indirect calorimetry from gas exchange.

The average figures:

maximal fat oxidation (MFO) — 0.59 ± 0.18 g/min, ranging from 0.17 to 1.27 g/min;
FATmax — at 49.3 ± 14.8% of VO₂max, ranging from 22.6 to 88.8%.

The second number is the central point of the whole paper. A range from 22.6 to 88.8 percent means that for one athlete peak fat metabolism falls at a very easy jog, while for another it is almost at threshold pace. And these are athletes, not random people off the street.

Within a single sport the spread also stayed large. Among footballers, fat oxidation per kilogram of fat-free mass varied from 4.1 to 20.5 mg/kg/min — a fivefold difference between teammates.

Sex differences work in both directions

Here is a curious twist that is easy to garble.

In absolute terms, men oxidised more: 0.61 versus 0.50 g/min (p < 0.001).

Per kilogram of fat-free mass, women oxidised more: 11.0 versus 10.0 mg/kg/min (p < 0.001).

Both figures are correct; they simply answer different questions. The average man is larger, so he burns more in total. A kilogram of female muscle, meanwhile, oxidises fat more actively.

A 2025 study in the American Journal of Human Biology reproduced this picture in a small sample of 20 people (12 men, 8 women) and added a detail. In the fitter group (VO₂max 56.5 ± 2.5 versus 46.9 ± 3.1 mL/kg/min), both fat oxidation per kilogram of fat-free mass (3.34 ± 1.44 versus 2.73 ± 0.87, p = 0.001) and speed at FATmax (7.67 ± 0.90 versus 7.00 ± 0.97 km/h, p = 0.044) were higher. But absolute fat oxidation did not differ significantly between the groups (0.67 versus 0.71 g/min, p = 0.124). Women were again ahead per kilogram of fat-free mass: 3.92 ± 1.35 versus 2.40 ± 0.46 (p = 0.015).

Half of the variation nobody can explain

The authors of the 2017 study checked what determines an individual's level of fat oxidation. Fat-free mass, VO₂max, FATmax itself, body fat percentage and — separately interesting — the duration of the fast before the test all turned out to be significantly associated.

All these variables together explain 47% of the differences between people. Which means more than half of the spread remains unexplained.

The practical conclusion is simple and inconvenient: you cannot calculate your own FATmax from a formula. Not from age, not from heart rate, not from VO₂max, not from body fat percentage. The number your app or watch shows you is a model estimate based on a population average, and here the population average explains less than half of reality.

Fasting shifts the result — which means it is not a constant

This point deserves particular attention, because it undermines the very idea of FATmax as a personal characteristic.

The duration of the fast before the test was among the significant predictors. A test done fasted after a night's sleep and a test done two hours after breakfast will give different FATmax values in one and the same person. A test on the back of a low-carbohydrate week is a third value.

Put differently, FATmax is not like your height or your shoe size. It is a state, not a constant. And it is measured under conditions (five or more hours of fasting) in which almost nobody trains seriously.

How much is that in calories

It helps to convert grams into something intuitive. The average 0.59 g/min is roughly 5.3 kcal/min from fat, that is, about 320 kcal per hour. Even for someone at the top of the range (1.27 g/min) it works out to around 685 kcal per hour — and that is in a fasted test at a perfectly chosen intensity.

Which leads to an honest conclusion about weight loss: fat oxidation is a narrow tap. An energy deficit is what solves that problem, and the choice of intensity only slightly changes which source the fuel is coming from right now. An hour at FATmax instead of an hour in a higher zone does not give you the kind of gain that would justify rebuilding your plan.

Where it genuinely matters

The real significance of fat metabolism for endurance is not weight loss but glycogen sparing. Your carbohydrate stores are finite, and the more of your energy at race intensity you take from fat, the later you hit the wall.

But note the condition: at race intensity. For most people, marathon pace is noticeably above FATmax, and at those intensities the picture is quite different. Training fat metabolism does make sense — but the way to aim at it is through volume and work in the aerobic zones, not through trying to calculate and sit on one magic number.

And one more important distinction: FATmax, zone two and the first lactate threshold are three different things. They often end up close together, but with FATmax ranging from 22.6 to 88.8% of VO₂max, that coincidence is guaranteed to nobody.

Being honest about the limitations

The 2017 study is cross-sectional. One test, one measurement, no follow-up over time. It describes the spread, but it does not tell us how stable FATmax is within one person from month to month.

A treadmill test. Transfer to the bike is not automatic: muscle recruitment and technique are different.

The 2025 study has only 20 people. It is consistent with the large sample, but on its own it is weak, and the split into fitness groups there was done by clustering within a small sample.

Indirect calorimetry has its own assumptions — it derives the contribution of each substrate from the ratio of oxygen consumed to carbon dioxide produced, and that is an estimate, not a direct measurement.

Key points

FATmax is the intensity of maximal fat oxidation, a real and measurable quantity.
A sample of 1,121 athletes: MFO 0.59 ± 0.18 g/min (from 0.17 to 1.27), FATmax at 49.3 ± 14.8% of VO₂max — with a range from 22.6 to 88.8%.
Within a single sport the spread persists: among footballers, from 4.1 to 20.5 mg per kg of fat-free mass per minute.
Men are higher in absolute terms (0.61 versus 0.50 g/min), women per kilogram of fat-free mass (11.0 versus 10.0 mg/kg/min).
Known factors explain only 47% of the differences — you cannot calculate your own FATmax from a formula.
Fasting duration shifts the result: FATmax is a state, not a constant.
The average 0.59 g/min ≈ 320 kcal/h from fat. For weight loss, what decides it is the energy deficit, not the choice of zone.
The real value of fat metabolism is glycogen sparing over long distances.
FATmax, zone two and LT1 are not synonyms, even though they often end up close together.

Sources: Randell R.K., Rollo I., Roberts T.J. et al., “Maximal Fat Oxidation Rates in an Athletic Population”, Medicine & Science in Sports & Exercise, 2017;49(1). https://doi.org/10.1249/MSS.0000000000001084. Jaén-Carrillo D., García-Pinillos F., Plaza-Florido A. et al., “Maximal Fat Oxidation Rate in Healthy Young Adults. Influence of Cardiorespiratory Fitness Level and Sex”, American Journal of Human Biology, 2025;37(1):e24212. https://doi.org/10.1002/ajhb.24212