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HOW TRAINING WORKS · 10 MIN

Fuel and intensity

Read the zones as a fuel story, and know why intensity changes what you need to eat.

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Fuel use across exercise intensityIllustrative curves for a trained athlete. Fat use peaks near 63% of VO2max and fades to near zero by about 91%. Carbohydrate use climbs steadily, then steeply. Carbohydrate overtakes fat as the main energy source near 55%. LT1 sits near 66% and LT2 near 86%. Marker at 44% of VO2max, Z1 recovery. About 58% of energy from fat, 42% from carbohydrate.Z1Z2Z3Z4Z5LT1LT20246g/min30405060708090100EasyIntensity, % of VO2maxMaxCarbohydrateFatFatmaxCrossover44%

Illustrative curves, typical trained athlete. Your thresholds move with training and are measured, not assumed.

At this intensity

Z1Recovery

44% of VO2max · Recovery jog

Fat 58%Carbohydrate 42%

Share of energy, approximate.

Carbohydrate burned
≈ 60 g/hApproximate, ~70 kg trained athlete. Burned, not what the gut can absorb.
Typically held for
Many hoursDepends on fitness, pacing and fuelling.
On the run
Recovery jogZ1 · Recovery

FuellingMostly fat, with a steady trickle of carbohydrate. Short sessions here rarely need fuel on the go.

STEP 1 OF 5

Two fuels, always both

You burn fat and carbohydrate at every intensity. What changes is the mix. At easy effort fat supplies a large share; as effort rises carbohydrate takes over.

Fat stores are large even in lean athletes. Carbohydrate stores last roughly 90 minutes to a couple of hours of hard work.

THE TAKEAWAY

The mix of fuel shifts with intensity.

Read the whole lesson

01 · Two fuels, always both

You burn fat and carbohydrate at every intensity. What changes is the mix. At easy effort fat supplies a large share; as effort rises carbohydrate takes over.

Fat stores are large even in lean athletes. Carbohydrate stores last roughly 90 minutes to a couple of hours of hard work.

The takeaway: The mix of fuel shifts with intensity.

02 · Where fat burning peaks

Fat burning in grams per minute peaks at a moderate intensity, often around 55–65% of VO2max in trained athletes. That is steady, conversational work, usually Zone 2.

Above that the body can still use fat, but carbohydrate is quicker to turn into energy, so its share rises fast.

The takeaway: Fat use peaks at a steady, moderate effort.

03 · The crossover

The crossover is the intensity where carbohydrate starts supplying more of the energy than fat. Past it, every step up in effort costs more carbohydrate.

This is why long races are fuelled with carbohydrate: even a steady marathon or long-course bike leg is above the crossover for most athletes.

The takeaway: Above the crossover, carbohydrate supplies most of the energy.

04 · Two thresholds, five zones

Turn on lactate. It stays low until the first threshold, then rises, and climbs steeply after the second. Most five-zone systems are built on these two turning points.

Below the first threshold you can go for hours. Near the second, about an hour. Above it, minutes.

The takeaway: Lactate turning points anchor the zones.

05 · Training moves the curves

Switch between trained and less trained. Training shifts fat burning and the thresholds to higher intensities, so the same pace or power costs less carbohydrate.

Thresholds are measured, not assumed. Form reads yours from your own data rather than a formula.

The takeaway: The curves are yours, and they move with training.

Sources for this lesson

Teaching models of typical patterns in healthy, trained adults. Individual responses vary. Not medical advice.

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