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← How it works

08

Making and spending energy

How cells produce usable energy — and what happens when demand repeatedly outruns supply.

Thinkingcosts energyStanding, movingEvery cellmakes itRest refills itCellsGut
SpendingRecoveringCellsGut

A schematic, not to scale — the places this mechanism works.

Part one — how this normally works

Energy is made continuously, not stored in bulk

Mitochondria convert fuel and oxygen into ATP on demand. Only a few seconds' worth is held at any moment, so supply has to track demand minute by minute — which means circulation, breathing and fuel delivery are all part of the energy system.

Recovery is an active process

After exertion the body repays an oxygen debt, clears metabolites and rebuilds. Adaptation happens during that recovery window, not during the effort itself.

Thinking costs energy too

Cognitive and emotional work, sensory filtering and masking all draw on the same budget as physical activity. They are not free.

Part two — what varies between bodies

  • Capacity changes with sleep, illness, iron status, thyroid function, deconditioning and hormonal phase.
  • Some bodies recover overnight; others need days, and that difference is not effort or motivation.
  • Standing itself is metabolically expensive when circulation is working harder to maintain pressure.

Part three — where it diverges

This is the first place conditions get named

  • When exertion is followed by a delayed, disproportionate and prolonged worsening across several systems — typically 12 to 48 hours later — that pattern is post-exertional malaise, the defining feature of ME/CFS and common after Covid-19 infection.
  • Where post-exertional malaise is present, graded exercise programs can cause harm; NICE guidance for ME/CFS recommends against them.
  • Orthostatic intolerance and energy limitation compound each other, because upright posture is itself a load.
  • Fatigue driven by pain, poor sleep or anemia is a different problem from post-exertional malaise and needs a different plan.

What moves this mechanism

  • Establish whether post-exertional malaise is present before any activity plan is written.
  • Pacing to stay inside the envelope on good days as well as bad, so the baseline stops sliding.
  • External data — heart rate, step counts, logs — where the body's own signals arrive late.
  • Treat the fixable contributors: iron, thyroid, sleep, orthostatic load.

Conditions that involve this mechanism