It's the Rate, Not the Duration

01The evidence

• The kinetics of ketone body metabolism in fasting humans were quantified decades ago, showing that the rise follows a characteristic time course and that individuals differ in how quickly they reach a given concentration.Metabolism, 1979
• Ketone concentrations during fasting have been used clinically as a marker of the fasting state, which presupposes that the concentration and its trajectory carry information about the state rather than just the elapsed time.Clinical reference

02The tension

Fasting research is organised by duration: sixteen hours, twenty-four hours, seventy-two hours. The results vary within a duration, and the variation is usually attributed to adherence or to baseline metabolism. Duration is easy to prescribe and easy to measure, which is probably why it has survived as the independent variable.

03The theory

The shift from glucose to ketones is a control transition with a rate as well as a threshold. When the rate is fast, the metabolic and signalling consequences appear together and quickly. When the rate is slow, the system lingers in an intermediate state where neither the glucose-dependent nor the ketone-dependent programme is fully engaged.

If the rate is the operative variable, then two people fasting for the same time can be in different states, and two people in the same state can have fasted for different times. That is exactly the pattern that duration-based studies report as noise.

The prediction is that a rate-accelerating intervention, such as exercise or a lower carbohydrate starting point, should reproduce the benefits of a longer fast in a shorter window, and that slow switchers should be poor responders regardless of duration.

This also explains the inconsistency of fasting trials in a way that adherence cannot: if the responders are the fast switchers, then a trial with a fixed duration is effectively a trial with variable dose.

04What this is not

05Predictions

• P1Metabolic benefits should correlate with the rate of ketone rise better than with fasting duration.
• P2Accelerating the switch should reproduce the benefits of a longer fast in a shorter time.
• P3Slow switchers should show little benefit even after prolonged fasting, and should be identifiable in advance.

06What would refute it

Benefits tracking duration at fixed rise rate would restore time as the dose.

07What it would change

It would personalise fasting protocols by metabolic rate rather than by clock, and change how trials are designed.