Individual variation and trainability
Evidence: strong
The same training and the same products work differently for different people. Treat published averages as a starting point to test on yourself, not a law.
The same training produces very different results in different people. In the HERITAGE Family Study, 20 weeks of an identical programme changed VO₂max by anywhere from about −5% to +48% across participants, and up to about half of that variation in trainability is heritable, an upper-bound estimate (Bouchard et al., HERITAGE). Genuine low responders and high responders exist for the same stimulus.
It is the most important caveat in all of endurance training. Group means describe the average runner, who does not exist. A training method, a shoe, or a supplement with a solid average effect can still do little or nothing for a given individual.
Where it shows up
- Super-shoes. Across studies, individual responses to carbon-plated shoes range from roughly 11% hindrance to 11% benefit, and no measured trait reliably predicts who responds (Knopp et al. 2023; Van Hooren 2025). The best shoe for one runner is often not the best for another.
- Dietary nitrate. The effect is blunted or absent in highly trained athletes and largest in recreational ones (McMahon et al. 2017).
- Caffeine. Response may vary with CYP1A2 genotype, though the evidence is contested (Guest et al. 2021).
- Training philosophies. No single training system is best for everyone; the right one depends on the event, training age, time available and injury history.
What to do about it
Treat published effects as a starting hypothesis, then test on yourself with enough signal to clear the noise. This is where honest use of wearable trends and the idea of a smallest worthwhile change matter: an intervention has worked for you only if the change it produces is larger than your own measurement and day-to-day variation.