Since the 1968 Mexico City Olympics, runners from Kenya and Ethiopia have dominated middle- and long-distance running, cross-country and the marathon. The temptation is to look for a gene. And there is a famous candidate: ACTN3, nicknamed the sprinter gene, with a variant that supposedly tips you toward power or endurance. The problem is that when scientists actually genotyped the best endurance runners on the planet, the story fell apart, and what replaced it is more useful for anyone who trains.
ACTN3, the sprinter gene
ACTN3 codes for alpha-actinin-3, a structural protein found only in fast-twitch muscle fibres, the ones that produce explosive force. A common variant, R577X, comes in two forms: R, which makes the working protein, and X, which does not. People with two X copies (XX, about 18% of Europeans) have no alpha-actinin-3 at all, and their muscles lean slightly toward slow-twitch, fatigue-resistant behaviour. A 2024 meta-analysis of more than 14,000 people confirmed the sprint side of the story: the R allele and the RR genotype are markedly more common in elite power athletes than in endurance athletes or non-athletes.
The test that failed
If X really were the endurance allele, East African distance runners should be full of it. Researchers led by the Pitsiladis group genotyped 284 elite Kenyan and 76 elite Ethiopian endurance runners against local controls. The result: the XX genotype was found in roughly 1% of Kenyans and 11% of Ethiopians, no different from the general population, and there was no association between R577X and running status. The best marathoners in the world mostly carry the so-called power version. ACTN3 nudges the sprint end of the spectrum; it does not decide who wins a marathon.
So what does explain it?
The 2012 review by Wilber and Pitsiladis in the International Journal of Sports Physiology and Performance summarises two decades of measurements, and the picture is physiological rather than genetic in the single-gene sense:
- Running economy, not VO2max. Elite Kenyans have a maximal oxygen uptake similar to elite Europeans. What differs is the oxygen cost of running at a given speed, which is lower.
- Body shape. Low body mass (BMI around 20), body fat around 5%, and above all long, slim lower legs (calf circumference around 34.5 cm). Larsen's work showed that when running economy is adjusted for lower-leg cross-sectional area, the difference between Kenyan and Danish runners disappears. A lighter shank swings with less effort, thousands of times per race.
- Altitude from birth. Most of these athletes were born and train between 2,000 and 2,500 m. Ethiopian highlanders also show a distinct adaptation: at 3,530 m they keep haemoglobin and oxygen saturation in the sea-level range, a pattern that differs from Andeans and Tibetans and whose genetic basis is still being mapped.
- Volume from childhood, then intensity. Running to school, moderate-volume high-intensity training at altitude, and a strong economic motivation to succeed.
Two more points matter. The dominance is regional, not continental: a few communities such as the Kalenjin in Kenya and the Arsi and Shewa highlands in Ethiopia account for most of it, while West African populations dominate the sprints. And a 2022 PLOS One study found that the genetic differences between these groups and their neighbours are small; whatever genetic contribution exists is spread over many variants of tiny effect, layered on top of altitude, morphology and training.
The genes your DNA report reads
- ACTN3: R577X, your fast-twitch protein status.
- ACE: the I/D variant, over-represented in endurance and high-altitude athletes.
- HIF1A: the master switch of the body's response to low oxygen.
- PPARGC1A: mitochondrial biogenesis, how readily muscle builds aerobic capacity.
- VEGFA: capillary growth in response to training.
Together these markers explain only a few percent of performance, and the report says so. Their value is in telling you where you start, not where you finish.
What to do with this
- XX is not a marathon handicap. The Kenyan data prove it. XX carriers may need a little more recovery after eccentric work (downhills, plyometrics), so progress those gradually.
- RR responds fast to strength and sprint work. Use it, but endurance is still built the same way: volume and consistency.
- Economy is trainable. Strides, hill repeats, plyometrics and a light shoe all reduce the oxygen cost of running, whatever your genotype.
- Altitude works for everyone, modestly. A few weeks at 2,000 m or more raises red-cell mass in most people; it is a tool, not a birthright.
Important: this article is educational and is not medical advice. Genetic tendencies explain a small share of athletic performance and are not a prediction of your results. Get a medical check before starting or intensifying an endurance programme.
See where your own power and endurance genes sit
The Fuel Your DNA DNA & Sport Performance report reads ACTN3, ACE, HIF1A, PPARGC1A, VEGFA and 35 other sport genes from the raw file you already have (23andMe, AncestryDNA, MyHeritage), with a power-to-endurance gauge, recovery, injury-risk and caffeine sections. See the report.