Introduction: Is an athlete born or made?

The classic debate between the influence of nature (genetics) and nurture (training, nutrition, environment) takes on a fascinating dimension in the realm of elite athletic performance. Historically, it has been assumed that hard work and perseverance were the only factors capable of forging a champion. However, contemporary sports medicine and functional genomics reveal a much more complex reality: while effort determines whether an athlete reaches their maximum individual potential, it is their underlying genetic makeup that precisely defines where that biological ceiling lies.

Interindividual variability in parameters such as aerobic capacity (VO2 max), muscle fiber composition, metabolic flexibility, susceptibility to musculoskeletal injuries, and post-exercise recovery capacity has an estimated hereditary component of between 50% and 80%. This discovery has revolutionized training planning and sports nutrition, giving rise to performance genetics. Far from the pseudoscience of simplistic commercial tests, current evidence supports the use of DNA profiling to design precision nutrition strategies and personalized training, minimizing risks and accelerating results.

The genetic foundation of muscle fiber: The ACTN3 gene

Strength and speed are fundamental physical capabilities directly influenced by the composition of our muscle fibers. In this field, the ACTN3 gene stands out as the absolute reference molecular marker in the scientific literature.

The R577X polymorphism and its impact on explosive strength

The ACTN3 gene encodes the alpha-actinin-3 protein, a critical structural component located exclusively in the sarcomeres of fast-twitch (type II) muscle fibers, which are responsible for generating fast, explosive, and high-power movements. The rs1815739 single nucleotide polymorphism in this gene dramatically alters its expression through a nonsense mutation (substitution of arginine with a stop codon, known as the R577X variant).

Individuals with the RR genotype possess full and functional expression of alpha-actinin-3 in their fast-twitch muscle fibers, which optimizes lateral force transmission, protects muscle fibers against damage induced by repeated eccentric contractions, and promotes a greater hypertrophic response to strength training. Elite athletes in sprinting, weightlifting, and jumping disciplines exhibit a significantly higher frequency of the RR genotype than the general population, solidifying ACTN3 as the true "sprint gene."

The XX variant: Endurance, muscle economy, and the "loser" gene

At the other end of the spectrum is the XX genotype. Individuals with this genetic profile have a complete deficiency of alpha-actinin-3 in their skeletal muscles. Surprisingly, this deficiency does not cause any muscle pathology, as it is partially compensated for by alpha-actinin-2 (present in slow-twitch fibers), but it profoundly alters the function of fast-twitch fibers.

Fast-twitch fibers deficient in alpha-actinin-3 exhibit increased mitochondrial enzyme activity, behaving metabolically more similarly to slow-twitch and endurance (type I) fibers. Individuals with the XX genotype demonstrate improved running economy, superior fatigue resistance, and optimized cardiovascular adaptation to prolonged stimuli, genetically predisposing them to excel in marathons, triathlons, and long-distance cycling. However, XX carriers also show a higher risk of delayed onset muscle soreness (DOMS) following high-intensity eccentric training, requiring specific adjustments to their training periodization and nutritional recovery.

Aerobic capacity and the ACE gene

The ability to efficiently transport and utilize oxygen during strenuous exercise is measured by VO2 max, a highly heritable parameter that differentiates elite endurance athletes from amateurs.

The insertion/deletion (I/D) polymorphism

The ACE gene encodes angiotensin-converting enzyme, a key regulator of the renin-angiotensin-aldosterone system that controls blood pressure, plasma volume, and vascular tone. This gene exhibits a functional polymorphism characterized by the presence (Insertion, I) or absence (Deletion, D) of a 287-base-pair repeated DNA fragment in intron 16 (rs4646994).

Performance variants in cardiovascular endurance

The resulting genotypic profile directly influences athletic performance:

• Genotype II (Homozygous insertion): It is associated with substantially lower circulating and tissue levels of the ACE enzyme. This low enzyme activity promotes greater peripheral vasodilation and increased capillary density in skeletal muscle. This results in greater efficiency of oxygen delivery to active muscle cells. Carriers of genotype II exhibit a greater increase in their VO2 max in response to endurance training and are overrepresented in high-altitude mountaineers and long-distance runners.
• DD Genotype (Homozygous Deletion): It is associated with elevated ACE levels, increased vasoconstriction, and a significantly enhanced hypertrophic response of the left ventricle to resistance training. This profile favors short-term muscle power and skeletal hypertrophy, and is common in sprinters and short-distance swimmers.

Injury prevention and post-training recovery

Genetics not only determines how fast we run or how strong we are at lifting weights; it also decisively influences our vulnerability to injury and the speed at which our body repairs damaged tissue.

Collagen genes and susceptibility to tendon injuries

Tendons and ligaments are predominantly composed of type I and type V collagen. The COL1A1 gene (rs1800012) and the COL5A1 gene (rs12722) regulate the synthesis and assembly of these structural collagen fibrils. Specific risk variants in the COL5A1 gene (such as the CC genotype) result in a less dense and mechanically weaker collagen structure. Individuals carrying these genetic variants exhibit up to a fourfold increased incidence of chronic tendinopathies (e.g., in the Achilles tendon) and anterior cruciate ligament (ACL) ruptures, necessitating preventive intervention based on controlled eccentric strengthening and tailored supplementation with collagen peptides and vitamin C.

Antioxidant capacity and the SOD2 gene

Strenuous exercise induces a massive production of reactive oxygen species (ROS), or free radicals, in skeletal muscle. The SOD2 gene encodes the enzyme manganese superoxide dismutase, the first line of antioxidant defense within the cell's mitochondria. The rs4880 polymorphism causes an amino acid change from alanine to valine (Ala16Val), which impairs the enzyme's transport into the mitochondria. Carriers of the low-cellular-activity variant (Val/Val genotype) experience markedly higher oxidative stress and prolonged cell membrane damage after exertion. For these athletes, the strategic use of dietary antioxidants from whole foods and optimized rest are essential to prevent overtraining.

Personalized sports nutrition based on DNA

Genetic information is useless if it is not translated into practical nutritional strategies. Sports nutrigenomics allows for the synchronization of nutrients with the enzymatic capabilities dictated by DNA.

Caffeine metabolism and the CYP1A2 gene

Caffeine is one of the most widely used ergogenic aids in sports, but its effectiveness is highly variable. The CYP1A2 gene encodes the cytochrome P450 1A2 enzyme, responsible for the hepatic clearance of caffeine. The rs762551 polymorphism (change -163A>C) classifies individuals into two groups with opposite responses:

• Rapid metabolizers (AA genotype): They eliminate caffeine efficiently and experience a clear ergogenic benefit from 3% to 7% in their strength and cardiovascular endurance performance when consumed 45 to 60 minutes before exercise.
• Slow metabolizers (AC and CC genotypes): They metabolize caffeine slowly. In these individuals, prolonged caffeine in the bloodstream induces coronary vasoconstriction and increased cardiovascular stress during exercise, which not only negates the ergogenic benefit but can also impair performance and increase the risk of tachycardia or early fatigue. Non-stimulant energy sources are recommended for them.

Inflammatory response and glycogen recovery

Genetics also modulates the kinetics of muscle glycogen replenishment and the post-exercise inflammatory cascade. Specific variants in the interleukin-6 gene (IL6, rs1800795) predispose individuals to an exacerbated and persistent pro-inflammatory response following exercise-induced muscle damage. Carriers of high-inflammatory response genotypes require nutritional guidelines rich in highly bioavailable omega-3 fatty acids (EPA and DHA) and bioactive polyphenols (such as anthocyanins from berries or curcumin) to modulate physiological inflammation without blocking natural muscle adaptations to training.

Conclusion: Genetics as a roadmap, not as a destination

Sports genomics does not promote biological determinism. Carrying a genotype unfavorable for endurance does not mean you cannot complete a marathon; it means that your metabolic pathway to that goal requires a different training and nutrition plan than that of an athlete genetically gifted for long-distance running.

Your genes don't choose whether you train today or not; that's your personal commitment. But your genes do unequivocally dictate how your muscles will respond to that effort, how quickly you'll recover, and what specific nutrients your cellular biology needs to maximize every drop of sweat. By analyzing your DNA profile using the Oorenji platform (https://oorenji.com), you transform genetics into your greatest competitive advantage, training and fueling with the precision your biology demands.

Scientific references

• Yang, N., MacArthur, D.G., Gulbin, J.P., Hahn, A.G., Beggs, A.H., Easteal, S., & North, K.N. (2003). ACTN3 genotype is associated with human elite athletic performance. American Journal of Human Genetics, 73(3), 627-631.
• MacArthur, D. G., & North, K. N. (2007). ACTN3: A genetic influence on muscle function and athletic performance. Exercise and Sport Sciences Reviews, 35(1), 30-34.
• Montgomery, H.E., Marshall, R., Hemingway, H., Myerson, S., Clarkson, P., Dollery, C., … & Humphries, S.E. (1998). Human gene for physical performance. Nature, 393(6682), 221-222.
• Guest, N.S., Horne, J., Vanderhout, S.M., & El-Sohemy, A. (2019). CYP1A2 genotype modulates the ergogenic effects of caffeine on physical performance: a systematic review. International Journal of Sport Nutrition and Exercise Metabolism, 29(1), 52-59.