How are ACTN3 and longevity related?

The relationship between ACTN3 and longevity is indirect and based on observational evidence. ACTN3 encodes α-actinin-3, a protein in fast-twitch muscle fibers. A common loss-of-function variant (R577X) results in no α-actinin-3. Carriers of the XX genotype tend to have better endurance performance and a more favorable metabolic profile (e.g., lower blood lipids, reduced inflammation). Some studies suggest a modest association with increased lifespan, especially in older adults. However, the effects are small, and the evidence is moderate at best. Mechanistically, a shift toward slow-twitch fibers may improve oxidative metabolism and counteract age-related muscle loss. Yet, clinical relevance remains unclear, as lifestyle and other genetic factors dominate. A DNA test alone is not a reliable predictor of longevity.