ACSL1 rs6565551: Cardiorespiratory Fitness and VO2max
rs6565551 is a single nucleotide polymorphism located within the ACSL1 gene on chromosome 4. Identified through exercise genomics research such as the HERITAGE Family Study, this variant is associated with individual differences in cardiorespiratory fitness and maximal oxygen uptake (VO2max) response following standardized endurance training. While it highlights genetic influences on aerobic conditioning, overall fitness is complex and driven primarily by exercise habits and broad lifestyle factors.
What each genotype means
Typical aerobic training response
Individuals with the CC genotype carry two copies of the common allele at rs6565551 in the ACSL1 gene. In exercise genetics research, such as the HERITAGE Family Study, this baseline genotype is associated with standard or average adaptations in VO2max following standardized aerobic endurance training. Because aerobic trainability is a polygenic trait influenced by numerous genetic loci and lifestyle factors, overall fitness improvements depend heavily on consistent exercise programming.
Carried by approximately 40% to 45% of individuals of European ancestry, representing the most common genotype.
Intermediate cardiorespiratory response
Individuals with the CT genotype carry one copy of the minor T allele at rs6565551. Research exploring ACSL1 variation indicates an intermediate cardiorespiratory response to endurance training relative to homozygote carriers. Genetic markers provide modest predictive utility on their own, meaning individual training gains are primarily driven by training volume, intensity, and recovery.
Carried by roughly 40% to 45% of people in European populations.
Altered VO2max trainability
Individuals with the TT genotype carry two copies of the minor T allele in the ACSL1 region. In cohorts evaluating exercise trainability, carrying homozygous minor alleles at key ACSL1 loci has been linked to differences in VO2max responsiveness compared to major allele carriers. However, these statistical associations reflect subtle tendencies rather than fixed physiological limits, and aerobic fitness still improves with consistent endurance training.
Carried by approximately 10% to 15% of individuals in European ancestries and varies across other continental populations.
Genomic Location and Variant Characteristics
The single nucleotide polymorphism designated rs6565551 is situated on chromosome 4 within the acyl-CoA synthetase long-chain family member 1 (ACSL1) gene. It represents an intronic non-coding variation where either an adenine (A) or a guanine (G) nucleotide occurs at this specific position. Because intronic variants do not directly alter the protein's amino acid sequence, researchers investigate whether rs6565551 or adjacent variants in linkage disequilibrium influence gene expression, pre-mRNA splicing efficiency, or local chromatin accessibility within metabolic tissues. Detailed reference information regarding this variant can be explored through [NCBI dbSNP](https://www.ncbi.nlm.nih.gov/snp/rs6565551) and the [GWAS Catalog](https://www.ebi.ac.uk/gwas/search?query=rs6565551).
Biological Role of the ACSL1 Gene
The ACSL1 gene encodes an essential metabolic enzyme called acyl-CoA synthetase long-chain family member 1. This enzyme converts free long-chain fatty acids into fatty acyl-CoA esters, a crucial regulatory step in cellular lipid utilization. By activating fatty acids, ACSL1 channels lipids toward mitochondrial beta-oxidation for aerobic ATP generation, as well as into complex lipid synthesis pathways. ACSL1 is prominently expressed in highly oxidative tissues, including skeletal muscle, cardiac tissue, and adipose deposits. In skeletal muscle, proper ACSL1 function is indispensable during sustained aerobic activity, when muscle fibers rely heavily on fatty acid oxidation to preserve glycogen stores and support continuous exercise performance, as detailed in [MedlinePlus Genetics](https://medlineplus.gov/genetics/).
Scientific Evidence on VO2max and Cardiorespiratory Fitness
Much of our understanding regarding ACSL1 and aerobic capacity originates from the Health, Risk Factors, Exercise Training and Genetics (HERITAGE) Family Study. HERITAGE investigated hundreds of sedentary individuals undergoing a strictly controlled 20-week endurance cycling protocol to assess inter-individual variance in VO2max gains. Genetic analyses revealed that polymorphisms within the ACSL1 locus were among the strongest individual predictors of VO2max trainability, with certain alleles correlating with significantly higher aerobic adaptations. However, scientific evidence indicates that aerobic trainability is highly polygenic. No single variant explains more than a modest fraction of training response. Subsequent investigations, including studies cited via [PubMed](https://pubmed.ncbi.nlm.nih.gov/?term=ACSL1+exercise), show that while candidate loci influence adaptations in untrained cohorts, findings do not always correlate directly with elite athletic status.
Population Distribution and Ancestry Patterns
Allele frequencies for rs6565551 vary across global ancestral backgrounds. In European populations, the minor allele frequency is estimated at approximately 0.35, resulting in a balanced distribution where heterozygous and both homozygous genotype combinations are regularly observed. Across other major continental ancestries, such as African, East Asian, and admixed American populations, frequency distributions differ and have been less comprehensively characterized in exercise genomic trials. Large population reference databases, such as [gnomAD](https://gnomad.broadinstitute.org/), track these global distributions across diverse human cohorts.
Practical Implications and Lifestyle Context
Genetic insights regarding rs6565551 offer educational context on how human metabolism adapts to aerobic stress, but they cannot predict individual physical capability or dictate fitness outcomes. Carrying a genotype linked to lower VO2max trainability does not mean a person cannot achieve high cardiovascular health, nor does a favorable genotype guarantee high performance without dedicated training. Cardiorespiratory fitness responds to consistent progressive exercise, adequate recovery, sleep quality, and balanced nutrition. Genetic testing results for this marker should be viewed strictly as informational insight into metabolic pathways rather than a clinical tool or training barrier.
How common is this variant?
The minor allele frequency for rs6565551 is approximately 0.35 in European populations, making both heterozygous and homozygous genotypes relatively common. Variations across other ancestral backgrounds exist and continue to be evaluated in large-scale population genetic databases.
Frequently asked questions
Does my rs6565551 genotype mean I cannot improve my VO2max?
No. Virtually all healthy individuals experience cardiovascular and metabolic benefits from consistent exercise regardless of genotype. Variations in ACSL1 are associated with statistical differences in the magnitude of training gains, not an inability to adapt or become fit.
Can rs6565551 determine whether I can become an elite athlete?
No, this genetic variant cannot predict athletic success. Research demonstrates that elite athletic status is influenced by thousands of genetic variants interacting with training volume, coaching, nutrition, and psychological factors. ACSL1 accounts for only a minor fraction of variance.
What is the relationship between ACSL1 and fat burning?
The ACSL1 gene encodes an enzyme necessary to convert long-chain fatty acids into forms that can enter mitochondria for fuel production. Because endurance exercise relies heavily on fatty acid breakdown, variations in this pathway may modulate how efficiently working muscles adapt to aerobic training.
Are there clinical medical guidelines based on rs6565551?
There are no medical or therapeutic guidelines associated with rs6565551. It is researched solely as a trait-associated genetic marker for physical fitness and exercise physiology, not as a diagnostic test or tool for medical decisions.
Sources & further reading
Educational information only, last refreshed 9/13/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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