Sources

1. [1]
   Mitochondrial DNA mutations dominate the small intestine — nature.com

   We observed substantial inter-individual variability in the rate of mtDNA heteroplasmy accumulation.

   Claim support: 80%Excerpt confirms 'substantial inter-individual variability' in heteroplasmy accumulation rates, matching the post's claim. Post adds 'across donor age cohorts' which isn't explicitly in excerpt, but is a reasonable specification of the variability described.