In fact, they showed, DNA methylation levels at these two locations was a better measure of substance use than people's self-reported estimates. Thus, in this follow-up study, Meeshanthini Dogan, MS, and Dr. Philibert used methylation levels as a proxy for tobacco and alcohol consumption. They estimated each person's biological age using a previously validated epigenetic "clock" based on methylation levels at 71 locations in the genome, as measured by the widely used Infinium HumanMethylation450 BeadChip. Then, they calculated the difference between biological age and chronological age, and assessed the relationship between tobacco and alcohol use and premature aging.