The common metric is that each dog year equals 7 human years, but a new study suggests that the correlation is not that simple, at least for Labrador retrievers. To evaluate relative dog age, the researchers examined epigenetic changes in dog DNA and compared them with similar age-related changes in human DNA. Epigenetic changes refer to processes which control the expression of our genes without changing our DNA sequences. These processes are critical for determining which genes are turned on and off in a given cell type. For example, why is a brain cell different than a skin cell even though both cells contain the exact same content of DNA? The reason is epigenetic differences that specify different gene sets to be turned on in these two cell types.
Among the known epigenetic processes are chemical modifications of DNA itself. In recent years there has been growing interest in using a particular epigenetic modification, called DNA methylation, as a marker of biological aging. DNA methylation is the addition of a methyl group (CH3) to cytosine (C) nucleotides in DNA (DNA is composed of 4 nucleotides: A, C, G, and T). Changes in the presence or absence of methylation at specific cytosines in our genome have a strong correlation with our age, suggesting the concept of an epigenetic clock. Interestingly, for any individual your epigenetic clock may run faster or slower than your chronological age, which may have significant implications for personal health and longevity (see Dupras et al.).
With a rich literature on DNA methylation in human aging, Dr. Ideker and colleagues set out to do a similar analysis in dogs. To avoid confounding differences that might exist between different breeds, they confined their analysis to 104 Labrador retrievers ranging in age from 5 weeks to 15.5 years. Their findings demonstrated a similar pattern of age-related methylation changes in dogs and humans. Using these results they were able to relate the dog DNA methylation changes to the human epigenetic clock. At least for Labradors, the age correlation doesn’t translate to a simple 1 to 7 formula and instead yields a more complex logarithmic relationship. Fortunately, you can go here for a simple calculator that converts dog age into human equivalent age. Labradors appear to age very rapidly in their early years (see graph) with a 1 year old dog being equivalent to a 31 year old human and a 5 year old dog being nearly 60 human years old (red area in graph). After year 5 their epigenetic aging slows greatly and they only age roughly 20 human years in the next 15 actual years (blue area in graph). It’s fun to play with this calculator, but remember this has only been verified for Labradors, so it may not be at all accurate for other breeds. Hopefully similar work will be done for other breeds so that we can all appreciate just how old our dogs really are in human years.