How Old Is Your Dog?

You may have heard of the old rule of thumb: multiply a dog's age by seven to get its age in human years. For example, a 2-year-old dog is said to be equivalent to a 14-year-old human.

This calculation assumes a simple linear relationship: if dogs live 10-12 years and humans live 70-80 years on average, then a 2-year-old dog has lived the same proportion of its life as a 14-year-old human. Unfortunately, this approach is simplistic and imprecise.

Tina Wang and colleagues at UC San Diego have proposed a more accurate method to determine the human-equivalent age of a dog. Using something called epigenetic clocks these scientists were able to determine that dogs achieve adulthood much sooner than would be implied by a linear relationship, but then stabilize in mellow middle age for years.

Source: Cell Press.

Using the epigenetic clock method, scientists estimate that a 2-year-old dog is equivalent to a 42-year-old human. The dog's rate of biological aging then slows dramatically, however. By age 9 that dog is 66 years old in human years, having aged at a rate of 3.4 human years per dog year in the interim.

These results make some intuitive sense. After all, dogs are able to get pregnant at six months of age on average. This would equate to a disconcerting 3.5-years-old human equivalent using the linear method, but a far more sensical 20-years-old human equivalent using the epigenetic clock method.

At this point, you may very well be wondering: what is an epigenetic clock? And, for that matter, what is epigenetics?

Different Types of Clocks

Chronological age is measured by the passage of time and is therefore the same for all people born in a given time period. Every person born in 2001 either already is or will turn 21 years old this year.

Biological age, on the other hand, is influenced by environmental factors, such as diet, exercise, and work exposure. For instance, the skin and lungs of a 50-year-old heavy smoker will likely show wear and tear well in excess of those of a 50-year-old nonsmoker. In fact, the smoker's organs may have more common with those of an 80-year-old nonsmoker: their biological age will be much greater than their chronological age.

Epigenetics & Clocks

Epigenetics is the study of how environmental factors such as diet, exercise, and profession impact how your genes work. The field is quite complex, so we will keep the discussion at a high level. See this page from the CDC for a good jumping-off point if you would like to learn more.

It is known that certain genetic processes change as we age. For example, one process called DNA methylation (more info here) has been shown to be a reliable indicator of biological age: it is highest in newborns and decreases as we get older.

The predictive value of DNA methylation seems to hold across species. So much so, that it was in fact the method chosen by Dr. Wang and her colleagues to estimate the different rates of aging between dogs and humans!

We have therefore arrived at a simple definition of an epigenetic clock: it is a method to measure biological age using DNA methylation.

Our understanding of epigenetic clocks continues to evolve. For instance, scientists like Steve Horvath at Altos Labs have developed sophisticated epigenetic clocks that look at biological age across different tissue types (muscle cells, nerve cells, etc.).

These methods can be used to more precisely measure the anti-aging benefits of new therapies, or of simple behavior changes like diet and exercise.

What Comes Next?

Understanding the process of biological aging and its causes is the first step. Next, scientists and their financial backers will want to find ways to slow down or - even better - reverse epigenetic clocks.

Altos Labs, mentioned above, has hired some of the world's greatest scientists to tackle aging. It does not hurt that the company is backed by $3 bn in seed capital - the world's best-funded startup ever.

Innovative approaches to delay and reverse - at least partially - the effects of aging are headed our way. Expect a few news headlines from Altos Labs and its competitors in the coming years.