# People always change

72 days from reading this, more than half of the cells in your body will be completely different cells than the cells in your body today.

Don’t ever let someone tell you “people never change.”

People are always changing. Sitting there, reading this, you—the mass of writhing, wiggling, cooperating, and competing cells that constitute your corporeal self—are changing. You are in flux. Millions of your cells have just died, and millions have just been born!

In talks, and on this blog, and in my papers, I often discuss this personal turnover because it leads to interesting biological questions. But I always paint this picture in the light of specific tissues. A recent conversation had me wondering—what about the entire body? What percent of our total cell number are different after a day? A week? A month?

Time for some more back of the envelope calculations!

Let’s say that 25 trillion (25 with 12 zeros after it, 25,000,000,000,000!) out of the 30 trillion of the cells in your body are red blood cells, as estimated by Sender et al. (2016). These red blood cells have an average lifetime—marking the time they are born until they are eventually recycled—of 120 days.  This turnover is a continuous process that keeps our blood fresh and functional each day.

So, every day, about $\frac{1}{120}$ of the $84\%$ of our cells are renewed, or $\frac{1}{120} \times \frac{84}{100} = 0.007$ , i.e. at least $0.7\%$ of our total cells are renewed daily! I stress at least because this estimate just includes the turnover of our red blood cells… our skin, our intestinal epithelium, and many other tissues that account for the 5 trillion cells that we didn’t include in the above calculation are continually renewed as well.

How long until half of the 30 trillion cells in your body are different from today? Again, just thinking about red blood cells, we need to calculate how long 15 trillion of these cells take to be recycled. 15 trillion is $\frac{15}{25} = 0.6 = 60\%$ of the total 25 trillion blood cells, and if the full batch of blood cells is renewed every $120$ days, this means that $60\%$ of the blood cells will be renewed in $0.6 \times 120 = 72$ days!