I attended a talk today where the speaker spoke of how millions of cells are replaced every day in our bodies. There is the thought that every part of our body is constantly being replaced by new cells, so that eventually, your body is completely made anew every seven years. Aging occurs when cells die faster than they are replaced, leading to degradation and eventually death (assuming you die of old age and not in a flaming ball of fire jumping off a cliff). While it sounds simple and seems to make sense, the scientist in me was crying.
Certainly, our hair and finger nails keep growing. We cut them off, they grow back, so we know that they are being replaced. The same goes for our skin cells: dead skill falls off, get rubbed off, we get a cut, our wounds heal. Yup, clearly our skin cells are also being replaced in most cases. (But what about scars and moles?)
But as a neuroscientist, I know that some neurons you are born with stay with you till the day you die (and may indeed die only after you are dead). So that alone disproves the 7-year-new-you theory.
So what about the number of cells replaced each day, is it in the millions? When I search for “How many cells are being replaced in your body every day?” somebody on wiki.answers.com tells me that in just red blood cells alone, “one million red blood cells are removed and replaced each second”. Wikipedia tells me it’s 2.4 million per second, (Sackmann 1995) so I’ll trust that to be the more accurate number. There are 86,400 seconds in a day, which means that 207.360 billion red blood cells are replaced everyday. Sounds incredible? Indeed, a quarter of the cells in the human body are red blood cells, averaging at about 20-30 trillion cells for an adult, so 200 billion is only 1% of all the red blood cells in our body.
The average life span of a red blood cell is about 4 months (~100-120 days). Other cells live for different amounts of time. For example, epidermal skin cells live for 2-4 weeks, while white blood cells, depending on the type, lasts any where from hours to years. And as mentioned before, some neurons last a lifetime. Different parts of your body regenerates at different rates, and perhaps someone had added the live span of all the cells together to arrive at 7 years as how long it would take for all the cells to get renewed in the human body.