You Are Being Rebuilt

What renews quickly

• The lining of your small intestine — about 5 days. The surface that meets your food is replaced more or less weekly. The colon’s lining is faster still, closer to three or four days.
• Your red blood cells — about 4 months. Roughly 120 days each, and about two hundred billion replaced every day.
• Your skin — one to two months. (You’ll see “28 days” everywhere. That comes from a single 1965 measurement, and a careful re-examination in 1972 put it closer to 40 to 56 days. One to two months is the honest range.)
• Your liver — under three years old, whatever your age. Some of its cells renew about yearly. On average the whole organ is younger than three, whether you are thirty or eighty.

What renews slowly

• Your skeleton — about a tenth of it a year. The dense outer bone goes slowest; the spongy inner bone turns over much faster.
• Your fat cells — about a twelfth a year. And here is something worth knowing: the number of fat cells you carry is largely settled by the end of adolescence and doesn’t change much in adulthood, even after significant weight loss. The cells change size. They don’t often change in number.

What is never replaced

This is the part almost nobody mentions, and it’s the part I keep thinking about.

So, about the seven years

You have probably heard that the body replaces itself entirely every seven years. It doesn’t, and it’s worth knowing why.

That number came from a real study, misread. Researchers worked out that the average age of a cell in the body is around seven to ten years — and an average of that kind is close to meaningless here, because almost nothing actually lives seven years. It’s a mixture of things that live a few days and things that live your whole life.

The true version is stranger and better. You are being rebuilt constantly, at enormous scale, and at the same time some of you has been there since the beginning and will never be replaced.

What it’s all made from

Here is the practical end of it, and I want to be careful to say it accurately.

Food does not make renewal go faster. The pace is set by the tissue, not by your plate. Nothing you eat speeds it up, and anyone selling you that is selling you something.

But renewal cannot be completed without the raw materials. That part isn’t in dispute at all. The body recycles most of what it needs — the majority of the iron used to build new red cells is salvaged from old ones — but the losses have to be made up, and they’re made up from what you eat.

You are, quite literally, being rebuilt out of last week’s groceries.

Stronger, or weaker

Which brings me to the question I keep coming back to, and it’s the one that turns all of this from an interesting fact into something that matters on a Tuesday.

The rebuilding happens either way. What changes is what it gets built out of — and the body builds with whatever is on hand at the time.

That isn’t a figure of speech. Some of it you can see. A red blood cell built while iron is short comes out smaller and paler than it should, and carries less — measurably, on a slide, under a microscope. Collagen built without enough vitamin C doesn’t hold together properly, because the step that locks the strands into a stable rope requires vitamin C to run. That’s what scurvy is. And the membranes around your cells are built out of the fats you’ve been eating.

So the real question isn’t whether you’re being rebuilt. It’s whether the version being assembled this month comes out a little stronger than the one it’s replacing, or a little weaker. Over a few months that’s a small difference. Over thirty years it’s a different person.

The provision came first. The body that knows what to do with it came first too.

Blessings,
Paul & Ann

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