MadSci Network: Biochemistry

Re: Can you verify that atoms are turned over every seven years in humans?

Date: Fri Aug 5 14:47:23 2005
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology
Area of science: Biochemistry
ID: 1121225635.Bc

Hi Mark,

Thanks for calling me on the issue of the turnover of atoms in the human body. I probably shouldn't have been so casual in my previous answer, so I'm glad to be able to follow up.

The idea that most if not all of the atoms in your body turn over on average every seven years has been around for quite a while. I've spent the last several weeks trying to track down its origin, but it has been tossed around rather casually (i.e., no primary references cited) for more than 80 years. I found a reference to this idea in a book that was written in 1922, the original edition of which was written in the 19th century.

I'm not positive yet, but it seems likely that the idea of a turnover of the atoms in the human body every seven years is the result of experiments observing the turnover of radioisotopes in animals. The figure of seven years is probably an extrapolation from small animals to humans. I'm still doing legwork on this, so I will update this page when I am able to track down more information.

So, while I can't yet identify the origin of this concept, I think that I can back-up the claim with contemporary evidence. A very nice paper was published in Cell last month, in which the authors (KL Spalding, RD Bhardwaj, BA Buchholz, H Druid, and J Frisen) determined the age of specific human tissues and cell types and compared those ages to the age of the person in question. Spalding et al found that most cell and tissue types in the human body are much younger than the person in which they are found, and that very few cells live for the entire life of the person.

They determined this by looking at the presence of the radioactive carbon 14 isotope (14C) in the DNA of different cell types and tissues. Atmospheric levels of 14C increased dramatically after the above-ground atomic tests of the mid-1950s and have been dropping since the last above-ground tests in 1963 (due to equilibration between the oceans and the atmosphere). Atmospheric 14C enters the biosphere when it combines with oxygen to form CO2, which is then fixed by plants. Because we eat plants and plant-eating animals, the 14C levels in our bodies are proportional to the level in the atmosphere (after accounting for growing seasons and harvest times); as the level of 14C in the atmosphere decreases, so does the 14C available to us in our food.

In their paper, Spalding et al note that "Most molecules in a cell are in constant flux, with the unique exception of genomic DNA, which is not exchanged after a cell has gone through its last division." So they used the level of 14C in the genomic DNA of particular tissues to date the time of the birth of the cells in that tissue, through comparison to the decreasing level of 14C in the atmosphere over the last ~40 years.

They found the average age of intestinal tissue to be about 11 years, and after accounting for epithelial cells (which have short lifespans of only 5 days) found the average age of non-epithelial intestinal tissue to be 15.9 years. Skeletal muscle tissue was found to have an average age of 15.1 years. In the brain, tissues appear to be much older. The average age of the cerebellum's gray-matter was only 2.9 years younger than the person, and the average age of occipital-cortex gray matter was about 10 years younger than the person, but the age of occipital-cortex neurons was found to be the same as the age of the person.

So, even though we think of our bodies as being permanent structures, most of our tissues (outside of our brains) are continually being turned over, renewed in a balance between the constant death of old cells (likely through the process of apoptosis) and the constant birth of new cells. In last week's New York Times article about this paper, Dr. Frisen (the senior author) suggested that the average age of the cells in an adult human may be as low as seven to ten years. Remember, this is an average value. As shown by Spalding et al, the value varies by tissue.

Spalding et al used genomic DNA as the measure of a tissue's age because genomic DNA has the lowest turnover rate of all of the molecules in the cell, and because the levels of 14C in that DNA reflected the atmospheric levels of 14C at the time when the cell was born. This tells us that the molecules that make-up new cells are recruited from outisde the body, and are not simply recycled from cells that just died. It also tells us that an average age of seven to ten years is an upper limit for most of the atoms in the body. Even the neuronal cells of the occipital-cortex are constantly making new proteins and RNA molecules, and are constantly consuming carbohydrates and lipids. So, it seems quite plausible to me that the average time for the turnover of the atoms in your body could be around seven years.

Spalding et al conclude that "The possibility to determine the age of cells can give us a map of the human body from a cell-renewal perspective." So, there is certainly much more information to come from future studies of this sort, and I am looking forward to seeing where they lead.

Thanks for your question!

Here are some references in case you want to pursue this further:

Spalding KL, Bhardwaj RD, Buchholz BA, Druid H, Frisen J. (2005) Retrospective birth dating of cells in humans. Cell. 122:133-43.

Wade N Your Body is Younger Than You Think New York Times August 2, 2005, page D1. This article contains some additional information about the lifespans of different tissues that isn't discussed in the paper by Spalding et al.

Here are a pair of seminal papers on apoptosis (pronounced "ah poe TOE sis"). Even though these papers are old, the authors recognized the role that apoptosis plays in the turnover of normal adult tissues.

Wyllie AH, Kerr JF, Currie AR. (1980) Cell death: the significance of apoptosis. Int Rev Cytol. 68:251-306.

Kerr JF, Wyllie AH, Currie AR. (1972) Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br J Cancer. 26:239-57.

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