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I got this random science fact from my mobile carrier.

Once a human reaches the age of 34, he/she will start losing approximately 7,000 brain cells a day. The cells will never be replaced.

Is the above claim true ?

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The claim is not true; neurons continue to form, and the connections between them to change, throughout our lives.

This paper on neurogenesis (i.e. forming of new brain cells) opens with this sentence:

It is now well documented that active neurogenesis does exist throughout the life span in the brain of various species including human.

However, it is true that the rate of forming new cells slows down with age. Here is a quote from a webpage of a lab dedicated specifically to researching neurogenesis:

The brain continues to produce new neurons throughout life; however, the Bartlett laboratory has shown that the rate at which these new nerve cells are produced declines with age and may contribute to age-associated memory loss.

I found no sources for the age of 34 to be critical, but in my experience a reputable neuroscience paper would never give a statement like that, because a myriad of additional factors are likely to be involved.

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    I thought neurogenesis only happened in specific parts of the brain though? – endolith May 9 '12 at 17:33
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    @endolith - It's controversial about some parts of the brain, and clearly present in others. This source claims for it to be present in widespread cortical areas, but I've found also found that some others still don't believe it, at least before it's replicated a few times. It's still pretty new, this research. – Ana May 9 '12 at 18:10
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    The claim is that 7,000 brain cells are lost per day. This answer suggests that brain cells are indeed lost faster than they regenerate. So I see this as not a debunking, but a possible verification. Without specific numbers (of cells lost/regenerated), it's not sufficient. – Flimzy May 10 '12 at 4:58
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    @Flimzy - I see. I understood the question to be a modern reintepretation of an old myth in neuroscience, that neurons only die and never regenerate. But you're right, the claim is different. I don't know about the balance of dying/regenerating, but I do know that nobody knows how many neurons there are, let alone that anybody can count how many die. There is - very roughly - an estimated 100.000 neurons under a random square millimeter of skull, but nobody can count them all up individually at this point. So I'm not sure how the claim could be tackled scientifically to begin with. – Ana May 10 '12 at 8:41
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    @Ana: I think it's very easy to interpret it that way. :) I don't think your answer is bad (thus I didn't downvote), but I think it needs some additional clarification--or a reference to the effect that it's impossible to count the exact numbers of affected cells, etc. – Flimzy May 10 '12 at 16:40

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