[set fair (OP)]

And where should we look?

As far as I know there is no evidence whatsoever that a typical regime of two periods per week of no food for 24 hours causes premature aging, albeit that probaby nobody has looked. However I think there is good reason to look.

My understanding is that intermittent fasting works by prompting the body to "eat" cells which are a little damaged and would soon be replaced anyway. There are chemicals released and measured to support this idea. my concern is that regular fasting will lead to a premature turnover of cells, cutting the average lifetime of cells. My understanding is that cells are programmed to divide a finite number of times, so replacing cells sooner rather than later on an ongoing and long term regime may have consequences in later life.

Really what I'm asking is where to look. I guess one would need people who have followed the regime for a few years. Or perhaps somebody here can suggest specific cells that a suitable to study over a period of months. Telomere shortening looks a likely place to look but obviously I'm no expert. So any ideas.

Also please feel free to tell me the biological terms I should have used.

[SenseWise]

No. Intermittent fasting is meant to get rid of fat cells. If you are destroying cells other than those that make up your fat, you are doing it for too long. That can cause permanent damage to muscles and organs. Also, it may stunt your growth but I doubt it could cause premature aging, as that is more or less due to loss or weakening of organ functions, not loss of fat cells.

[evan_au]

The original work was done in worms, and showed that reduced food intake led to a slower metabolism and longer life.
- Since then, it has been replicated in many species
- But it is hard to do controlled experiments on humans (partly because it takes so long to collect the results; partly because it would not get past the ethics committees or the funding committees).

See: https://en.wikipedia.org/wiki/Calorie_restriction#Life_extension

[alancalverd]

Concentration camp survivors often exceed normal life expectancy, but it's not easy to establish causality: would those individuals have survived as long in normal circumstances?

I like to introduce "disposable life" into ethical discussions. Disposable income is money you can spend as you like, rather than to meet essential needs and obligations. Similarly with life. If you enjoy food, then restricting your diet is reducing your disposable life (or at least the way you choose to dispose it) to extend your total life. "Live fast, love hard, die young."

[Bored chemist]

It might be linked to your ears turning green on your 101st birthday.
We don't have the data to rule that out.
But I doubt we have any evidence to support it either.

[alancalverd]

My great-grandfather said, of fish and chips (the family business) "If you eat that for a hundred years, you will live a long time."  No need for an experiment.

[Deecart]

My understanding is that intermittent fasting

24 hours with no food is not what we call fasting.
You need at least 3 days for that.
After this period of 3 days the metabolisme of cells adapt to the new conditions, using new metabolic paths and this mean using some other DNA portions to produce different proteins so as to use new metabolic paths.
No cell is harmed at this point.
But because we are not used to fast in our civilisations, the selection of people capable of fasting and other who dont, has not taken place since a long period. Therefore the alternative metabolic path could be damaged (and you dont know it before you fast), producing per example some intermediate molecules of the metabolism remaining into the cells (killing them or producing some illness, if too much of the molecules stay here in the cells) and therefore fasting is not everytime a good thing. 

What we know of the 24 hour "false fasting", is that the body produce in this case the multiplication of the fat cells so as to be able to store fats better.
These cells are never destroyed after that (unless like you say some cell damaged, but it is anecdoctical)
More cells leads to potentialy more cells with fat, so more diseases due to weight disorder.
So lower life duration (but no premature aging).

[evan_au]

24 hours with no food is not what we call fasting.

Fasting means different things to different people.
- A Moslem man once commented to me that Moslems have a greater caloric intake during the fast of Ramadan than they do when they are not fasting.

Levels of dietary hormones like Leptin & Ghrelin are affected by periods of fasting as short as 12-24 hours.
- Leptin levels even change during sleep...
https://en.wikipedia.org/wiki/Leptin
https://en.wikipedia.org/wiki/Ghrelin