Thymus aging mitigation strategies
OK time to start thinking about the thymus. First start with this blog post by Josh Mitteldorf (an aging theorist) which is a good introduction to the thymus and aging. The short answer is the Thymus generates memory T-cells ('memory' meaning they remember past exposure to viruses, and T meaning "thymus"). The Thymus gets smaller with age, and we also generate fewer T-cells. From the post, we don't know why the Thymus shrinks, except that it is not from typical aging senescence.
One way to increase its size (presuming that larger = better) is possibly through human growth hormone (HGH). Supplementing with GH comes with a host of other problems - at the moment it's best not to attempt this. In the comments the author, who is a friend of Greg Fahy's, discusses Gregs 1 year experiment with HGH. At the end he did have a larger thymus
So AFAIK we don't know yet how to fix the root cause of aging thymus degeneration, but we do know (clinically verified in many studies) how to mitigate the result which is poor T-cell function. In this way we can address the effect, if not the cause, similar to addressing failing adrenal function by hormone supplementation. This is something I've mentioned here several times, because it surprises me that it's not bigger in the aging community, bu which is 3+ day water only fasting. There's a lot of top drawer research behind this, the most approachable is to read The Longevity Diet by Valter Longo who is one of the primary researchers, working at UCLA. No he's not trying to make a buck, I think all the money goes to a research foundation, he's reporting on his work which started as a PhD student working for Roy Walford.
Roy believed that long term calorie restriction prolonged life, but now we know that unfortunately it does not. Aubrey DeGrey frequently cites this, it turns out that chronic calorie restriction in long lived species does not confer the same benefit it does in short lived species. We can hypothesize about this from an evolutionary standpoint, but regardless it's an established fact. Now Valter continued Roy's work and has found that instead of chronic restriction intermittent calorie restriction does have tremendous benefits. Far more that what many of us are doing here via pharmaceuticals and nutraceuticals.
It's spelled out in that book, but back to the topic at hand, at around 3-4 days of a water only fast what happens is about half of our T-cells are killed off. These are old programmed T-cells hanging around, after a lifetime of exposure we have this old programming still around. But what happens is when you re-feed (start eating again), your stem cells (which enjoy a rejuvenating effect) regenerate those T-cells, but now they're baby unprogrammed cells. So - stem cell therapy, simply by not eating for a few days.
If anybody is interested we can discuss why's and how's. I do it - been doing water only fasts for years, twice a year which coincidentally what Valter recommends for those with 'elite health'. Anecdotal evidence of it's benefits; since I started I haven't gotten sick. Not once. Used to get sick every year, but no longer. I attribute this to probable T-cell rejuvenation which is more responsive to infection. Note there are many, many other benefits too, which Bill Faloon recently reiterated in lecture.
I would recommend for everybody here to deeply consider adding multi-day water only fasting to their regime. It's the cheapest, simplest, easiest, strongest with no side effect way to improve longevity.
Dan, mostly great post, but I think you have slightly mischaracterized a few important things....
it turns out that chronic calorie restriction in long lived species does not confer the same benefit it does in short lived species. We can hypothesize about this from an evolutionary standpoint, but regardless it's an established fact.
I don't think this is settled science to the point of wide agreement as you suggest. Aubrey & most of the SENS community, including Reason of FightAging.org certainly believe that CR confers minimal benefit to humans as you say, and I agree. But it's important to note that many aging scientists still believe there is a possibility for much bigger benefit from CR or its mimetic drugs. This is important because if the issue were more settled, it would likely effect amounts of research funding flowing to different subparts of the field.
Also notable on this issue is Michael Rae for being both a CR practitioner and one of the staunchest defenders of the idea that human CR is worthwhile but also squarely in the SENS community (co-author of Ending Aging with Aubrey, and officially the SENS science writer). He is a very well-informed guy who knows the literature and the arguments on both sides.
One of the most interesting places to read some of the debate on this topic is on the CRSociety's own web forums where several practicing members of that community discuss relatively recent evidence suggesting weaker overall human benefit in a thread called https://www.crsociety.org/topic/11699-will-serious-cr-beat-a-healthy-obesity-avoiding-diet-lifestyle/
As for Intermittent Fasting, I completely agree that it is probably a great thing for long-term health for most people, but I think it might be implying too much benefit to make it the focus of a thymus regeneration thread. It's true that Longo's work showed elimination of WBCs from long enough fasts and that WBC count recovers at least somewhat on refeeding, but I don't recall seeing any demonstration (your n=1 anecdotal data aside) yet of improved infectious disease resistance or any biomarkers for overall immune system function showing long-term improvement (though admittedly I don't follow that literature closely). And it's also true that organs shrink on 4+ day fasts and recover size on refeeding, but I'm not aware of any demonstration of the thymus getting bigger than before a fast. I've also heard some people worry about stem cell "exhaustion" as a potential long-term problem if they are called on to regenerate new WBCs too many times---I think this goes into the category of no one knows yet if that's a problem.
Longo himself often presents one of the main benefits as getting a fraction of the CR benefit but in a way that is easier to do. So of note I'm not sure how valuable IF is for those already very healthy or even doing CR. Most of the scientific data on IF uses controls that are typical humans eating mostly pretty bad diets or ad-lib rodents and thus the results don't speak to the level of benefit from a healthier starting point. I think one of the main benefits of fasting is from its anti-cancer properties (cancers like glucose and don't like ketones, plus they like protein especially met & cys) and likely senolytic effects (but this area needs more research).
And of course, in the category of nice to know about but doesn't help what people can do themselves now, Reason himself now has a startup called Repair one of whose research programs is thymus regeneration, which he's talked about on FightAging.