For centuries, writers and poets have told stories of the Fountain of Youth, said to turn back the clock if you drink or bathe in its waters.
But instead of aquatic alchemy, could we slow the inexorable march of time with broccoli and lentil sludge?
A recent thread doing the rounds on Twitter highlighted tech entrepreneur Bryan Johnson's "Project Blueprint", which he claims reduced his "epigenetic age by 5.1 years in 7 months".
He says he did this by following an incredibly regimented routine involving meals that were "methodically crafted based on gold standard scientific evidence for optimal nutrition" (including the aforementioned sludge), a swag of supplements, daily exercise and a commitment to sleep hygiene, including a temperature-controlled bed.
Mr Johnson, founder of the company Braintree, which was bought by PayPal in 2013 for $US800 million ($1.18 billion), even enlisted his family to his plan.
And according to its website, Project Blueprint is "scientifically rigorous".
So … is it?
Well, no, says Emma Beckett, a molecular nutritionist at the University of Newcastle.
She says it's an example of research from very specific niches of biology being turned into general health advice.
"There are studies about this idea of epigenetic clocks in development and cancer, but it's not the intent that you would then use them as constant monitoring markers to design an extreme diet," Dr Beckett says.
What is 'epigenetic ageing'?
The "ageing" that Mr Johnson's Project Blueprint is trying to tackle is more than counting the years — that's chronological ageing — or simply getting a few extra wrinkles. (That said, he follows an exhaustive full-body skincare routine too.)
This is ageing at a biological level, and there are a few ways we can measure the "age" of various organs and tissues.
Pretty much every cell in our body has the same DNA blueprint, but they don't need all of our genes switched on all of the time. For instance, genes that are switched on in liver cells might be silenced in hair follicles.
So we have what's called the epigenome — an array of chemical markers that control which genes are switched on or off, and when.
One of these switching tools is called methylation. It works by adding a simple molecule comprising a carbon and three hydrogen atoms — called a methyl group — to certain letters in our genetic code, locking genes in the "off" position.
And as the years wear on, some parts of our genome can become more methylated, and others less.
Depending on what the affected genes do, changes in methylation might mean our cells accumulate damage and age faster.
Just how meaningful is epigenetic age to our health?
"That's a question that is still largely open right now," says Christian Nefzger, who studies epigenetics and ageing at the University of Queensland.
"It's reasonably accepted that accelerated DNA methylation age is, in a way, a risk factor for premature death, morbidities and disorders."
So being "younger", at least from a DNA methylation standpoint, is normally considered beneficial.
But, Dr Nefzger says, current studies investigating DNA methylation and human chronological age only show a correlative link.
Whether there is causation — and if changing DNA methylation has any effect on the cell's ageing process — is yet to be determined.
"Another thing that's still unknown is when someone lowers their 'DNA methylation age' via these kinds of intervention strategies [adopted by Mr Johnson], whether that automatically also lowers their risk for premature death or developing diseases," Dr Nefzger says.
Dr Beckett agrees.
"Adding something back or taking something away won't necessarily give us the desired effect [on ageing], because these things are not necessarily direct cause and effect.
"It's a super-complex system, and lots of things affect those epigenetic markers."
Measuring methylation via blood tests, the most common method, also comes with issues.
White blood cells are a convenient and accessible way to check where methyl groups are hanging off our DNA.
But blood isn't necessarily where methylation changes are happening, and can't be used as a proxy for the methylation status of all your organs, Dr Beckett says.
"We know from the science that doesn't necessarily work. Organs have different epigenetic markers. Even different white cell populations will have different markers."
Plus methylation is just one of a whole toolbox of epigenetic markers in our cells, and measuring methylation doesn't mean you're measuring the lot, Dr Beckett adds.
So … can I reverse the 'age' of my organs?
If you don't have the time, resources, willpower, ability or even the slightest desire to try Mr Johnson's plan, or can't afford getting regular, pricey DNA methylation tests, don't worry — there are easier and more affordable ways to check up on the health of your insides using other biomarkers.
Dr Beckett recommends seeing a GP and getting a general check for things like blood pressure, plus a broad blood screen, including cholesterol, vitamin D levels and the like.
"They're biomarkers in the exact same way that methylation is a biomarker. They're markers of nutrition and health," Dr Beckett says.
"Then you can see a dietitian who can explain your results in the context of your life and the changes you can make that are affordable and palatable.
"[Mr Johnson's program] is not sustainable. It's not life-friendly. It's lacking in diversity and joy."
Indeed, it's impossible to say which aspect of Mr Johnson's routine might have contributed to his lower "methylation age", Dr Nefzger says, or if the results will stick.
"Time will tell if it will do him any good."