Rapamycin Extended Mouse Lives by 26%. The First Human RCT Couldn't Move Visceral Fat.

Three hundred and thirty-three. That is how many adults an online survey published in GeroScience found willing to report they were taking rapamycin off-label for anti-aging purposes. They represented a fraction of the actual number. Longevity clinics across the United States now prescribe the generic immunosuppressant to thousands of clients at $30-80 per month, banking on the strongest preclinical aging dataset any drug has ever produced. In mice, rapamycin reliably extends median lifespan by 14-26% depending on dose and timing, confirmed across three independent laboratories in the NIA's Interventions Testing Program. No other compound in geroscience comes close to that level of replication.

Then, in April 2025, the first large randomized controlled trial in humans reported its results. The PEARL trial (Participatory Evaluation of Aging with Rapamycin for Longevity) enrolled healthy adults ages 50-85, randomized them to placebo, 5 mg weekly, or 10 mg weekly compounded rapamycin, and followed them for 48 weeks. Its primary endpoint was change in visceral adiposity measured by DXA scan, the fat compartment most tightly linked to metabolic aging, cardiovascular risk, and insulin resistance.

The result: partial eta-squared of 0.001, p = 0.942. Visceral fat did not budge. Not in the 5 mg group. Not in the 10 mg group. Not by any clinically detectable amount.

What PEARL Actually Found

The headline result was a null. But PEARL was a well-designed study with a rich secondary endpoint battery, and two findings did clear statistical significance.

Women taking 10 mg rapamycin weekly gained lean tissue mass (partial eta-squared = 0.202, p = 0.013) and reported less pain (partial eta-squared = 0.168, p = 0.015). Women in the 5 mg group reported improved emotional well-being (partial eta-squared = 0.108, p = 0.023) and general health (partial eta-squared = 0.166, p = 0.004). Men showed no significant changes on any measure at either dose.

Adverse events were similar across all three groups, confirming what transplant physicians have known for decades: low-dose rapamycin is not dangerous at these levels. Blood biomarkers remained within normal ranges. Safe, yes. Effective at moving the primary aging biomarker, no.

Endpoint	Group	Effect Size (η²p)	p-value	Interpretation
Visceral adiposity (primary)	All	0.001	0.942	No effect
Lean tissue mass	Women, 10mg	0.202	0.013	Moderate-large
Self-reported pain	Women, 10mg	0.168	0.015	Moderate
Emotional well-being	All, 5mg	0.108	0.023	Small-moderate
General health	All, 5mg	0.166	0.004	Moderate
Adverse events	All	N/A	NS	No difference from placebo

These secondary findings are interesting but require caution. With multiple comparisons across subgroups, some significant results are expected by chance alone. PEARL did not pre-register corrections for multiple testing on secondary endpoints. A lean-mass gain in women at one dose but not the other, with no corresponding effect in men, could reflect a real sex-specific mTOR biology or could be a statistical artifact in a modestly powered trial.

The Translation Gap: From Mouse to Human

Here is the calculation nobody in the rapamycin community wants to confront. In the NIA's Interventions Testing Program, rapamycin at 14 parts per million extended median mouse lifespan by 14% in males and 9% in females, even when started at 600 days of age (roughly equivalent to 60 human years). At 42 ppm, the effect grew to 23% in males and 26% in females. These are massive effect sizes by any biomedical standard.

Now compare that to PEARL's primary endpoint. Visceral adiposity change after 48 weeks of weekly rapamycin: effectively zero. A partial eta-squared of 0.001 means rapamycin explained 0.1% of the variance in visceral fat. For context, measurement noise in DXA scanning typically accounts for more variance than that.

Geroscience has a name for this pattern. It is called the species translation gap, and it plagues every calorie-restriction mimetic that has moved from rodent to primate models. Calorie restriction extends mouse lifespan by roughly 30-40%. In rhesus macaques, the NIA's 25-year calorie restriction study showed improved healthspan metrics but no statistically significant lifespan extension in the intention-to-treat analysis. In humans, epidemiological data suggests calorie restriction may add a few years at best, not the decades implied by mouse effect sizes.

Rapamycin is a calorie-restriction mimetic. It works by inhibiting mTOR (mechanistic target of rapamycin), which upregulates autophagy, the cellular recycling system that calorie restriction also activates. If calorie restriction itself loses 80-90% of its effect size crossing the species barrier, we should expect rapamycin to follow the same trajectory. PEARL's null primary endpoint is consistent with this prediction, not an anomaly.

Intervention	Mouse Lifespan Extension	Best Human Evidence	Translation Ratio
Calorie restriction	30-40%	~2-5 years (estimated)	~5-15% of mouse effect
Rapamycin (ITP)	14-26%	Primary endpoint null (PEARL)	Unmeasurably small on fat
Metformin	4-6%	TAME trial ongoing	Unknown

The Dosing Problem Nobody Solved

PEARL tested 5 mg and 10 mg weekly. Transplant patients take 2-5 mg daily, producing blood trough levels of 5-15 ng/mL for immunosuppression. The longevity community has converged on intermittent low-dose protocols (typically 3-6 mg once weekly) based on the theory that pulsed mTOR inhibition captures the autophagy benefit without sustained immunosuppression. This theory has never been validated in a dose-response clinical study.

That is exactly what prompted the National Institute on Aging to fund a new trial at UT Health San Antonio, announced March 26, 2026. Led by Ellen Kraig, PhD, Dean Kellogg Jr., MD, PhD, and Brett Ginsburg, PhD, the study takes a fundamentally different approach from PEARL.

Phase one establishes biological baselines by measuring immune and metabolic markers in younger adults, defining what "optimal" function looks like before aging degrades it. Phase two performs systematic dose-finding in older adults, testing multiple dosing schedules to identify the minimum effective dose that shifts biomarkers toward younger-adult levels without immunosuppressive side effects. Phase three runs a randomized, placebo-controlled trial with approximately 84 older adults comparing daily rapamycin, intermittent dosing, and placebo over six months of treatment plus six months of follow-up.

"Rapamycin is widely discussed in popular culture as a longevity drug," Kraig told UT Health San Antonio. "But there is a difference between something that is biologically plausible and something that has been rigorously tested in people."

The study is being conducted at the Sam and Ann Barshop Institute for Longevity and Aging Studies, part of the same UT system that hosts one of the three ITP sites where rapamycin's mouse lifespan data was generated. If anyone has institutional credibility to close the mouse-to-human loop, it is this group.

Why People Won't Wait

The 333 adults in the GeroScience survey represent one pole of a growing tension in geroscience. Generic rapamycin (sirolimus) costs $30-80 per month with a GoodRx coupon. Any physician can prescribe it off-label. Longevity medicine practices like AgelessRx (which funded PEARL) have built business models around matching willing patients with prescribers. Bryan Johnson's "Blueprint" protocol, which attracted global media coverage in 2023-2024, included rapamycin and helped normalize the practice of self-optimizing pharmaceutical regimens based on preclinical data.

Case for self-dosing is straightforward: mice live 14-26% longer, the drug is safe at low doses, I am aging now and cannot wait 10 years for Phase III results. Its counterargument is equally simple: the primary endpoint of the first real trial was null, nobody knows the right dose for humans, and "safe" at the population level does not mean "beneficial" at the individual level.

A February 2026 paper in PMC added one more data point to the mechanistic argument. Rapamycin appears to help aging human immune cells resist DNA damage by enhancing cellular resilience pathways. This is plausible. It is also an in-vitro finding in immune cells, not a clinical outcome in living people. The gap between "rapamycin does something interesting in a dish" and "rapamycin makes you live longer" remains unpaved.

Strongest Counterargument

PEARL may have been set up to fail on its primary endpoint. Visceral adiposity is a downstream metabolic marker that takes years to shift meaningfully. Forty-eight weeks may simply not be long enough to detect changes in a compartment that accumulated over decades. The ITP mouse studies ran for the animals' remaining natural lifespan. A more appropriate primary endpoint might have been an epigenetic clock (GrimAge, DunedinPACE) or immune function panel, both of which can detect biological age changes over shorter intervals. The null result on visceral fat tells us rapamycin did not shrink belly fat in one year. It does not tell us rapamycin failed to slow aging.

Additionally, PEARL was crowdfunded and decentralized, with participants self-administering compounded rapamycin at home. Compliance monitoring relied on self-report. Blood trough levels were not routinely measured. If a substantial fraction of participants were under-dosing (compounded drugs have known potency variability), the null result could reflect pharmacological undershoot rather than biological failure. The UT San Antonio trial's focus on pharmacokinetics, measuring how the drug is actually absorbed and cleared in older bodies, directly addresses this weakness.

And the secondary endpoints are not noise. A partial eta-squared of 0.202 for lean mass in women on 10 mg is a large effect size. Lean tissue preservation is independently associated with reduced mortality in older adults. If rapamycin's primary benefit in humans turns out to be sarcopenia prevention in women rather than visceral fat reduction in everyone, that would still be clinically meaningful, just not the universal longevity drug the community wanted.

Limitations

This analysis relies on the PEARL trial's published data, which has not yet been independently replicated. The 333-person survey was self-selected and self-reported, making it a floor estimate of off-label use rather than a reliable prevalence measure. Our translation-gap comparison uses different outcome measures across species (lifespan in mice vs. visceral fat in humans), which limits the precision of the ratio calculation. A true translation ratio would require a completed human lifespan study, which does not exist and may never be feasible. The UT San Antonio trial has launched but has no results yet. Mouse-to-human dose conversion is an imperfect science, and whether PEARL's doses (5-10 mg weekly) correspond to the ITP's effective mouse doses (14-42 ppm in chow) is debated. Finally, rapamycin's effects may be highly individual, varying by sex, age, baseline mTOR activity, and genetic background in ways that population-level trials cannot detect.

What You Can Do

If you are considering rapamycin for longevity: The honest assessment is that no clinical trial has demonstrated rapamycin extends human healthspan or lifespan. PEARL confirmed it is safe at low doses, which is useful. But "safe and unproven" is different from "safe and effective." If you choose to proceed, insist on blood trough monitoring (target: 3-8 ng/mL for longevity protocols vs. 5-15 ng/mL for immunosuppression), baseline and periodic lipid and glucose panels (rapamycin can cause hyperglycemia and dyslipidemia at higher doses), and an honest conversation with your prescriber about the evidence grade, which is currently preclinical-plus.

If you follow geroscience research: Watch the UT San Antonio trial's Phase 2 dose-finding results, expected in late 2027 or 2028. This will be the first study to establish pharmacokinetically validated dosing for anti-aging rapamycin in humans. Also watch the TAME trial (Targeting Aging with Metformin), which will provide a comparison point for a different mTOR-adjacent pathway.

If you are a physician prescribing rapamycin off-label: Inform patients that the strongest clinical evidence (PEARL) showed a null primary endpoint. Secondary findings in women at 10 mg are promising but unreplicated. Prescribing off-label is legal. Prescribing off-label without communicating the evidence grade is a liability.

The Bottom Line

Rapamycin remains the most replicated longevity intervention in animal models. Nothing else in geroscience has 14-26% lifespan extension confirmed across three independent labs. But the first rigorous human trial found the primary aging biomarker unmoved after a full year of treatment. The secondary findings, lean mass and well-being gains in specific subgroups, are real but narrow. Thousands of adults are dosing themselves with a drug whose optimal human dose has never been established by a pharmacokinetic study. The NIA-funded trial at UT San Antonio is designed to fix that, but results are years away. Until then, the gap between what rapamycin does in a mouse and what it does in a human is the most expensive open question in longevity science.