Reading Longevity Studies: Cohort, Trial and Mouse Findings

Why the type of study matters

A headline such as "compound X helps mice live longer" can describe very different kinds of evidence. A study in people who simply report what they do is not the same as a trial that assigns a treatment, and neither is the same as an experiment in mice. This page explains the three, using real results from aging research. It is information only and gives no advice about what to take.

Cohort and other observational studies

Observational studies watch what happens to people in normal life. The National Institute on Aging says they do not test a medical intervention, such as a drug or device, but may help identify new treatments or prevention strategies to test in clinical trials. [1]

The weakness is that people who choose a habit or supplement often differ from those who do not. In one analysis of 4,286 women aged 60 to 79, adverse socioeconomic circumstances across the life course were linked to a lower chance of ever using hormone replacement therapy. The authors suggested that leftover confounding may explain some of the gap between observational studies and randomized trials of that therapy. [2] The lesson is general: an association between a habit and long life is not proof that the habit caused it. Label for most supplement claims built only on observation: insufficient evidence.

Randomized controlled trials

Clinical trials test a medical, surgical or behavioral intervention. [3] Random assignment is meant to make the groups alike, so differences in outcomes are more likely to come from the treatment itself. Even then, read what was measured. A change in a blood marker is a biomarker result, not proof of a longer or healthier life; see surrogate endpoints and aging biomarkers in trials.

Large trials can overturn hopeful ideas. A Cochrane review of 78 randomized trials with 296,707 participants found no evidence to support antioxidant supplements for preventing death, and concluded that beta-carotene and vitamin E seem to increase mortality. [4][5][6] The US Preventive Services Task Force likewise recommends against beta carotene or vitamin E supplements to prevent cardiovascular disease or cancer. [7] Label: evidence of harm for those two, no good evidence of benefit for the group. More in antioxidant supplements and aging.

Mouse lifespan studies

Mouse studies are valuable for finding ideas, but they are preclinical (cells or animals only). Four things limit what they tell us about people.

• Genetic variety. One researcher notes that most studies in model organisms use strains of limited genetic diversity, which do not apply well to human populations. [8]
• Poor overlap with human genes. Of more than 100 mouse genes linked to aging, 51 had life-extending effects, yet none of these was associated with human longevity in the large study discussed. [9]
• Effects differ by sex and by test. In the NIA Interventions Testing Program, aspirin and nordihydroguaiaretic acid (NDGA) increased lifespan in male mice only. [10] A later reanalysis found three compounds, including metformin, were effective only in males, and two others only in females. [11]
• Many ideas fail. Five widely sold ingredients (resveratrol, green tea extract, curcumin, oxaloacetic acid and medium-chain triglyceride oil) had no statistically significant effect on lifespan at the concentrations tested. [12]

What a strong mouse study looks like

The Interventions Testing Program was built to reduce these problems. Each compound is tested in parallel at three sites, [13] using genetically heterogeneous mice, chosen to avoid genotype-specific effects on disease susceptibility. [14] Its best-known positive result is rapamycin, started at 600 days of age, which raised the age at 90% mortality by 14% in females and 9% in males. [14][15] The program has so far identified 12 compounds that extend lifespan in these mice by its standard test. [16]

That is a solid result in mice. It is still not a human result. Rapamycin (sirolimus) is an approved medicine for a different purpose: its FDA label lists uses such as preventing organ rejection in kidney transplant patients, and aging does not appear in the label text we read. [17] What is known about it in people is covered in rapamycin and mTOR inhibitors in aging research.

A quick checklist

When you see a longevity claim, ask:

1. Who or what was studied? Cells, worms, mice, or people.
2. What was measured? Lifespan, a disease, or only a marker such as a blood level.
3. What was it compared with? A placebo, another treatment, or nothing.
4. How many, for how long? Small and short studies can mislead.
5. Do the products match? Supplements bought in stores may differ in important ways from products tested in studies. [18]
6. Who paid, and did anyone else repeat it? Look for independent confirmation and for negative results.

Where to go next

See how we label evidence in longevity research, aging in mice, worms and humans, the Interventions Testing Program, the claim check on mouse results, and how to find longevity studies and trial records.