Pills, paychecks, and the healthy user
People who buy omega-3 and vitamin D look employable. The bottles are not why.
Folk claim
Supplement users have lower unemployment and better jobs. Therefore the pills cause the career.
What the papers say
This is the textbook Healthy User Effect (Shrank, Patrick, Brookhart 2011). Mendelian randomization of 25(OH)D (Meng 2019; Jiang 2021) does not produce socioeconomic or general-disease effects. VITAL (Manson 2019) does not produce the promised cancer or cardiovascular benefit in a general population. Autier et al. (2014) already warned that observational D is a marker of ill health. Condition on income, education, and outdoor time and the career arrow disappears. What remains are clinical-deficiency corrections.
If a health input can be bought, income is on the backdoor. Genetic instruments and RCTs are how this atlas takes the backdoor away.
Name the bias
Healthy users floss, show up to work, fill preventive prescriptions, and buy fish oil. Their kids' schools and their parents' degrees came first. An observational regression of job quality on pill count without those covariates is a press release, not an estimate.
What genetics and trials take away
MR asks: do people whose genomes push 25(OH)D higher have different lives? In UK Biobank-scale data, not in the ways wellness marketing needs. VITAL asks a related question with a pill and a randomization. The primary endpoints did not move. Together they identify the observational D–success link as smoke.
What remains after the demolition
Treat deficiency. B12 in the deficient, iron in the anemic, D in the dark and institutionalized. Those are medical arrows onto fatigue and bone, maybe mood. They are not arrows onto title, comp, or mate value. This atlas will keep those clinical edges if and when a paper identifies them — and will refuse to let them smuggle a career claim.
Edges in this case
Observational · High confidence
Vitamin / mineral supplementation→Employment status
The archetypal smoke. Control for education, parental SES, and baseline income and the supplement→career effect goes to zero.
Observational · High confidence
Omega-3 supplementation→Employment status
Fish-oil buyers are not a random sample of the labor force.
Mendelian randomization · High confidence
25-hydroxyvitamin D→Employment status
Genetic instruments for 25(OH)D do not buy you a job. The observational D–success link was a marker.
Observational · High confidence
25-hydroxyvitamin D↔Disposable income
People with money and outdoor lives have higher 25(OH)D. That is not D causing the money.
Observational · High confidence
Disposable income→Vitamin / mineral supplementation
Money buys bottles. Reverse causation is the whole trick of the wellness-to-wealth genre.
Observational · High confidence
Time outdoors→25-hydroxyvitamin D
Sun makes 25(OH)D. This is the boring, real arrow that makes D a terrible instrument for success.
Sources
- W. H. Shrank et al. (2011). Healthy user and related biases in observational studies of preventive interventions: a primer for physicians
- X. Meng et al. (2019). Phenome-wide Mendelian-randomization study of genetically determined vitamin D on multiple health outcomes using the UK Biobank study
- X. Jiang et al. (2021). The causal role of circulating vitamin D concentrations in human complex traits and diseases: a large-scale Mendelian randomization study
- J. E. Manson et al. (2019). Vitamin D supplements and prevention of cancer and cardiovascular disease
- P. Autier et al. (2014). Vitamin D status and ill health: a systematic review
Cite this page
What Causes What. “Pills, paychecks, and the healthy user.” https://whatcauseswhat.org/cases/supplements-healthy-user (atlas updated 2026-08-31).