A Pill Is Not the Sun

On September 10, The Washington Post reported on a study asking whether some of the most popular supplements might be doing older brains no favors. The vitamin D result caught a lot of attention, including a widely shared thread from pulmonologist and sleep physician Dr. Roger Seheult. Here is what the study found, read from the paper itself.

What the study found

The paper is a longitudinal cohort analysis published in BMJ Open in 2026, drawn from the U.S. Health and Retirement Study. Researchers followed 5,065 adults for six years (baseline 2014, cognitive assessments through 2020). Vitamin D supplement use was self-reported at baseline.

• Compared with non-users, supplement users showed a slightly faster decline in global cognition (−0.052 points per year; 95% CI −0.092 to −0.013) and in executive function (−0.021 points per year).
• The effect was concentrated in people whose blood level was already ≥30 ng/mL at baseline (−0.088 points per year). In people below 30 ng/mL there was no association at all.
• The authors' conclusion: the findings do not support vitamin D supplementation to prevent or slow cognitive decline in older people who already have adequate vitamin D, and clinicians should be cautious recommending it without a clear indication.

What it does not show

This is an observational study, and the authors say so plainly. It cannot show that supplements cause cognitive decline. The limitations they list matter for how you read it:

• Supplement use was self-reported once, at baseline. Doses and formulations were unknown.
• The cognitive tests were screening-level assessments, and the effect sizes are small.
• The sample was mostly white Americans.
• There was no data on sunlight exposure. The study cannot distinguish someone who is replete because they spend time outdoors from someone who is replete because they take a capsule every morning.

The safest reading is the authors' own: if you are deficient, correcting it matters; if you are already replete, more vitamin D from a pill is not a free lunch, and the burden of proof for taking it is on the pill.

Why the route matters

The part of Dr. Seheult's thread that resonated with us is the question underneath the headline: is vitamin D from a pill biologically the same as vitamin D your skin makes from sunlight? His answer, and ours, is no.

When UVB hits skin, 7-dehydrocholesterol becomes previtamin D3, which then slowly converts to vitamin D3. That process is self-limiting: with continued exposure, excess previtamin D3 and vitamin D3 are photoisomerized into inactive products (lumisterol, tachysterol, and others). This is why nobody has ever developed vitamin D toxicity from sunlight — the skin has a ceiling built in. A capsule bypasses that regulation entirely; the only ceiling is how many you take.

Sunlight is also not merely a vitamin D delivery system. Skin exposure releases nitric oxide, daylight sets circadian timing, and there are immune and mitochondrial effects that a capsule of cholecalciferol does not reproduce. As Dr. Seheult put it, “Route matters. Physiology matters. Context matters.” Nutrition professor Marion Nestle, quoted in the Post, said she votes for short exposures of skin to sunlight, especially in summer.

What this means if you use dminder

This study is, almost exactly, the case dminder was built for. The app's job is to get you to your target blood level from the sun when the sun can do it, and to make supplements the fallback rather than the default.

• If your estimated level is already at your goal, the study's replete-and-still-supplementing group is the one you don't want to be in. Use the D window instead of another capsule.
• If you are below your goal and the sun can't get you there — winter above ~37°N, a schedule that keeps you indoors at midday — a supplement is exactly what the authors say it is for: correcting a deficiency, not topping up a full tank.
• Know your number. The estimate is an estimate. A blood test tells you whether you are in the ≥30 ng/mL group or the <30 group, and this study says that distinction is the whole story. See Get Tested.

None of this is a reason to stop a supplement your doctor prescribed. It is a reason to ask which group you are in before you keep taking one out of habit.

Sources

1. Association between vitamin D supplement use and cognitive functioning: a longitudinal cohort study. BMJ Open, 2026. PMC12829399
2. Scientists are investigating whether popular supplements could harm the aging brain. The Washington Post, September 10, 2026. washingtonpost.com
3. Roger Seheult, MD. Thread on X, September 10, 2026. x.com/RogerSeheult
4. Holick MF, MacLaughlin JA, Doppelt SH. Regulation of cutaneous previtamin D3 photosynthesis in man: skin pigment is not an essential regulator. Science. 1981;211(4482):590–593. PubMed 6256855
5. Webb AR, DeCosta BR, Holick MF. Sunlight regulates the cutaneous production of vitamin D3 by causing its photodegradation. J Clin Endocrinol Metab. 1989;68(5):882–887. PubMed 2541158