Reviewed
Reviewed on 2026-09-19 against the evidence rules: every claim carries a grade, every claim and every safety flag carries a source, and every citation was re-resolved against the record it names. The read was delegated by the owner on 2026-09-18 and signed by the Lead on his behalf (D-295) — it is a delegated review, not the owner's own reading of the page. Every claim on it carries an evidence grade and a named weakest link. It is educational and is not medical advice.
Last reviewed
The largest review in this section found no vitamin or mineral strategy that maintains cognition in healthy adults. The one trial here that found an effect studied women who were deficient. That difference is the whole entry: correcting a shortfall is not the same claim as topping up.
Safety
Adverse effects
Iron: national guidance puts requirements at 8.7 mg a day for men and women over 50 and 14.8 mg for women aged 19 to 49. Doses over 20 mg can cause constipation, nausea, vomiting and stomach pain; 17 mg or less a day is unlikely to cause harm. These are ceilings, not targets.
Adverse effects
Vitamin D: do not take more than 100 micrograms — 4,000 IU — a day. Too much over a long period causes calcium to build up, which can weaken bones and damage the kidneys and heart.
Interaction
Three ordinary medicines appear in the B12 risk list: proton pump inhibitors, metformin and nitrous oxide. Anyone taking one of them has a reason to raise B12 at a routine appointment.
No data in your group
The Cochrane null covers adults aged 40 and over. Younger adults are outside it, which cuts both ways: there is no evidence of benefit for them either, only less evidence of anything.
What the evidence says, and how strong it is
A grade belongs to one claim, in one population, for one preparation — never to a compound, a food or a page. Open a grade to see the four links between the evidence and the thing you would actually do, and which of them is doing the least work.
Across 28 studies and more than 83,000 people, a Cochrane review found no evidence that any vitamin or mineral supplementation strategy has a meaningful effect on cognitive decline or dementia in cognitively healthy adults. This is the largest result on the page and it is a null.
Evidence grade B: May supportIn whom: Cognitively healthy adults aged 40 and over
What form: Any vitamin or mineral supplementation strategy
Measured as: Cognitive decline and dementia
Evidence does not support
Why this grade
- Weakest link
- Population — the people studied are not the people reading. The review covers adults aged 40 and over, so the null is powerful about them and says much less about someone in their twenties, where almost nobody has looked.
- What was actually studied
- Cognitively healthy adults aged 40 and over; 28 studies (n = 83000)
- Detail worth knowing
- Found no evidence that any vitamin or mineral supplementation strategy for cognitively healthy adults in mid or late life has a meaningful effect on cognitive decline or dementia. `n` is the reported 'over 83,000 participants', rounded down to the figure the abstract states.
In a trial of women aged 18 to 35 of varied iron status, iron status was a significant factor in task performance, and deficiency affected accuracy across several tasks. It is a single trial with 113 people at follow-up.
Evidence grade B: May supportIn whom: Women aged 18–35 who were iron deficient
What form: Iron supplementation over 16 weeks, with iron status measured throughout
Measured as: Accuracy and speed on laboratory attention and memory tasks
Why this grade
- Weakest link
- Study quality — design, size, blinding or replication. One trial with 113 people at follow-up, never replicated — the pattern across three iron-status groups is what makes it interesting, and it is still one trial.
- What was actually studied
- Women aged 18–35 of varied iron status; 149 at baseline, 113 at follow-up (n = 113, retested after 16 weeks of treatment, of 149 measured at baseline)
- Detail worth knowing
- Concludes that iron status is a significant factor in cognitive performance in women of reproductive age, with iron deficiency affecting accuracy across diverse laboratory tasks. A single trial.
Across 67 trials and 8,506 women, daily iron reduced anaemia and iron deficiency, raised haemoglobin and iron stores, improved exercise performance and reduced symptomatic fatigue. Gastrointestinal side effects increased. These are the established outcomes; thinking is not among them.
Evidence grade B: May supportIn whom: Menstruating women aged 12–50
What form: Daily iron supplementation
Measured as: Anaemia, haemoglobin, iron stores, symptomatic fatigue and exercise performance
Why this grade
- Weakest link
- Population — the people studied are not the people reading. The 8,506 participants are menstruating women aged 12 to 50, which is the group with a reason to be short of iron and not the general adult reader.
- What was actually studied
- Menstruating women aged 12–50; 67 trials (n = 8506)
- Detail worth knowing
- Daily iron supplementation reduces anaemia and iron deficiency, raises haemoglobin and iron stores, improves exercise performance and reduces symptomatic fatigue; gastrointestinal side effects increase. Cognitive outcomes were sparsely reported.
Cognitive outcomes were sparsely reported in the Cochrane review, and the trial that found an effect found it where iron was low. Nothing here supports taking iron on the assumption of a shortfall, and iron has a documented dose above which harm is expected.
Evidence grade D: Human evidence insufficientIn whom: Adults whose iron status is normal
What form: Iron supplementation without a measured shortfall
Measured as: Any cognitive measure
Why this grade
- Weakest link
- Preparation — what was tested is not what is sold. Iron given to somebody with a normal ferritin is not the intervention either source tested; both recruited on iron status or in a group where shortfall is common.
- What was actually studied
- Menstruating women aged 12–50; 67 trials (n = 8506); Women aged 18–35 of varied iron status; 149 at baseline, 113 at follow-up (n = 113, retested after 16 weeks of treatment, of 149 measured at baseline); UK general population
- Detail worth knowing
- Daily iron supplementation reduces anaemia and iron deficiency, raises haemoglobin and iron stores, improves exercise performance and reduces symptomatic fatigue; gastrointestinal side effects increase. Cognitive outcomes were sparsely reported.
- Concludes that iron status is a significant factor in cognitive performance in women of reproductive age, with iron deficiency affecting accuracy across diverse laboratory tasks. A single trial.
- Requirements 8.7 mg/day for men 19 and over, 14.8 mg/day for women 19–49, 8.7 mg/day for women 50 and over. Doses over 20 mg can cause constipation, nausea, vomiting and stomach pain; 17 mg or less a day is unlikely to cause harm.
National guidance names the risk groups: pernicious anaemia, diets without meat, fish or dairy including vegan diets without fortified food or a supplement, stomach surgery, Crohn's disease, and the medicines proton pump inhibitors, metformin and nitrous oxide.
Evidence grade B: May supportIn whom: UK adults
What form: No supplement — a blood test discussed with a clinician
Measured as: Identification of who has a reason to have their vitamin B12 measured
Why this grade
- Weakest link
- Outcome — what was measured is not what you would notice. What this establishes is who has a reason to have a level checked, which is a triage question for a clinician rather than a measurement of anything on this page.
- What was actually studied
- UK general population
- Detail worth knowing
- Names the risk groups this entry lists: pernicious anaemia (the most common UK cause), diets without meat, fish or dairy including vegan diets without fortified foods or a supplement, stomach surgery, Crohn's disease, and the medicines proton pump inhibitors, metformin and nitrous oxide. Stores last 2–5 years.
Everything above, and where it comes from
- Rutjes AW, Denton DA, Di Nisio M, et al. (2018). Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life. Cochrane Database of Systematic Reviews 2018(12):CD011906.pub2
- Murray-Kolb LE, Beard JL (2007). Iron treatment normalizes cognitive functioning in young women. American Journal of Clinical Nutrition 85(3):778–787
- Low MS, Speedy J, Styles CE, De-Regil LM, Pasricha SR (2016). Daily iron supplementation for improving anaemia, iron status and health in menstruating women. Cochrane Database of Systematic Reviews 2016(4):CD009747.pub2
- National Health Service (UK) (2020). Iron — Vitamins and minerals. nhs.uk, page last reviewed 3 August 2020
- National Health Service (UK) (2023). Causes — Vitamin B12 or folate deficiency anaemia. nhs.uk, page last reviewed 20 February 2023
- National Health Service (UK) (2020). Vitamin D — Vitamins and minerals. nhs.uk, page last reviewed 3 August 2020
The one thing worth doing
Write down which risk groups you are actually in. For most people the list is empty, and an empty list is the result. Take a short list to your next appointment.
Nothing in this entry is medical advice, and no dose is recommended anywhere in it. The two figures quoted are upper limits from national guidance. Deficiency questions belong to a clinician with a blood result in front of them.