Supplement fact check: every claim, checked against its source
The headline claims in our dosing guides, each with a plain verdict and a direct link to the health authority document or published trial behind it. Where the evidence does not support a popular claim, we say so.
Claims and verdicts
Adults should not exceed 40 mg of zinc a day from food and supplements.
Verdict: Yes — that is the published upper limit
The NIH Office of Dietary Supplements sets the adult tolerable upper intake level for zinc at 40 mg a day from food and supplements combined. Many single capsules supply 50 mg.
Source: NIH Office of Dietary Supplements — Zinc fact sheet · Zinc dosage guide
Zinc definitively shortens the common cold.
Verdict: Partly — dose, timing and format dependent, and contested
Meta-analyses of lozenge trials delivering high elemental doses within about a day of symptom onset reported shorter illness. A 2024 Cochrane review was more cautious. The evidence does not support a swallowed high-dose capsule taken on day four.
Source: Hemilä H. Zinc lozenges and the common cold: a meta-analysis. JRSM Open. 2017 · Zinc dosage guide
Taking 1,000 mg of vitamin C in one dose delivers roughly ten times as much as 100 mg.
Verdict: No — absorption saturates
Vitamin C absorption uses saturable transporters. Pharmacokinetic work shows the fraction absorbed falling and plasma levels plateauing as single doses rise, with the excess excreted in urine.
Source: Levine M, et al. Vitamin C pharmacokinetics in healthy volunteers. PNAS. 1996 · Vitamin C dosage guide
Vitamin C prevents colds in the general population.
Verdict: No — not in the general population
The Cochrane review found regular supplementation did not reduce cold incidence in ordinary adults, though it modestly shortened illness. A clear reduction in incidence appeared only in people under extreme physical stress.
Source: Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane. 2013 · Vitamin C dosage guide
A three-year trial tested 180 mcg a day of MK-7 in postmenopausal women.
Verdict: Yes — that is the published protocol
Knapen and colleagues ran a three-year randomised placebo-controlled trial at 180 mcg a day of MK-7, reporting reduced age-related decline in bone mineral density at the spine and femoral neck.
Source: Knapen MHJ, et al. Osteoporosis International. 2013 · Vitamin K2 dosage guide
Vitamin K2 is proven to keep calcium out of arteries and prevent heart events.
Verdict: No — mechanism is plausible, outcomes are not shown
Matrix Gla protein requires vitamin K to inhibit soft-tissue calcification, which is real biochemistry. Trials measuring arterial outcomes have been mixed, and no health authority concludes that supplementation prevents cardiovascular events.
Source: NIH Office of Dietary Supplements — Vitamin K fact sheet · Vitamin K2 dosage guide
People on warfarin must keep vitamin K intake consistent.
Verdict: Yes — this is standard clinical advice
Warfarin works by blocking vitamin K recycling, so changes in vitamin K intake shift the INR. Supplements should be discussed with the anticoagulation clinic before starting.
Source: NIH Office of Dietary Supplements — Vitamin K fact sheet · Vitamin K2 dosage guide
Selenium supplements prevent prostate cancer.
Verdict: No — the large trial was negative
SELECT randomised more than 35,000 men to 200 mcg a day of L-selenomethionine and found no reduction in prostate cancer risk. The trial was stopped early for futility.
Source: Lippman SM, et al. SELECT. JAMA. 2009 · Selenium dosage guide
A single brazil nut can supply more selenium than an adult needs in a day.
Verdict: Yes — often well above the 55 mcg requirement
The NIH ODS puts one brazil nut at roughly 68–91 mcg of selenium, against an adult requirement of 55 mcg and an upper limit of 400 mcg. Content varies with soil selenium.
Source: NIH Office of Dietary Supplements — Selenium fact sheet · Selenium dosage guide
Folic acid before conception reduces the risk of neural tube defects.
Verdict: Yes — one of the strongest findings in nutrition
The MRC Vitamin Study demonstrated a substantial reduction in neural tube defect recurrence with periconceptional folic acid. The CDC advises 400 mcg daily for anyone who could become pregnant, starting at least a month before conception.
Source: MRC Vitamin Study Research Group. Lancet. 1991 · Folate and folic acid guide
High-dose folic acid can hide a vitamin B12 deficiency.
Verdict: Yes — this is why the 1,000 mcg limit exists
Folic acid can correct the anaemia of B12 deficiency while neurological damage continues. The adult upper limit of 1,000 mcg of synthetic folic acid exists largely to reduce that masking risk.
Source: NIH Office of Dietary Supplements — Folate fact sheet · Folate and folic acid guide
Anyone with an MTHFR gene variant needs methylfolate instead of folic acid.
Verdict: No — not established
MTHFR variants reduce enzyme activity, but most people reach adequate folate status on ordinary folic acid, and public health advice for pregnancy still specifies folic acid because that is the form the prevention trials used.
Source: NIH Office of Dietary Supplements — Folate fact sheet · Folate and folic acid guide
Calcium is best absorbed in single doses of 500 mg or less.
Verdict: Yes — splitting the dose absorbs more
Absorption efficiency is highest at single doses of 500 mg or less, so two smaller doses deliver more calcium than one large tablet.
Source: NIH Office of Dietary Supplements — Calcium fact sheet · Calcium dosage guide
Calcium plus vitamin D supplements reliably prevent hip fractures.
Verdict: No — the largest trial found no significant reduction
The Women's Health Initiative trial of 1,000 mg calcium plus 400 IU vitamin D in over 36,000 postmenopausal women found a small bone density gain, no significant hip fracture reduction overall, and more kidney stones.
Source: Jackson RD, et al. N Engl J Med. 2006 · Calcium dosage guide
Calcium can stop levothyroxine and some antibiotics from working properly.
Verdict: Yes — separate them in time
Calcium reduces absorption of levothyroxine, quinolone and tetracycline antibiotics and bisphosphonates. Spacing the doses apart is standard advice.
Source: NIH Office of Dietary Supplements — Calcium fact sheet · Calcium dosage guide
Rhodiola rosea is a proven treatment for fatigue.
Verdict: No — evidence is limited and inconclusive
Two small placebo-controlled trials of the standardised SHR-5 extract reported reduced fatigue. NCCIH states the studies are small and of variable quality and the evidence is insufficient to conclude rhodiola is effective for any condition.
Source: National Center for Complementary and Integrative Health — Rhodiola · Rhodiola rosea guide
The rhodiola night-duty trial used a low daily amount of a standardised extract.
Verdict: Yes — a repeated low-dose SHR-5 regimen
Darbinyan and colleagues used a repeated low-dose regimen of the standardised SHR-5 extract over two weeks in physicians on night duty, reporting improvement on a mental fatigue index.
Source: Darbinyan V, et al. Phytomedicine. 2000 · Rhodiola rosea guide
Lion's mane improves memory in healthy adults.
Verdict: No — that population has not been shown to benefit
The most cited trial studied older adults who already had mild cognitive impairment, over 16 weeks, and scores fell back after supplementation stopped. There is no good evidence of memory benefit in healthy people.
Source: Mori K, et al. Phytotherapy Research. 2009 · Lion's mane dosage guide
Lion's mane regenerates nerves in people.
Verdict: No — laboratory and animal findings only
The nerve growth factor results come from cell culture and rodent studies. They justify further research; they are not evidence of nerve regeneration in humans.
Source: National Center for Complementary and Integrative Health — Mushrooms · Lion's mane dosage guide
Ginkgo biloba prevents dementia.
Verdict: No — two large long trials found no benefit
The Ginkgo Evaluation of Memory study followed over 3,000 adults aged 75+ for a median of about six years on 240 mg a day and found no reduction in dementia incidence. The five-year GuidAge trial also found no reduction in progression to Alzheimer's disease.
Source: DeKosky ST, et al. JAMA. 2008 · Ginkgo biloba dosage guide
Ginkgo should usually be stopped before surgery.
Verdict: Yes — because of bleeding concern
Ginkgo affects platelet function and bleeding events have been reported, so surgical teams commonly ask for it to be stopped one to two weeks beforehand.
Source: National Center for Complementary and Integrative Health — Ginkgo · Ginkgo biloba dosage guide
Raw ginkgo seeds are safe to eat.
Verdict: No — they are toxic
Fresh and roasted ginkgo seeds contain a toxin; ingestion has caused seizures and deaths. Supplements use a standardised leaf extract, which is a different product.
Source: National Center for Complementary and Integrative Health — Ginkgo · Ginkgo biloba dosage guide
Vitamin D needs a blood test to decide the right amount.
Verdict: Partly — the RDA applies to everyone, blood level guides correction
Health authorities publish a daily requirement and a 4,000 IU adult upper limit that apply generally. Correcting a suspected deficiency, rather than meeting the requirement, is what a 25-hydroxyvitamin D test informs.
Source: NIH Office of Dietary Supplements — Vitamin D fact sheet · Vitamin D daily dose guide
Creatine damages the kidneys in healthy people.
Verdict: No — not shown in healthy adults
Creatine raises serum creatinine, a marker used to estimate kidney function, without evidence of kidney damage in healthy people. Anyone with existing kidney disease should ask their doctor first.
Source: NIH Office of Dietary Supplements — Dietary supplements for exercise and athletic performance · Creatine dosage guide
How we check a claim
A claim is marked supported only when a named health authority document or a published trial states it directly. Where a claim rests on laboratory or animal work, or on trials too small or too short to answer the question, it is marked not supported or partly supported — even when the supplement itself is popular and well tolerated.
Amounts quoted anywhere in the guides are figures reported in those sources, not recommendations from us. Any decision about what you personally take belongs with your doctor or pharmacist, who knows your medicines and history.
This page is reference material, not medical advice. Supplements interact with prescription medicines and with each other. Talk to a doctor or pharmacist before starting anything, especially if you are pregnant, breastfeeding, managing a health condition or taking prescription medicine.