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Selenium for Cancer Prevention

Verdict: Selenium does not prevent cancer

Large randomized trials show that selenium supplements do not lower the risk of any cancer, and they carry a real signal of harm, including a higher risk of type 2 diabetes. Get selenium from a normal diet rather than from supplements taken for cancer prevention.

D 🔴 D Counter-Evidence Counter-Evidence

🔬Why this grade7-layer evidence engine

The evidence here is decisively negative. The landmark SELECT trial randomized 35,533 men and found no reduction in prostate cancer or any secondary cancer (HR 1.04), and it was stopped early for futility plus safety concerns (PMID 19066370). Extended follow-up confirmed the null selenium result (HR 1.09) and, separately, a 17% rise in prostate cancer with vitamin E alone (PMID 21990298). The 2018 Cochrane review of pooled RCTs reached the same conclusion: any-cancer incidence RR 1.01 and cancer mortality RR 0.97 (PMID 29376219).

There is also a consistent harm signal. The Cochrane review flagged increased type 2 diabetes (RR 1.06), echoing an earlier NPC analysis where selenium raised diabetes risk (HR 1.55, PMID 17620655). The older Clark/NPC trial's early positive hints were later withdrawn, with extended follow-up instead showing more squamous-cell and non-melanoma skin cancer (PMID 8971064). The SU.VI.MAX antioxidant-mix trial showed no overall cancer reduction and cannot isolate selenium's effect (PMID 15557412).

Authorities align with this grade. The American Cancer Society and Mayo Clinic state plainly that selenium does not prevent prostate cancer, the Cleveland Clinic says most people simply do not need it, and the FDA calls the cancer evidence "limited and not conclusive." The NHS notes a balanced diet supplies enough selenium, and WHO endorses it only as deficiency prophylaxis in endemic (Keshan disease) regions, not for cancer prevention.

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Scoring transparency

All scores computed by a 7-layer evidence engine — fully auditable
Raw score 0.26
D
C
B
A
S
← counter-evidence / ineffectiveeffective / strong evidence →
Final grade
D · Counter-Evidence
Confidence
87%
Highly consistent evidence
Evidence level
E1
Cochrane high-quality SR/MA

How strongly each layer supports this effect

lower = less supportive
L2 PubMedPrimary literature
0.20
L3 MechanismPlausibility
0.20
L5 Clinical bodiesAuthoritative stance
0.29
L11 AI re-checkIndependent read
0.30
L1 ExamineGlobal benchmark
0.50
Against Mixed Supports
▸View the full decision path (audit trail)
  1. compute_raw_score — 加權公式: L2×0.30 + L3×0.25 + L5×0.25 + L11×0.10 + L1×0.10 = 0.264
  2. tier_from_score — 依分數區間映射至 tier letter
  3. apply_hec_rules — 無高階證據可裁決
  4. tier_strict_requirement_check — Tier 條件達標，未降階
  5. detect_disputes — 偵測到 0 個 hard + 0 個 soft dispute
  6. decide_status — 依 tier + dispute 結果決定 status

📄PubMed studies (6)L2 · primary research & systematic reviews

Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
PMID: 19066370 2009 RCT (double-blind) n = 35,533
Finding: Selenium did not reduce prostate cancer (HR 1.04, 99% CI 0.87-1.24) nor any secondary cancer (lung, colorectal, all-cancer); trial halted early for futility and safety signals (non-significant increases in T2DM with selenium and prostate cancer with vitamin E).
🟢 High quality Government Effect size: HR 1.04 (99% CI 0.87-1.24) for prostate cancer; null across all secondary cancer endpoints
View on PubMed
Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
PMID: 21990298 2011 隨機對照試驗 n = 35,533
Finding: Updated analysis confirmed no benefit of selenium for prostate cancer (HR 1.09, 99% CI 0.93-1.27) and revealed a statistically significant 17% increase in prostate cancer with vitamin E alone (HR 1.17, 99% CI 1.004-1.36); selenium+E combination null. Reinforces decisively negative selenium signal.
🟢 High quality Government Effect size: Selenium HR 1.09 (99% CI 0.93-1.27); vitamin E alone HR 1.17 (99% CI 1.004-1.36, harm)
View on PubMed
Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer Study Group (Clark)
PMID: 8971064 1996 RCT (double-blind) n = 1,312
Finding: No effect on primary endpoint of NMSC (RR 1.10, 95% CI 0.95-1.28). Initial reports showed reduction in total cancer incidence/mortality and prostate/lung/colorectal cancer; however extended follow-up (Duffield-Lillico 2003) attenuated/abolished most signals and showed increased risk of squamous cell skin cancer (HR 1.25) and total NMSC (HR 1.17). Subsequent NPC analyses revoked the prostate cancer benefit in higher-baseline-selenium subjects.
Government Effect size: NMSC RR 1.10 (NS); later analyses: SCC HR 1.25 (95% CI 1.03-1.51, harm); total NMSC HR 1.17 (95% CI 1.02-1.34, harm); initial all-cancer signal not replicated
View on PubMed
Selenium for preventing cancer (Cochrane Systematic Review, Vinceti et al.)
PMID: 29376219 2018 Cochrane SR n = 27,232
Finding: RCT evidence does not support a beneficial effect of selenium supplementation in preventing cancer in humans. Pooled RCTs: any cancer incidence RR 1.01 (95% CI 0.93-1.10); cancer mortality RR 0.97 (95% CI 0.88-1.08); no site-specific benefit. Evidence of harm: increased risk of type 2 diabetes (RR 1.06, 95% CI 1.02-1.11) and some non-melanoma skin cancers; observational data showing inverse associations not confirmed in RCTs and likely confounded.
🟢 High quality Academic Effect size: Any cancer RR 1.01 (95% CI 0.93-1.10); mortality RR 0.97 (95% CI 0.88-1.08); T2DM RR 1.06 (95% CI 1.02-1.11, harm)
View on PubMed
Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial (NPC secondary analysis, Stranges)
PMID: 17620655 2007 隨機對照試驗 n = 1,202
Finding: Selenium supplementation associated with statistically significant increased risk of type 2 diabetes (HR 1.55, 95% CI 1.03-2.33); risk concentrated in upper tertile of baseline plasma selenium. Establishes safety signal that contributed to halting SELECT and informs negative risk-benefit balance for cancer prevention indication.
🟢 High quality Government Effect size: HR 1.55 (95% CI 1.03-2.33) for incident T2DM (harm)
View on PubMed
The SU.VI.MAX Study: a randomized, placebo-controlled trial of the health effects of antioxidant vitamins and minerals (Hercberg)
PMID: 15557412 2004 RCT (double-blind) n = 13,017
Finding: No overall reduction in total cancer incidence (RR 0.90, 95% CI 0.76-1.06) or all-cause mortality (RR 0.63, 95% CI 0.42-0.95 in men only; null in women). Effect, where present, was sex-specific and confined to men with lower baseline antioxidant status; cannot be attributed specifically to selenium given combination formula.
Government Effect size: Total cancer RR 0.90 (95% CI 0.76-1.06, NS overall); men-only subgroup signals not confirmed in subsequent selenium-specific trials
View on PubMed

🏛️Regulatory & authoritative positionsL4/L5 · FDA / EMA / NIH ODS / Cochrane / Mayo …

L4a US FDA
Cautious
Selenium may reduce the risk of certain cancers. Some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer. However, FDA has determined that this evidence is limited and not conclusive. source↗
L4b EU EFSA
Supportive
Selenium contributes to normal thyroid function; contributes to the normal function of the immune system; contributes to the protection of cells from oxidative stress; contributes to normal spermatogenesis; contributes to the maintenance of normal hair; contributes to the maintenance of normal nails. source↗
L4c UK NHS
Cautious
75μg a day for men (19 to 64 years); 60μg a day for women (19 to 64 years). You should be able to get all the selenium you need by eating a varied and balanced diet that includes meat, fish or nuts. Taking 350μg or less a day of selenium supplements is unlikely to cause any harm. Too much selenium causes selenosis, a condition that, in its mildest form, can lead to loss of hair and nails. source↗
L4d TW TFDA / 衛福部
Supportive
形態屬膠囊狀、錠狀且標示有每日食用限量之食品，在每日食用量中，其硒之總含量不得高於200 μg。限於補充食品中不足之營養素時使用。 source↗
L4e WHO
Supportive
Prophylaxis consisting of oral administration of selenium 3 months before the periods of highest anticipated risk is highly effective. Once the disease is established, selenium is of little or no therapeutic value. source↗
L5a NIH Office of Dietary Supplements
Cautious
Selenium is a trace element that is naturally present in many foods, added to others, and available as a dietary supplement. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection. source↗
L5b Mayo Clinic
Against
Research shows that vitamin E and selenium supplements don't prevent prostate cancer. There also is concern that vitamin E supplements might raise the risk of prostate cancer. source↗
L5c Cleveland Clinic
Cautious
Lower selenium intakes have been associated with increased cancer diagnoses. Selenium has been known to decrease cancer risk, as it helps to repair DNA damage that occurs in our bodies. ... Thinking about selenium supplements? Depending on where you live, you probably don't need them. ... high levels of selenium can be dangerous, and it can also interact with some medications. source↗
L5e Specialty Society (condition-mapped)
Against
a large study known as the Selenium and Vitamin E Cancer Prevention Trial (SELECT) found that neither vitamin E nor selenium supplements lowered prostate cancer risk. source↗
✓PMID 100% verifiedevery citation checked via NCBI Entrez
🔬6 PubMed studiesindependently re-checked by multiple sub-agents
engine_version: v1.0 claim_id: CLM-COND-cancer-prevention-INT-selenium-001 繁體中文版 →