Vitamin B2 (Riboflavin)
Riboflavin supports energy metabolism. Supplemental benefits are clearest for migraine prevention and correcting low intake.
Overview
What is vitamin B2?
Riboflavin (vitamin B2) is a vitamin required in many bodily processes including the maintenance of skin, gut and brain health. Vitamin B2 is essential for producing two cofactors: FAD and FMN. These cofactors are critical for many enzymes called flavoproteins that drive energy production and metabolism. Riboflavin deficiency, though rare, causes symptoms affecting the skin, mucous membranes, and eyes. Supplementation benefits include cardiovascular health improvements for people with the MTHFR 677TT gene variant and may help migraines at higher doses. Riboflavin is important for those with poor diets, certain genetic conditions, and those on iron therapy to optimize nutrient use.
Dosage
The recommended dietary allowance (RDA) of riboflavin is 1.3 mg per day for males and 1.1 mg per day for females.
For pregnant women, the RDA rises to 1.4 mg per day.
During lactation, the RDA rises to 1.6 mg daily.
Effect graph
Effect heatmap
SupplementDEX Database
27 conditions for , and rated at least stars. All purchase bands. No minimum star filter.
Database Snapshot
Most vitamin b2 (riboflavin) research is concentrated in a handful of conditions.
- Migraine accounts for 15 of 63 studies (24%).
- Cardiovascular health accounts for 5 of 63 studies (8%).
- Esophageal cancer accounts for 5 of 63 studies (8%).
- Sparse coverage remains for Oral leukoplakia, Acne, Anxiety.
Dietary Sources
Top dietary sources
Ranked by amount per 100 g
- 1 mg100 g
- 0.9 mg100 g
- 0.5 mg100 g
- 0.5 mg100 g
- 0.5 mg100 g
- 0.4 mg100 g
- 0.3 mg100 g
- 0.2 mg100 g
Safety
Oral riboflavin is likely safe at a dose of 400 mg per day, and this dose is used for the purpose of reducing migraines. A tolerable upper intake level is not known.
Frequently Asked Questions
References
Sources55 references
SupplementDEX uses only high-quality sources to determine supplement effectiveness.
SupplementDEX Database · Riboflavin deficiency
3 references1.SupplementDEX Database: Riboflavin deficiency — Biochemical indices and neuromuscular function tests in rural Gambian schoolchildren given a riboflavin, or multivitamin plus iron, supplement1994
A randomized trial in children/adolescents found that 5 mg for 1 year although overall the arm tremor and related neuromuscular function tests did not respond significantly to the supplements, significant improvement was seen in the boys for the arm-tremor test in both the supplemented groups.
2.SupplementDEX Database: Riboflavin deficiency — Effects of pyridoxine and riboflavin supplementation on physical fitness in young adolescents1990
A study in children/adolescents found that in both supplemented groups there was a significant increase in physical fitness in subjects with initially deficient biochemical vitamin status whereas supplementation had no effect on physical fitness in subjects with initially high biochemical values.
3.SupplementDEX Database: Riboflavin deficiency — Riboflavin status of adolescent southern Chinese: riboflavin saturation studies1985
A randomized trial in children/adolescents found that 1.5 mg/day after 14 days' supplementation it was significantly increased only in the groups receiving 1.0 and 1.5 mg daily.
SupplementDEX Database · Cardiovascular health
5 references4.SupplementDEX Database: Cardiovascular health — Riboflavin Intake Inversely Associated with Cardiovascular-Disease Mortality and Interacting with Folate Intake: Findings from the National Health and Nutrition Examination Survey (NHANES) 2005-20162022
A study of 10480 adults found that those with a high folate intake (Q4) and low intake of riboflavin (Q1) had the highest risk of CVD mortality (HR 4.38, 95% CI 1.79-10.72), as compared with a high intake of both riboflavin and folate.
5.SupplementDEX Database: Cardiovascular health — Riboflavin offers a targeted strategy for managing hypertension in patients with the MTHFR 677TT genotype: a 4-y follow-up2012
A randomized trial of 31 patients with cardiovascular disease found that 1.6 mg/day for 16 weeks these findings, if confirmed in the general population, could have important implications for the prevention of hypertension.
6.SupplementDEX Database: Cardiovascular health — Riboflavin lowers blood pressure in cardiovascular disease patients homozygous for the 677C-->T polymorphism in MTHFR2010
A randomized trial of 60 found that 1.6 mg/day for 16 weeks among patients taking one or more antihypertensive drugs at recruitment (82%), we observed that target blood pressure (<140/90 mmHg) had been achieved in only 37% patients with the TT genotype compared with 59% with the CT and 64% with the CC genotype (P < 0.001).
7.SupplementDEX Database: Cardiovascular health — Riboflavin supplementation and biomarkers of cardiovascular disease in the elderly2009
A randomized trial of 21 older adults found that 10 mg/day for 28 days riboflavin supplementation significantly decreased plasma tHcy (P=0.005) and EGRAC (P=0.014), but not plasma ferritin, uric acid or C-reactive protein.
8.SupplementDEX Database: Cardiovascular health — Riboflavin lowers homocysteine in individuals homozygous for the MTHFR 677C->T polymorphism2005
A randomized trial of 28 healthy adults found that 1.6 mg/day for 12 weeks individuals homozygous for the methylenetetrahydrofolate reductase (MTHFR) 677C-->T polymorphism have reduced MTHFR enzyme activity resulting from the inappropriate loss of the riboflavin cofactor, but it is unknown whether their typically high homocysteine levels are.
SupplementDEX Database · High blood pressure
2 references9.SupplementDEX Database: High blood pressure — Inverse Association Between Riboflavin Intake and New-Onset Hypertension: A Nationwide Cohort Study in China2020
An observational study of 245 adults found that our results emphasized the importance of maintaining relatively higher riboflavin intake levels for the prevention of hypertension.
10.SupplementDEX Database: High blood pressure — Blood pressure in treated hypertensive individuals with the MTHFR 677TT genotype is responsive to intervention with riboflavin: findings of a targeted randomized trial2013
A randomized trial of 1427 hypertensive patients found that 1.6 mg/day for 16 weeks these results show that riboflavin supplementation targeted at hypertensive individuals with the MTHFR 677TT genotype can decrease BP more effectively than treatment with current antihypertensive drugs only and indicate the potential for a personalized approach to the.
SupplementDEX Database · Hyperhomocysteinemia
3 references11.SupplementDEX Database: Hyperhomocysteinemia — Effect of riboflavin status on the homocysteine-lowering effect of folate in relation to the MTHFR (C677T) genotype https://pubmed.ncbi.nlm.nih.gov/12560354/2003
A study of 126 (ages 20–63) found that plasma tHcy was 2.6 micromol/L higher in the lowest plasma riboflavin quartile compared with the highest (P or =1.4 from 52% to 65% after supplementation (P <0.05).
12.SupplementDEX Database: Hyperhomocysteinemia — Impaired functioning of thermolabile methylenetetrahydrofolate reductase is dependent on riboflavin status: implications for riboflavin requirements https://pubmed.ncbi.nlm.nih.gov/12145019/2002
A study of 286 healthy adults found that this may have important implications for governments considering new fortification policies aimed at the prevention of diseases for which this genotype is associated with increased risk.
13.SupplementDEX Database: Hyperhomocysteinemia — Riboflavin as a determinant of plasma total homocysteine: effect modification by the methylenetetrahydrofolate reductase C677T polymorphism https://pubmed.ncbi.nlm.nih.gov/10926884/2000
A study found that riboflavin determined tHcy independently in a multiple linear regression model with adjustment for sex, age, folate, cobalamin, creatinine, and MTHFR genotype (P = 0.008). tHcy was 1.4 micromol/L higher in the lowest compared with the highest riboflavin quartile.
SupplementDEX Database · Migraine
15 references14.SupplementDEX Database: Migraine — Effects of selected dietary supplements on migraine prophylaxis: A systematic review and dose-response meta-analysis of randomized controlled trials https://pubmed.ncbi.nlm.nih.gov/39404918/2024
A meta-analysis of 22 migraine trials found alpha-lipoic acid reduced attack frequency by 1.24 and severity by 0.38 versus control, among several supplements showing prophylactic benefit.
15.SupplementDEX Database: Migraine — Nutritional intake of riboflavin (vitamin B2) and migraine: a cross-sectional analysis of the National Health and Nutrition Examination Survey (NHANES) 2001–2004 https://pubmed.ncbi.nlm.nih.gov/36175363/2022
An observational study of 3439 adults found that odds ratios and 95% confidence intervals were calculated for riboflavin intake quartiles using an adjusted logistic regression model.
16.SupplementDEX Database: Migraine — Effect of Vitamin B2 supplementation on migraine prophylaxis: a systematic review and meta-analysis https://pubmed.ncbi.nlm.nih.gov/33779525/2022
A meta-analysis of 673 participants found that 400 mg/day vitamin B2 supplementation significantly decreased migraine days ( p = .005, I 2 = 89%), duration ( p = .003, I 2 = 0), frequency ( p = .001, I 2 = 65%), and pain score ( p = .015, I 2 = 84%).
17.SupplementDEX Database: Migraine — Dietary intake of thiamine and riboflavin in relation to severe headache or migraine: A cross-sectional survey https://pubmed.ncbi.nlm.nih.gov/36047917/2022
An observational study of 13439 adults found that dietary thiamine intake was significantly inversely associated with severe headache or migraine (odds ratio [OR] = 0.93, 95% confidence interval [CI] = 0.88-1.00, p = 0.046).
18.SupplementDEX Database: Migraine — Retrospective Observational Study on Riboflavin Prophylaxis in Child and Adolescent Migraine https://pubmed.ncbi.nlm.nih.gov/33189027/2021
An observational study of 42 children/adolescents found that riboflavin prophylaxis also reduced migraine intensity and duration.
19.SupplementDEX Database: Migraine — Efficacy, Safety, and Acceptability of Pharmacologic Treatments for Pediatric Migraine Prophylaxis: A Systematic Review and Network Meta-analysis https://pubmed.ncbi.nlm.nih.gov/32040139/2020
A meta-analysis of 2217 participants found that in the short term (<5 months), propranolol (standard mean difference, 0.60; 95% CI, 0.03-1.17) and topiramate (standard mean difference, 0.59; 95% CI, 0.03-1.15) were significantly more effective than placebo.
20.SupplementDEX Database: Migraine — Effectiveness of low-dose riboflavin as a prophylactic agent in pediatric migraine https://pubmed.ncbi.nlm.nih.gov/32336482/2020
A study of 43 children/adolescents found that 40 mg/day however, non-responders were more likely to have co-morbid non-migraine headaches (odds ratio, 4.11; 95% confidence interval [CI], 1.27-13.33; p = 0.02); this variable was also significant in a multivariate analysis (adjusted odds ratio, 3.8; 95% CI, 1.16-12.6; p = 0.03).
21.SupplementDEX Database: Migraine — Improvement of migraine symptoms with a proprietary supplement containing riboflavin, magnesium and Q10: a randomized, placebo-controlled, double-blind, multicenter trial https://pubmed.ncbi.nlm.nih.gov/25916335/2015
A randomized trial of 130 adults found that migraine symptoms and burden of disease, however, were statistically significantly reduced compared to placebo in patients with migraine attacks.
22.SupplementDEX Database: Migraine — Medium-dose riboflavin as a prophylactic agent in children with migraine: a preliminary placebo-controlled, randomised, double-blind, cross-over trial https://pubmed.ncbi.nlm.nih.gov/20974610/2010
A crossover trial of 42 adults found that 50 mg/day for 16 weeks secondary outcome measures were mean severity and mean duration of migraine and tension-type headaches in the last 4 weeks at the end of the riboflavin and placebo phase, compared with the preceding baseline or wash-out period.
23.SupplementDEX Database: Migraine — High-dose riboflavin for migraine prophylaxis in children: a double-blind, randomized, placebo-controlled trial https://pubmed.ncbi.nlm.nih.gov/18984840/2008
A randomized trial of 27 children/adolescents found that 200 mg/day a 50% or greater reduction in headaches was seen in 14/21 patients in the placebo group and 12/27 patients in the riboflavin group (not significant P = .125).
24.SupplementDEX Database: Migraine — High-dose riboflavin treatment is efficacious in migraine prophylaxis: an open study in a tertiary care centre https://pubmed.ncbi.nlm.nih.gov/15257686/2004
A study found that 400 mg/day for 6 months in contrast, headache hours and headache intensity did not change significantly.
25.SupplementDEX Database: Migraine — A combination of riboflavin, magnesium, and feverfew for migraine prophylaxis: a randomized trial https://pubmed.ncbi.nlm.nih.gov/15447697/2004
A randomized trial of 49 found that 400 mg riboflavin + 300 mg magnesium + 100 mg feverfew/day for 3 months similarly, there was no significant difference in secondary outcome measures, for active versus placebo groups, respectively: 50% or greater reduction in migraine days (33% and 40%, P=.63); or change in mean number of migraines, migraine days, migraine index, or triptan d
26.SupplementDEX Database: Migraine — Prophylactic treatment of migraine with beta-blockers and riboflavin: differential effects on the intensity dependence of auditory evoked cortical potentials https://pubmed.ncbi.nlm.nih.gov/10759900/2000
A study of 15 patients with migraine found that after the treatment with beta-blockers, the intensity dependence of the auditory evoked cortical potentials was significantly decreased (before: 1.66+/-1.02 microV/10 dB; after: 0.79+/-1.06 microV/10 dB, P=.02).
27.SupplementDEX Database: Migraine — Effectiveness of high-dose riboflavin in migraine prophylaxis. A randomized controlled trial https://pubmed.ncbi.nlm.nih.gov/9484373/1998
A randomized trial of 55 patients with migraine found that 400 mg/day regarding the latter, the proportion of patients who improved by at least 50%, i.e. "responders," was 15% for placebo and 59% for riboflavin (p = 0.002) and the number-needed-to-treat for effectiveness was 2.3.
28.SupplementDEX Database: Migraine — High-dose riboflavin as a prophylactic treatment of migraine: results of an open pilot study https://pubmed.ncbi.nlm.nih.gov/7828189/1994
A study of 49 adults found that 400 mg/day mean global improvement after therapy was 68.2% and there was no difference between the two groups of patients.
SupplementDEX Database · Acne
1 reference29.SupplementDEX Database: Acne — Topical riboflavin versus 5-aminolevulinic acid photodynamic therapy for the treatment of mild to moderate acne: A split-face randomized study https://pubmed.ncbi.nlm.nih.gov/39709022/2025
A randomized trial of 30 found that both ALA and riboflavin-PDT significantly reduced non-inflammatory and inflammatory lesions at weeks 4, 6, and 10 compared to baseline (P all <0.001).
SupplementDEX Database · Anxiety
1 reference30.SupplementDEX Database: Anxiety — Effect of White Potatoes on Subjective Appetite, Food Intake, and Glycemic Response in Healthy Older Adults2020
A randomized trial of 20 older adults found that 13.7 g subjective appetite and glycemic response were measured for 120 min using visual analogue scales and capillary blood samples, respectively.
31.SupplementDEX Database: Carpal tunnel syndrome — Clinical usefulness of nutraceutics with acetyl-L-carnitine, α-lipoic acid, phosphatidylserine, curcumin, C, E and B-group vitamins in patients awaiting for carpal tunnel release during COVID-19 pandemic: a randomized controlled open label prospective study2023
An open-label RCT in 147 mild-to-moderate carpal tunnel patients awaiting surgery found a nutraceutical blend with acetyl-L-carnitine improved pain and symptom scores after 60 days, but not hand function.
SupplementDEX Database · Cataracts
3 references32.SupplementDEX Database: Cataracts — Long-term nutrient intake and 5-year change in nuclear lens opacities2005
An observational study in women found that long-term use of vitamin E supplements and higher riboflavin and/or thiamin intake may reduce the progression of age-related lens opacification.
33.SupplementDEX Database: Cataracts — Diet and cataract: the Blue Mountains Eye Study2000
The Blue Mountains Eye Study of 2,900 adults (ages 49–97) found higher protein, vitamin A, niacin, thiamin, and riboflavin intake were associated with reduced nuclear cataract prevalence.
34.SupplementDEX Database: Cataracts — The Linxian cataract studies. Two nutrition intervention trials1993
A randomized trial of 3249 (ages 45–74) found that in the second trial, the prevalence of nuclear cataract was significantly lower in persons receiving riboflavin/niacin compared with persons not receiving these vitamins.
SupplementDEX Database · Cervical cancer
1 reference35.SupplementDEX Database: Cervical cancer — Diet and premalignant lesions of the cervix: evidence of a protective role for folate, riboflavin, thiamin, and vitamin B122003
An observational study in women found that the odds ratios for LSIL and HSIL were reduced by 50-90% for the highest compared to the lowest levels of intake of these nutrients.
36.SupplementDEX Database: Chronic kidney disease (CKD) — Association between riboflavin intake and the risk of all-cause mortality of patients with chronic kidney disease: A retrospective cohort study2024
An observational study of 3750 patients with hyperlipidemia found that after excluding potential confounders, lower intake of riboflavin was associated with the higher risk of all-cause mortality (Q1: HR = 1.33, 95% CI: 1.05-1.69).
37.SupplementDEX Database: Cognitive performance — Investigating the Effects of a Multinutrient Supplement on Cognition, Mood and Biochemical Markers in Middle-Aged Adults with 'Optimal' and 'Sub-Optimal' Diets: A Randomized Double Blind Placebo Controlled Trial2022
A 12-week RCT of 141 middle-aged adults found a multinutrient formula with bacopa raised B vitamin status but did not improve memory overall, though attention improved in those with optimal baseline diets.
SupplementDEX Database · Colorectal cancer
1 reference38.SupplementDEX Database: Colorectal cancer — Dietary B vitamin and methionine intakes and risk for colorectal cancer: a case-control study in China2020
An observational study of 2502 women found that the multivariable OR for the highest quartile v. the lowest quartile were 0·62 (95 % CI 0·51, 0·74; Ptrend < 0·001) for folate, 0·46 (95 % CI 0·38, 0·55; Ptrend < 0·001) for vitamin B2, 0·55 (95 % CI 0·46, 0·76; Ptrend < 0·001) for vitamin B6 and 0·72 (95 % CI 0·60, 0·86;.
39.SupplementDEX Database: Gestational diabetes — Pregnancy thiamine and riboflavin intake and the risk of gestational diabetes mellitus: A prospective cohort study2023
A study in women found that higher intake of thiamine and riboflavin during pregnancy is associated with a lower incidence of GDM.
SupplementDEX Database · Diabetic neuropathy
1 reference40.SupplementDEX Database: Diabetic neuropathy — Efficacy and safety of the combined metabolic medication, containing inosine, nicotinamide, riboflavin and succinic acid, for the treatment of diabetic neuropathy: a multicenter randomized, double-blind, placebo-controlled parallel group clinical trial (CYLINDER)2022
A randomized trial of 109 adults found that the combination of SINR effectively alleviates DPN symptoms in patients with type 2 diabetes.
SupplementDEX Database · Esophageal cancer
5 references41.SupplementDEX Database: Esophageal cancer — Riboflavin deficiency and esophageal cancer: a case control-household study in the Caspian Littoral of Iran2005
An observational study of 57 found that 0.43 mg/day odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were also calculated.
42.SupplementDEX Database: Esophageal cancer — Effects of vitamin/mineral supplementation on the prevalence of histological dysplasia and early cancer of the esophagus and stomach: results from the General Population Trial in Linxian, China1994
In 391 Linxian adults, beta-carotene, vitamin E, and selenium reduced total and stomach cancer mortality during intervention but did not lower esophageal/gastric dysplasia prevalence at endoscopy.
43.SupplementDEX Database: Esophageal cancer — Prevention of esophageal cancer: the nutrition intervention trials in Linxian, China. Linxian Nutrition Intervention Trials Study Group1994
A randomized trial of 391 adults found that we have reported separately that cancer mortality over the entire 5.25-year period was significantly reduced among those receiving beta-carotene, vitamin E, and selenium.
44.SupplementDEX Database: Esophageal cancer — Studies on medicamentous inhibitory therapy for esophageal precancerous lesions--3- and 5-year inhibitory effects of antitumor-B, retinamide and riboflavin1990
A randomized trial of 2531 (ages 3–5) found that treatment was continued for 3 or 5 years (administration rate greater than 90% in all groups) and esophageal cytology reexamined (reexamination rates were 94.1% and 92.5% respectively).
45.SupplementDEX Database: Esophageal cancer — No effect of riboflavine, retinol, and zinc on prevalence of precancerous lesions of oesophagus. Randomised double-blind intervention study in high-risk population of China1985
A randomized trial of 610 found that 50 mg the intervention did not affect the prevalence of oesophageal lesions: after one year, the prevalence of oesophagitis with or without atrophy or dysplasia was 45.3% in the placebo group and 48.9% in the vitamin/zinc treated group.
SupplementDEX Database · Kwashiorkor
1 reference46.SupplementDEX Database: Kwashiorkor — Antioxidant supplementation for the prevention of kwashiorkor in Malawian children: randomised, double blind, placebo controlled trial2005
A randomized trial of 2372 children/adolescents found that secondary outcomes were the rates of change for weight and length and the number of days of infectious symptoms. 62 children developed kwashiorkor (defined by the presence of oedema); 39/1184 (3.3%) were in the antioxidant group and 23/1188 (1.9%) were in the placebo group.
SupplementDEX Database · Liver cancer
1 reference47.SupplementDEX Database: Liver cancer — Chemoprevention of primary liver cancer: a randomized, double-blind trial in Linxian, China2007
A randomized trial of 151 healthy adults found that factor C reduced liver cancer death, albeit with only borderline statistical significance in males (HR = 0.70, 95% CI = 0.47 to 1.02) but not in females (HR = 1.30, 95% CI = 0.72 to 2.37).
SupplementDEX Database · Lung cancer
1 reference48.SupplementDEX Database: Lung cancer — Lung cancer chemoprevention: a randomized, double-blind trial in Linxian, China2006
A randomized trial of 147 healthy adults found that no significant interactions were seen for age, sex, or smoking status.
SupplementDEX Database · Malaria
1 reference49.SupplementDEX Database: Malaria — Effect of supplementary vitamins and iron on malaria indices in rural Gambian children1987
A study of 190 children/adolescents found that no significant difference in malarial incidence was observed between the 2 groups, despite a major improvement in biochemical indices of nutrient status in the supplemented group, especially for riboflavin and vitamin C.
SupplementDEX Database · Metabolic health
2 references50.SupplementDEX Database: Metabolic health — Riboflavin supplementation to patients with multiple sclerosis does not improve disability status nor is riboflavin supplementation correlated to homocysteine2013
A randomized trial in healthy adults found that 10 mg/day the mean ± SD of EDSS score was significantly decreased in both groups over the six months of the study (2.3 ± 0.7 vs. 1.6 ± 0.6 for the riboflavin group and 2.8 ± 1.1 vs. 2.3 ± 1.3 for the placebo groups.
51.SupplementDEX Database: Metabolic health — Effect of riboflavin supplementation on plasma homocysteine in elderly people with low riboflavin status2002
A randomized trial in older adults found that 1.6 mg/day for 6 weeks although riboflavin supplementation significantly improved riboflavin status in both parts 1 and 2 of the study (P<0.001 for each), tHcy concentrations were unaffected (P=0.719).
SupplementDEX Database · Oral leukoplakia
2 references52.SupplementDEX Database: Oral leukoplakia — Chemoprevention of oral leukoplakia and chronic esophagitis in an area of high incidence of oral and esophageal cancer1993
A randomized trial of 532 men found that 100000 IU/day for 20 months risk of progression of chronic esophagitis was also lower in men with a high blood concentration of beta-carotene, odds ratios being 0.30 (95% CI: 0.10 to 0.89) and 0.49 (95% CI: 0.15 to 1.58) for medium and high levels, respectively.
53.SupplementDEX Database: Oral leukoplakia — Chemoprevention of oral and esophageal cancer in Uzbekistan, Union of Soviet Socialist Republics1985
A study of 1569 found that of the men from whom blood was drawn for analysis, 4%, 66%, and 86% had low levels of retinol, carotene, and riboflavin, respectively.
SupplementDEX Database · Sickle cell disease
1 reference54.SupplementDEX Database: Sickle cell disease — Clinical trial of riboflavin in sickle cell disease1993
A study of 18 found that 5 mg 2×/day for 8 weeks the total iron binding capacity (TIBC) and serum ferritin (SF) were significantly higher (p < 0.01), but GSH level, haemoglobin and transferrin saturation (TS) were significantly lower (p < 0.001) in SCD patients than in normal subjects.
SupplementDEX Database · Stroke
1 reference55.SupplementDEX Database: Stroke — Do nutritional supplements lower the risk of stroke or hypertension?1998
In 29,584 adults in Linxian, China, beta-carotene, vitamin E, and selenium supplementation for 5 years modestly reduced total and stroke mortality but did not lower hypertension prevalence.