Which Dietary Supplements Actually Work: A Graded Evidence Review

The supplement industry generates over £500 million annually in the UK and over $40 billion in the US. A very small proportion of this revenue goes to products with genuine, replicated clinical evidence. Most supplements are sold based on preliminary data, plausible mechanisms, or no evidence at all. This review applies an evidence grading framework to the most common supplement categories, separating those that genuinely work from those that do not.

Evidence Grading Framework

Each supplement is assessed against clinical evidence for its primary claimed use:

Grade A — Strong evidence: Multiple independent randomised controlled trials (RCTs) and/or systematic reviews/meta-analyses confirming efficacy. The effect is well-established and reproducible.

Grade B — Moderate evidence: Consistent RCT evidence from multiple studies; some inconsistency between trials or limitations in study design, but overall positive direction.

Grade C — Limited evidence: Single RCTs or mixed results; plausible mechanism but insufficient replication to draw firm conclusions.

Grade D — Weak/no evidence: Animal data only, in vitro data only, or human trials consistently failing to show benefit.

Vitamins and Minerals

Vitamin D — Grade A (for deficiency correction)

Use: Bone health, immune function, muscle function, mood

Evidence: NICE and most national health bodies recommend vitamin D supplementation for UK adults through autumn/winter due to insufficient sun exposure. Cochrane reviews confirm benefit for bone density, fracture prevention (in combination with calcium), respiratory infection risk, and several other outcomes in deficient individuals.

Critical nuance: Evidence is strong for people who are deficient (very common in the UK — estimated 40%+ of adults in winter). Evidence for supplementation in people with adequate vitamin D levels is substantially weaker.

Dose: 10 mcg (400 IU) recommended by NHS; 25 mcg (1,000 IU) is widely used and considered safe; doses above 100 mcg/day sustained carry toxicity risk.

Omega-3 Fatty Acids (EPA/DHA) — Grade A (for triglyceride reduction); Grade B (for general cardiovascular health)

Use: Cardiovascular health, inflammation reduction, brain function

Evidence: Very strong evidence for triglyceride reduction at 3–4g EPA/day (REDUCE-IT trial showed 25% reduction in cardiovascular events in high-risk individuals at 4g EPA). General cardiovascular benefits at standard supplement doses (1–2g EPA+DHA) are more modest but consistently positive in multiple meta-analyses.

Who benefits most: People who rarely eat oily fish (salmon, mackerel, sardines, herring); the benefit in people with high dietary omega-3 intake is smaller.

Dose: 1–2g EPA+DHA daily from combined fish oil or algae-derived supplement.

Safety: At doses above 3g/day, anticoagulant effect increases — discuss with GP if taking warfarin, aspirin, or other anticoagulants.

Folate (B9) — Grade A (in pregnancy)

Use: Neural tube defect prevention; cell division; DNA synthesis

Evidence: One of the strongest supplement evidence bases in medicine. 400 mcg/day before conception and through the first trimester of pregnancy reduces neural tube defect risk by approximately 70% (Medical Research Council Vitamin Study, 1991). This is a near-universal recommendation from all national health authorities.

Who needs it: All people who could become pregnant; those taking methotrexate or certain anticonvulsants; those with MTHFR genetic variants (may benefit from methylated folate form).

Vitamin B12 — Grade A (for deficiency correction)

Use: Red blood cell production; neurological function; DNA synthesis

Evidence: B12 is found exclusively in animal products. Deficiency is virtually inevitable in vegans/strict vegetarians without supplementation; also common in people over 65 (reduced gastric acid) and those taking metformin or long-term PPIs. Supplementation in deficiency: clear benefit, reverses deficiency symptoms.

Dose: 10–50 mcg/day as maintenance; 1,000–2,000 mcg/day oral for confirmed deficiency (high oral dose compensates for reduced absorption).

Magnesium — Grade B

Use: Sleep quality, blood pressure, insulin sensitivity, bone health, muscle function

Evidence: Magnesium deficiency is common in Western diets. A 2016 meta-analysis found significant blood pressure reduction from supplementation in hypertensive individuals. Evidence for sleep quality improvement (magnesium glycinate particularly) is positive but from smaller trials. Consistent evidence for improvements in insulin sensitivity in magnesium-deficient individuals.

Best-absorbed forms: Magnesium glycinate, citrate (significantly better than oxide, which is the cheapest and most common but least bioavailable).

Dose: 200–400 mg/day elemental magnesium; maximum tolerable supplement dose 350 mg/day (higher from food is fine; high supplement doses cause loose stools).

Iron — Grade A (for deficiency); Grade D (without deficiency)

Use: Haemoglobin production; energy; cognitive function

Evidence: Definitive evidence for supplementation in confirmed iron deficiency anaemia. Evidence for supplementation without deficiency: no benefit and potential harm (iron accumulates and promotes oxidative stress).

Critical rule: Test before supplementing. Iron supplementation without confirmed deficiency is not beneficial and carries real risk. A ferritin blood test (GP or private) confirms deficiency before any supplementation decision.

Calcium — Grade B

Use: Bone mineralisation; muscle contraction; blood pressure

Evidence: Clear evidence for dietary adequacy for bone health; evidence for supplements specifically is more nuanced. Combined calcium + vitamin D reduces fracture risk in institutionalised elderly. Supplementation in community-dwelling adults without deficiency is less clearly beneficial and some research associates high-dose calcium supplements (>1,500 mg/day) with elevated cardiovascular risk.

Approach: Prioritise dietary sources (dairy, fortified plant milks, kale, broccoli, sardines with bones). Supplement only to close the gap to 1,000–1,200 mg/day total.

Sports and Performance Supplements

Creatine Monohydrate — Grade A

Use: Strength, power, lean mass gain

Evidence: The most evidence-supported performance supplement — over 500 published studies. Meta-analyses confirm approximately 8% strength improvement and 1.1 kg greater lean mass gain vs placebo at matched training volumes. One of the few supplements where independent, industry-free research consistently confirms the marketed effect.

Dose: 3–5g/day indefinitely (no need for a loading phase, though 20g/day for 5 days achieves saturation faster). Monohydrate form has the strongest evidence; no superior alternatives despite marketing claims for other forms.

Safety: Excellent safety profile across 30+ years of research; extensively studied for kidney function (no adverse effects in healthy individuals); the creatine/kidney concern is unsupported by evidence in non-kidney-disease individuals.

Protein Powders (Whey, Casein, Plant-Based) — Grade A (for lean mass)

Use: Meeting protein targets; lean mass preservation; post-exercise recovery

Evidence: Extensive RCT evidence confirms protein supplementation supports lean mass gain when combined with resistance training, and lean mass preservation during calorie restriction. The source of protein (whey vs plant) matters less than total daily protein intake — achieving 1.6–2.2g/kg body weight/day from any complete protein source produces equivalent outcomes.

When useful: When dietary protein targets cannot be met through whole food alone. Post-workout protein appears particularly effective for muscle protein synthesis — 20–40g complete protein within 2 hours of training.

Caffeine — Grade A

Use: Endurance performance, strength, cognitive function, thermogenesis

Evidence: Caffeine is among the most evidence-supported ergogenic compounds across both performance and metabolic domains. Multiple meta-analyses confirm 2–3% endurance performance improvement at 3–6 mg/kg body weight. Thermogenic effect: 50–100 kcal/day at 400 mg/day.

Tolerance: Develops with regular use. Cycling (taking breaks from regular caffeine use) maintains sensitivity.

Beta-Alanine — Grade B

Use: Muscular endurance in high-intensity exercise (1–4 minutes duration)

Evidence: Multiple RCTs and a 2012 meta-analysis (Amino Acids) confirm significant performance improvement (approximately 2.85%) in the specific duration range where pH buffering from carnosine is the fatigue-limiting factor. Limited benefit for very short (<60 second) or very long (>10 minute) efforts.

Side effect: Harmless tingling/flushing sensation (paraesthesia) at doses above 800mg — dose-splitting or extended-release forms manage this.

Weight Loss Supplements

Glucomannan — Grade B

Use: Appetite reduction; modest weight loss support

Evidence: The only OTC supplement ingredient with an EFSA-approved health claim for weight loss: "glucomannan contributes to weight reduction in the context of an energy-restricted diet." Meta-analysis confirms approximately 0.79 kg additional weight loss vs placebo. Modest but genuinely replicated effect.

Dose: 1g with large glass of water, 15–30 minutes before each meal. Pre-meal timing and adequate water are essential.

Caffeine and Green Tea Extract (EGCG) — Grade B (thermogenics)

Use: Increasing energy expenditure

Evidence: As above for caffeine; combined with EGCG from green tea extract, additional 78–90 kcal/day expenditure vs caffeine alone (Obesity Reviews meta-analysis). Realistic total from both: 150–200 kcal/day additional thermogenesis.

Safety concern for green tea extract: Hepatotoxicity (liver damage) documented at high doses (>800mg EGCG/day). Use at moderate doses; avoid combining with other supplements known to stress liver function.

Raspberry Ketones — Grade D

Use: Fat loss

Evidence: No human RCT evidence. Animal studies used doses not achievable through supplementation. No mechanism validated in humans at supplemental doses.

Garcinia Cambogia (Hydroxycitric Acid) — Grade D

Use: Weight loss, appetite suppression

Evidence: Multiple RCTs exist — the largest meta-analysis found approximately 0.88 kg additional weight loss vs placebo, which is within measurement error and not clinically significant. Some hepatotoxicity reports at high doses.

L-Carnitine — Grade C (athletic performance); Grade D (weight loss)

Use: Fat oxidation, weight loss

Evidence: Carnitine transports fatty acids into mitochondria. In deficient individuals (rare), supplementation improves fat oxidation. In non-deficient individuals with adequate protein intake: multiple RCTs show no significant weight loss benefit. Athletic performance evidence is mixed and modest.

Herbal and Botanical Supplements

Ashwagandha (Withania somnifera) — Grade B (stress/cortisol)

Use: Stress reduction, cortisol management, testosterone support, sleep quality

Evidence: Multiple RCTs demonstrate significant cortisol reduction and validated stress score improvement vs placebo. A 2019 RCT (Medicine): 300mg twice daily produced significant reductions in cortisol and stress scales at 8 weeks. Multiple trials confirm improvement in sleep quality.

Safety: Well-tolerated in trials; rare reports of liver injury at very high doses; check with GP if taking thyroid medications (may enhance thyroid hormone activity).

Black Cohosh — Grade B (menopausal symptoms)

Use: Hot flash reduction during menopause

Evidence: A 2012 systematic review found significant reductions in hot flash frequency and severity in most included trials. Mechanism is likely serotonergic rather than oestrogenic. Effect size: modest (20–30% reduction in hot flash frequency).

Safety concern: Rare hepatotoxicity documented; German Commission E recommends limiting use to 6-month courses with monitoring.

St John's Wort — Grade A (mild-moderate depression); caution for drug interactions

Use: Depression, anxiety

Evidence: A 2008 Cochrane review found comparable efficacy to standard antidepressants for mild-moderate depression with fewer side effects. Effect on severe depression is not established.

Critical interaction warning: St John's Wort is a potent CYP450 enzyme inducer — it significantly reduces blood levels of oral contraceptives (may cause contraceptive failure), HIV medications, anticoagulants, immunosuppressants, and several other drug classes. Must be discussed with GP before use if taking any prescription medications.

Probiotics — Grade B (condition-specific)

Use: Gut health, immune function, specific conditions

Evidence: Evidence is strain-specific and condition-specific. Lactobacillus acidophilus and Bifidobacterium strains have evidence for IBS symptom management and antibiotic-associated diarrhoea prevention. General "gut health" claims for commercial probiotics are less well-supported. The British Medical Journal (2022) noted that most commercial probiotics are not tested for clinical outcomes.

Summary Table: Evidence Grades

Conclusion

The gap between supplement marketing claims and evidence-graded clinical data is large. Grade A and B supplements address documented deficiencies (vitamin D, B12, folate, iron), provide measurable performance benefits with replicated evidence (creatine, caffeine, protein), or offer condition-specific benefits in relevant populations (omega-3 for cardiovascular risk, probiotics for IBS). The majority of marketed supplements — particularly proprietary weight loss blends — have Grade D evidence or below. Before purchasing any supplement, identifying the evidence grade for each active ingredient against its specific claimed use is the most important step in informed decision-making.

Disclaimer: This article is for informational and educational purposes only and does not constitute medical advice. Supplements can interact with prescription medications and are not appropriate for everyone. Consult a qualified healthcare professional or pharmacist before beginning any new supplement regimen, particularly if you have existing health conditions or take prescription medications.