Caffeine
VO2max: No change in fitness, but may slightly raise a test result on the day
The best-supported legal endurance aid, but it improves performance, not your underlying VO2max
See the evidence →Supplements
Supplements and VO2max: what works.
Most supplements don't raise VO2max. A few improve endurance performance; others only help if you are deficient. Here is the evidence for each, with sources.
| Supplement | Effect on VO2max | Endurance evidence | Studied dose | |
|---|---|---|---|---|
| Caffeine Performance aids | No change in fitness, but may slightly raise a test result on the day | Strong | 3–6 mg per kg body weight, about 60 minutes before exercise | Get on iHerb ↗ |
| Beetroot (dietary nitrate) Performance aids | No increase; makes submaximal exercise cheaper in oxygen | Moderate | About 5–9 mmol nitrate (typically a concentrated beetroot shot) 2–3 hours before exercise, sometimes for several days | Get on iHerb ↗ |
| Beta-alanine Performance aids | No direct effect | Moderate | 4–6 g a day in divided doses for at least 2–4 weeks | Get on iHerb ↗ |
| Iron Health & deficiencies | Only if you are deficient | Moderate | Prescribed after a blood test; doses and schedules vary by deficiency and are set by a doctor | Get on iHerb ↗ |
| Protein Health & deficiencies | Small effect in some studies | Moderate | About 1.4–2.0 g per kg body weight a day in total (ISSN), in servings of about 0.25 g per kg or 20–40 g every 3–4 hours | Get on iHerb ↗ |
| Sodium bicarbonate Performance aids | No direct effect | Moderate | 0.2–0.3 g per kg body weight, 60–180 minutes before exercise | Get on iHerb ↗ |
| Citrulline and arginine Performance aids | No direct effect | Mixed | Most often a single 8 g dose of citrulline malate about an hour before exercise; pure citrulline up to 3 g twice a day in dosing studies | Get on iHerb ↗ |
| Cordyceps Adaptogens & trending | Mixed: one small positive study, others no change | Mixed | About 1–4 g a day for 3–12 weeks in published trials | Get on iHerb ↗ |
| Creatine Performance aids | No increase, possibly a small negative effect | Mixed | 3–5 g a day (sometimes after a 5–7 day loading phase of 4 × 5 g a day) | Get on iHerb ↗ |
| Vitamin D Health & deficiencies | Mixed results in small trials | Mixed | The trials on this page used 2,000–3,000 IU a day for 12 weeks (not personal advice) | Get on iHerb ↗ |
| Ashwagandha Adaptogens & trending | Small effect in a few small trials | Limited | Roughly 300–600 mg of root extract a day for 8–12 weeks in most trials | Get on iHerb ↗ |
| CoQ10 Health & deficiencies | No effect in healthy people | Limited | About 100–300 mg a day in most trials, for weeks rather than a single dose | Get on iHerb ↗ |
| Electrolytes Performance aids | No direct effect | Limited | Personalised from sweat rate and race length rather than a fixed dose; drinking to thirst is the starting point (not personal advice) | Get on iHerb ↗ |
| Fenugreek Testosterone boosters | No effect | Limited | Typically a few hundred mg of seed extract a day, e.g. 600 mg for 8 weeks in one key trial (not personal advice) | Get on iHerb ↗ |
| Magnesium Health & deficiencies | No good evidence | Limited | Intake from food first; trial doses vary widely, so check with a doctor before supplementing (not personal advice) | Get on iHerb ↗ |
| Omega-3 (fish oil) Health & deficiencies | No clear effect | Limited | From 2.6 g of omega-3 a day for 3 weeks to 8 g of fish oil a day for 8 weeks in athlete trials (not personal advice) | Get on iHerb ↗ |
| Probiotics Gut health | No direct effect | Limited | Strain-specific; trials typically used about 1–100 billion CFU a day for 4 weeks to several months | Get on iHerb ↗ |
| Rhodiola Adaptogens & trending | Small effect in some studies | Limited | About 200 mg of extract, or 3 mg per kg body weight, taken about an hour before exercise in key trials | Get on iHerb ↗ |
| Tart cherry Health & deficiencies | No direct effect | Limited | Montmorency tart cherry juice or powdered concentrate, from about 7 days to 1.5 hours before exercise, often continued for 2–3 days after | Get on iHerb ↗ |
| Tongkat ali Testosterone boosters | No effect | Limited | About 100–600 mg a day of standardised root extract in trials (not personal advice) | Get on iHerb ↗ |
| Boron Testosterone boosters | No effect | Insufficient | About 2.5–10 mg a day in the few human trials (not personal advice) | Get on iHerb ↗ |
| D-aspartic acid Testosterone boosters | No effect | Insufficient | 3–6 g a day in trials (not personal advice) | Get on iHerb ↗ |
| Zinc Testosterone boosters | No effect shown in trials | Insufficient | Men need about 11 mg a day; deficiency trials used about 30 mg a day (not personal advice) | Get on iHerb ↗ |
Evidence ratings are our summary of the research cited on each page. Not medical advice. iHerb links are referral links (code OEF2285).
Performance aids
Beetroot (dietary nitrate)
VO2max: No increase; makes submaximal exercise cheaper in oxygen
A small, real endurance boost for recreational athletes, but it does not raise VO2max and does little for elites
See the evidence →Beta-alanine
VO2max: No direct effect
Helps hard efforts lasting about 30 seconds to 10 minutes, but does little for VO2max or long steady cardio
See the evidence →Sodium bicarbonate
VO2max: No direct effect
A proven buffer for hard efforts of 30 seconds to 12 minutes, but the gut side effects mean you must test it in training
See the evidence →Citrulline and arginine
VO2max: No direct effect
Citrulline did not improve aerobic performance in a 2022 meta-analysis, so dietary nitrate is the better-supported nitric oxide option
See the evidence →Creatine
VO2max: No increase, possibly a small negative effect
Great for strength, sprints and surges, but it will not raise VO2max and the extra weight can cost runners
See the evidence →Electrolytes
VO2max: No direct effect
Useful for long or hot events and heavy sweaters, unnecessary for most runs under an hour, and never a reason to drink more than thirst asks for.
See the evidence →Health & deficiencies
Iron
VO2max: Only if you are deficient
Test, don't guess. Correcting iron deficiency can restore aerobic capacity, but extra iron does nothing for people with normal stores.
See the evidence →Protein
VO2max: Small effect in some studies
Getting enough protein from food or powder supports recovery and adaptation, but carbohydrate remains the main performance fuel for endurance
See the evidence →Vitamin D
VO2max: Mixed results in small trials
Worth correcting if a blood test shows you are low, but there is no reliable evidence that vitamin D raises VO2max in people with normal levels.
See the evidence →CoQ10
VO2max: No effect in healthy people
Not a performance aid for healthy people; may lower muscle damage markers, and has a separate role in heart failure under medical care
See the evidence →Magnesium
VO2max: No good evidence
Make sure your diet supplies enough magnesium, but don't expect supplements to stop muscle cramps or raise VO2max.
See the evidence →Omega-3 (fish oil)
VO2max: No clear effect
A reasonable health supplement if you eat little oily fish, but not a proven way to raise VO2max or endurance performance.
See the evidence →Tart cherry
VO2max: No direct effect
A reasonable option around races or hard training blocks for faster strength recovery, with only a small and uncertain endurance benefit
See the evidence →Adaptogens & trending
Cordyceps
VO2max: Mixed: one small positive study, others no change
Weak, inconsistent evidence; no effect in trained cyclists, so don't expect a VO2max boost
See the evidence →Ashwagandha
VO2max: Small effect in a few small trials
Promising but thin evidence; training will do far more, and safety questions are real
See the evidence →Rhodiola
VO2max: Small effect in some studies
Might make hard efforts feel slightly easier, but trials are small and quality varies widely
See the evidence →Gut health
Probiotics
VO2max: No direct effect
Worth considering for athletes prone to colds or race-day gut problems, using a strain tested in trials, but not as a performance booster
See the evidence →Testosterone boosters
Fenugreek
VO2max: No effect
At best a small rise in total testosterone with very low certainty, no effect on free testosterone or VO2max
See the evidence →Tongkat ali
VO2max: No effect
Modest testosterone rise in men with low levels, weak in healthy men, real quality concerns, no VO2max benefit
See the evidence →Boron
VO2max: No effect
Interesting hormone data from one tiny study, no proven benefit for testosterone, muscle or VO2max
See the evidence →D-aspartic acid
VO2max: No effect
Not effective in trained men, with one trial showing lower testosterone at 6 g a day; no VO2max benefit
See the evidence →Zinc
VO2max: No effect shown in trials
Fix a real deficiency, otherwise extra zinc does not raise testosterone, strength or VO2max
See the evidence →