Do supplements raise VO2max? Caffeine, beetroot and iron

The supplement aisle is full of products promising more "oxygen", "endurance" or "VO2max". The honest answer is that almost no legal supplement raises VO2max in a healthy, well-fed person. A few do improve performance — which is not the same thing — and one nutrient, iron, can restore VO2max if you are deficient.

Performance is not VO2max

VO2max is the ceiling of your oxygen delivery and use, set mostly by the heart and blood (see what limits VO2max). Endurance performance also depends on how much of that ceiling you can sustain, your running or cycling economy, and how hard the effort feels. A supplement can help any of those without touching VO2max at all. The things that genuinely raise oxygen-carrying capacity quickly — EPO and blood transfusions — are banned for good reason, and dangerous.

Caffeine: the best-supported aid

The International Society of Sports Nutrition position stand (Guest et al., 2021) concludes that caffeine consistently improves performance in many types of exercise, with endurance benefits among the most reliable.

• Dose: about 3–6 mg per kg of body weight, taken roughly 60 minutes before exercise. Lower doses can also work, and higher doses add side effects without more benefit.
• How it works: mainly through the brain — it blocks adenosine receptors, lowering perceived effort and pain.
• VO2max: largely unchanged. You go further or faster at the same ceiling.
• Downsides: jitters, stomach upset, a raised heart rate and poor sleep, especially after afternoon or evening use. Individual responses vary, partly due to genetics and habitual intake.

Dietary nitrate (beetroot juice)

Nitrate from beetroot and leafy greens is converted in the body to nitric oxide, which affects blood flow and muscle efficiency. Studies by Larsen, Bailey and others found that nitrate lowers the oxygen cost of submaximal exercise, a surprising result at the time. Andrew Jones's 2014 review summarised the field:

• Benefits are clearest in time-to-exhaustion tests and in recreational athletes; effects in highly trained athletes are smaller and inconsistent.
• Typical protocols use a concentrated beetroot juice shot taken about 2–3 hours before exercise, sometimes for several days beforehand.
• Again, VO2max is generally not raised — exercise just becomes slightly more economical.
• Antibacterial mouthwash can blunt the effect, because mouth bacteria help convert nitrate. Beetroot also turns urine pink, which is harmless.

Iron: fix a deficiency, don't chase a boost

Iron is needed to make haemoglobin, which carries oxygen in the blood. Iron-deficiency anaemia lowers VO2max and performance, and even low iron stores without anaemia can cause fatigue. Endurance athletes, women with heavy periods, vegetarians and frequent blood donors are at higher risk.

However, a meta-analysis by Burden and colleagues found that iron treatment in iron-deficient but non-anaemic athletes improved iron stores and, on average, aerobic capacity (VO2max) — a moderate benefit, but only because those athletes started out low. In people with normal iron, extra iron does not help — and it can cause constipation, nausea and, rarely, iron overload.

Test first: ask for a blood test including ferritin, haemoglobin and transferrin saturation before taking iron, and supplement only under medical guidance.

The evidence at a glance

Supplement	Performance effect	Raises VO2max?	Evidence
Caffeine	Clear benefit	No	Strong
Dietary nitrate	Small to moderate, mainly non-elite	No	Moderate
Iron (if deficient)	Restores lost capacity	Yes, back to normal	Strong
Iron (if normal)	None	No	Strong
"Oxygen" drinks, most blends	None shown	No	Weak or absent

The IOC consensus statement on supplements lists caffeine and nitrate among the few performance aids with good evidence, and warns about contamination: some supplements contain banned substances. Competitive athletes should use batch-tested products.

Where the real gains are

Training changes VO2max far more than anything in a capsule. A weekly 4×4 interval session, mostly easy volume as in 80/20 training, and enough food and sleep will do more than any stack — see how to improve VO2max. Losing excess fat also raises relative VO2max (see VO2max and body weight). For racing, caffeine on race day is a sensible extra once the basics are in place. Track progress with the VO2max calculator and browse more studies on the research page.

Safety: high-dose caffeine can be dangerous with heart rhythm problems, high blood pressure or pregnancy; check with a doctor before using supplements if you have a medical condition or take medication.

Sources

• Guest NS, et al. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr, 2021.
• Jones AM. Dietary nitrate supplementation and exercise performance. Sports Med, 2014.
• Larsen FJ, et al. Effects of dietary nitrate on oxygen cost during exercise. Acta Physiol, 2007.
• Bailey SJ, et al. Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans. J Appl Physiol, 2009.
• Burden RJ, et al. Is iron treatment beneficial in iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. Br J Sports Med, 2015.
• Maughan RJ, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med, 2018.