Beetroot juice can lift endurance, but caffeine may cancel it out

Beetroot juice has become one of the most popular endurance supplements, and the rationale is physiological: it raises levels of nitric oxide, a molecule involved in blood flow, gas exchange, mitochondrial biogenesis and efficiency, and the strength of muscle contraction. Those mechanisms suggest it should help cardiorespiratory endurance, but mechanisms are not performance. Athletes also rarely take one supplement in isolation, so whether beetroot juice still works alongside other products, caffeine above all, is a live question this review set out to answer.

The authors searched five databases for publications from 2010 to 2016 on beetroot juice supplementation and cardiorespiratory endurance in athletes. After excluding reviews and meta-analyses, animal studies, papers without accessible full text, and studies that either did not supplement with beetroot juice or did not adequately assess endurance, 23 articles remained for analysis. The review examined beetroot juice alone and in combination with other supplements.

The available results suggest beetroot juice can improve cardiorespiratory endurance in athletes by increasing efficiency, which translated into better performance at various distances and longer time to exhaustion at submaximal intensities. It may also improve performance at anaerobic threshold intensities and maximum oxygen uptake. The literature contained contradictory data, and the authors hypothesise, rather than conclude, that beetroot juice could soften the endurance-sapping effects of hypoxia. For combinations with other supplements, no positive or negative effect could be confirmed, but the review flags that caffeine may undermine the benefit.

This review consolidates a fast-growing body of work behind one of the few endurance supplements with a plausible and well-described mechanism. It supports the case for beetroot juice as an ergogenic aid while being honest that the studies do not all agree and that no pooled effect size was calculated. The biggest open questions it leaves are practical: how beetroot juice interacts with caffeine and other supplements, and whether the hinted benefit under hypoxia holds up in dedicated trials.

If you train for endurance, this is a reason to take beetroot juice seriously as one of the better-evidenced supplements for efficiency and time to exhaustion, while remembering the studies here were done in athletes and did not all agree. The more surprising message is about stacking: the review raises the possibility that caffeine, the most common training supplement of all, may blunt beetroot juice's effect. Anyone combining the two is operating in territory the evidence has not yet mapped.