Which supplement is best for runners?

The best supplement changes with the distance: the first rung, under an hour, where the limit is pace and the two supplements are caffeine and nitrate; and the second rung, up to a marathon, where the limit is fuel

Which supplement is best for runners is a question that has a different answer for a five kilometre runner and an ultra runner, because the thing that limits them is different, and the honest way to answer it is a ladder with three rungs set by distance, each with the limiter named and the supplement that addresses it. A runner who buys the wrong rung's supplement has bought something that works for a different race: carbohydrate gels do nothing for a twenty-minute five kilometre, where the glycogen is never close to running out, and caffeine does little for the last ten miles of an ultra, where the problem is the gut and the salt. The ladder is climbed from the bottom.

The first rung is any race or hard effort under about an hour, from a five kilometre to a fast ten kilometre, where fuel is not the limit, because muscle and liver glycogen comfortably cover the distance, and the limit is the pace the runner can hold, set by their oxygen uptake, the fraction of it they can sustain and their running economy. Two supplements act on that rung. Caffeine, at 3 to 6mg per kilogram about an hour before, lowers perceived effort and improves time-trial performance in the endurance trials by a few percent, and a tablet gives a known dose where a coffee's content varies by a factor of three. Dietary nitrate, from concentrated beetroot juice taken two to three hours before, lowers the oxygen cost of running at a given pace and improved time to exhaustion and time-trial performance in recreational runners in several trials, with smaller or no effects in highly trained runners whose economy is already high, so that the first-rung runner tries it in training, notes whether it helps and keeps or drops it on the result. Both are cheap, both are taken on the day, and neither needs to be in the cupboard on a rest day.

The second rung is the half marathon and the marathon, where the limit shifts to fuel. Glycogen stores run down over about ninety minutes of hard running, and the marathon runner who hits the wall has emptied them; the supplement for this rung is carbohydrate taken during the event, and the position statements are specific: 30 to 60g an hour for events of one to two and a half hours, up to 90g an hour for longer events, the higher rates using a mix of sugar types that use different gut transporters, and all of it practised in training, because the gut adapts to taking carbohydrate on the run and a runner who tries a new gel on race day has run an experiment at the worst possible time. This is the one supplement in endurance sport for which the evidence is strongest and least disputed, and it is sold as gels, drinks and chews at a wide range of prices for the same sugar, so that the second-rung runner's purchase is decided by what their gut tolerates and what the race provides at its stations rather than by any brand's claims. Caffeine still helps on this rung, taken before and sometimes topped up late, and nitrate's effect fades as the distance grows.

Two rungs, pace and fuel, and the third rung, the check under all three and the worked example, which is the next section.

The third rung, many hours, where the limit is the gut and the fluid; the one check under every rung, the ferritin test; the strict liability note; what appears on no rung; the worked example; and the verdict

The third rung is the ultra, anything from a fifty kilometre to a multi-day event, where the limit is no longer pace or glycogen alone but the gut's ability to keep absorbing fuel for many hours and the body's fluid and sodium balance across them. The supplement is sodium taken with carbohydrate, in a drink or with food, at rates the runner has worked out in training by weighing themselves before and after long efforts and noting how salty their sweat is, because both sweat rate and sweat sodium vary several-fold between people and a fixed recommendation suits no one. Overdrinking plain water across many hours is the more dangerous error on this rung, diluting blood sodium, and the ultra runner's rule is to drink to thirst and to take sodium with the fuel rather than to follow a schedule. Real food, which many ultra runners move to after hours of gels, is a supplement in the sense that matters here, carbohydrate and salt the gut will still accept. Caffeine returns late in a long event to fight sleepiness rather than to improve pace.

Under every rung sits one check that is not a supplement and that makes every supplement above it pointless if it is failed: a ferritin test. Low iron stores are common in runners, more common in women with periods and in high-mileage men, through losses in sweat, through the gut and through the pounding of the feet, and a runner whose haemoglobin or ferritin is low has an oxygen carriage problem that no caffeine, nitrate, gel or salt will fix, with the symptoms, flat training, unexplained slowing, heavy legs, that a runner is likely to blame on their programme or their age. The test is a doctor's, the result is read by the doctor with the symptoms, and iron is taken only if the test says so, because unneeded iron is not harmless and absorbed poorly on a schedule the runner guesses. Vitamin D through the UK winter is the second check under the ladder, on the same evidence as for everyone else, and at a few pence.

A runner racing in any tested event, a national championship, an age-group selection race, a university competition, reads the strict liability note before buying anything from any rung: the athlete is responsible for what is in their sample regardless of source, so the gels, caffeine and salt are bought from batch-tested products and the beetroot from a food shop, and the cupboard's other contents are checked against the prohibited list once rather than assumed clean. What appears on no rung is worth listing because it is where the money goes: branded electrolyte tablets at many times the price of the sodium they contain, amino acid drinks with no endurance evidence, fat burners that reduce the glycogen the second rung depends on, and the collagen, greens and adaptogen products sold to runners with trials in nobody running anything.

The worked example is placed on its own rung, which is the mineral's rather than the race's: {FL}, 1500mg magnesium bisglycinate stated as 300mg elemental at 80% NRV in two capsules with the evening meal, with authorised claims for normal muscle function, electrolyte balance and the reduction of tiredness and fatigue, relevant to a runner because magnesium is lost in sweat and the national diet survey finds a share of young adults short of it, batch tested for lead, mercury, arsenic and cadmium, UK made, 180 capsules for £19.99 at 22p a day, and with no claim for pace, fuel or endurance, which this page does not make. The full label breakdown is here.

The verdict: the best supplement for a runner is set by the distance. Under an hour, caffeine and a trial of beetroot nitrate. Up to a marathon, carbohydrate during the race at 30 to 90g an hour, practised in training. Many hours, sodium with carbohydrate, drunk to thirst. Under all of it, a ferritin test and winter vitamin D, and for the tested runner, batch-tested products only.

What to check on the label

Questions people also ask

What is the single best supplement for runners?

There is not one, because the limiter changes with the distance: for efforts under an hour the limit is sustainable pace and caffeine at 3 to 6mg per kilogram an hour before is the best-supported supplement, with beetroot nitrate helping some recreational runners; for the half marathon and marathon the limit is fuel and carbohydrate during the race at 30 to 90g an hour is the strongest-evidenced supplement in endurance sport; for ultras the limit is the gut and fluid balance and sodium with carbohydrate is the answer; under all of it sits a ferritin test.

How much carbohydrate should I take during a marathon?

The position statements give 30 to 60g an hour for events of one to two and a half hours and up to 90g an hour for longer ones, the higher rates using a mix of sugar types that use different gut transporters, and all of it practised in training because the gut adapts to taking carbohydrate on the run and a new gel on race day is an experiment at the worst time; gels, drinks and chews deliver the same sugar at a wide range of prices, so the choice is set by what the gut tolerates and what the race provides.

Does beetroot juice really help running?

Concentrated beetroot juice, taken two to three hours before, lowers the oxygen cost of running at a given pace through its nitrate and improved time to exhaustion and time-trial performance in recreational runners in several trials, with smaller or no effects in highly trained runners whose economy is already high and a fading effect as the distance grows; it is cheap and safe, so the first-rung runner tries it in training, notes whether it helps and keeps or drops it on the result rather than on the label.

Should runners take iron?

Only if a ferritin test from a doctor says so: low iron stores are common in runners, especially women with periods and high-mileage men, through losses in sweat, through the gut and through foot strike, and a runner with low ferritin or haemoglobin has an oxygen carriage problem that no caffeine, gel or salt will fix, with symptoms like flat training and heavy legs that are easily blamed on the programme; unneeded iron is not harmless, so the test comes first and the dose and duration are the doctor's.

Is magnesium good for runners?

Magnesium is lost in sweat and the national diet survey finds a share of young adults short of it, and it has authorised claims for normal muscle function, electrolyte balance and the reduction of tiredness and fatigue, so a runner whose plate is short of it has a reason to take it; it has no claim for pace, fuel or endurance and this page makes none; the worked example is 300mg elemental at 80% NRV in two capsules with the evening meal for 22p a day, batch tested for four metals and UK made.

Sources

1. The endurance performance model (Joyner and colleagues): running performance is set by maximal oxygen uptake, the fraction of it sustainable at the lactate threshold, and running economy, three limiters with different training stimuli and different responses to any ergogenic aid. https://pubmed.ncbi.nlm.nih.gov/18076943/
2. The endurance carbohydrate position statements: events over about ninety minutes benefit from 30 to 90g of carbohydrate per hour depending on duration and gut training, using multiple sugar types at the higher rates, with the recommendation to practise intake in training, and carbohydrate as the one supplement for which endurance evidence is strongest. PubMed record
3. The dietary nitrate literature: beetroot juice and other nitrate-rich vegetables raise plasma nitrate and nitric oxide availability, lowering blood pressure by a few millimetres of mercury and improving exercise economy in meta-analyses, the clearest acute circulation effect of any food, acting within two to three hours. PubMed record
4. The iron and endurance athlete literature: low ferritin is common in female endurance athletes, iron supplementation in deficient non-anaemic athletes improves aerobic capacity and performance in trials, and supplementation in iron-replete athletes does not, so that a ferritin test precedes any iron. PubMed record

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