Category: For You
Iron is the one supplement question runners should take seriously, and it is more misunderstood than any other. Here is what the evidence actually shows about iron, creatine, carbohydrate and the rest of the shelf.
Category: Demographic | Reading time: ~12 min | Level: Beginner
Open the supplement page of any running specialist and you will find a shelf built almost entirely on hope: BCAA blends promising faster recovery, ketone esters promising a metabolic edge, antioxidant megadoses promising to mop up the damage of a hard session, "recovery" powders in flavours that owe more to branding than physiology. Runners are a good market for this because they are disciplined people primed to believe the last few percent of performance is sitting in a tub. Occasionally it is. Usually it is a rounding error next to training load, sleep and how much food you actually ate that week.
That is the uncomfortable starting point for this article, and it is not the whole story. Buried inside the noise is one nutrient that deserves real attention, not because it is trendy but because endurance training itself creates a documented risk around it: iron. Distance runners, and menstruating women in particular, sit among the highest-risk groups for iron deficiency of any healthy adult population, for reasons that have little to do with how well someone eats. Foot-strike impact breaks down red blood cells, iron is lost through sweat, and inflammation from repeated hard training interferes with absorption. Yet iron is also one of the most misused supplements on the market: under-addressed in runners who actually need it, and taken speculatively, sometimes dangerously, by those who do not.
This piece is deliberately narrow. It gives iron the serious treatment it deserves as a repletion tool rather than an energy product, it is straight about where creatine, carbohydrate and sodium genuinely fit into a training plan, and it debunks the BCAAs, ketone esters and antioxidant megadoses that the evidence does not support.
For most runners, the supplement question should start and mostly end with food, training and sleep. Iron is the one nutrient worth actually checking, because iron-deficient endurance athletes, especially menstruating women and vegetarians, are common, and a review of 18 randomised trials found that correcting a documented deficiency reduced self-reported fatigue, though it did not improve measured physical capacity such as VO2max [1]. Iron should never be taken speculatively: health authorities set a firm upper daily limit, and unnecessary supplementation can irritate the gut, reduce zinc absorption, and in people with the inherited condition haemochromatosis, cause serious organ damage [2]. Creatine has reasonable evidence for sprinting, hill work and strength training but only marginal support for steady-state endurance [3]. Carbohydrate and sodium during long sessions do more for in-race performance than any capsule [4]. BCAAs, ketone esters and antioxidant megadoses are the weakest-evidenced products on the shelf [5][6].
Endurance training is not neutral with respect to nutrient status, and iron is the clearest example. Every footstrike sends a small shockwave through the feet that ruptures a proportion of circulating red blood cells, a process called foot-strike haemolysis, and runners lose further iron through sweat and, sometimes, minor gut bleeding during long efforts. Hard training also raises a hormone called hepcidin for a day or so afterward, which temporarily blocks the gut from absorbing dietary iron. None of this is dangerous in isolation, but stacked across a training block it explains why runners, and especially menstruating women who already lose iron monthly, run a genuinely elevated risk of drifting into deficiency even on a reasonable diet.
That is a different problem from most of what the supplement industry sells. It is not "more is better" for a vague sense of vitality; it is a specific, testable, correctable gap that shows up on a blood test. Understanding that distinction is the difference between using iron sensibly and using it the way most runners are marketed to, which is not sensibly at all.
The honest answer is: it helps if, and only if, you are actually deficient. A systematic review pooling 18 randomised controlled trials and over 1,100 patients found that iron supplementation in non-anaemic, iron-deficient adults produced a moderate reduction in self-reported fatigue [1]. That is a meaningful finding for someone who has been dragging through sessions and turns out to be low on iron. But the same review found no significant improvement in objective physical capacity, including VO2max, the standard laboratory measure of aerobic fitness [1]. In plain terms: iron repletion can make a deficient runner feel less exhausted, but it will not, by itself, make a healthy runner faster.
That distinction cuts against how iron is marketed. It is not an energy supplement in the way caffeine is, and it will not turn an adequately-fed runner into a better one. It corrects a specific physiological shortfall, and its benefit is proportional to how deficient you actually were.
The population-level picture, from the US National Institutes of Health's Office of Dietary Supplements, is clear about who is at genuine risk: menstruating women need substantially more iron than men (the RDA for women aged 19 to 50 is 18mg a day, against 8mg a day for men), and endurance athletes are flagged as a higher-risk group independent of sex [2]. Vegetarians and vegans face a further complication: nonheme iron from plant foods absorbs less efficiently than the heme iron in meat, fish and poultry, so plant-based eaters need roughly 1.8 times the intake to reach the same effective target [2].
None of that is licence to supplement on a hunch. The same fact sheet sets a Tolerable Upper Intake Level of 45mg a day for adults, and is explicit that supplementing iron without a documented deficiency provides no benefit while carrying real downsides: gastrointestinal irritation, reduced absorption of zinc, and in people with haemochromatosis, an inherited iron-overload condition present in roughly 1 in 200 white adults in its homozygous form, a genuine risk of organ damage [2]. Iron is also flagged by regulators as a leading cause of fatal accidental poisoning in children under six, a plain reason to keep any supplement stored well out of reach [2]. The correct sequence for any runner wondering whether iron is worth trying is a blood test first (typically ferritin, alongside a full blood count), a conversation with a prescriber about the result, and only then a supplement, at a dose matched to the actual deficit.
Creatine monohydrate has one of the largest, most consistent evidence bases of any sports supplement, and the International Society of Sports Nutrition's position stand describes it as an effective way to increase high-intensity exercise capacity, typically at a maintenance dose of around 3 to 5g a day, with an optional faster loading phase of around 20g a day for 5 to 7 days [3]. The mechanism is straightforward: creatine helps regenerate ATP, the molecule muscles burn during short, hard bursts of effort, so raising muscle creatine stores extends the capacity for repeated high-intensity contractions.
For a distance runner, the honest read is that creatine's strongest evidence sits with strength training, hill repeats and short, hard efforts, not steady-state aerobic running, where the position stand describes the evidence as limited and the benefit as marginal at best [3]. That makes it relevant to a runner who lifts weights or does structured hill sessions, just not as a direct lever on marathon pace. It is well tolerated in healthy adults, without the kidney or dehydration risks online chatter sometimes attaches to it, though anyone with kidney disease should discuss it with a clinician first.
If there is a genuine performance lever available in a packet or a bottle for runners, it sits here, not in a capsule. The joint position stand from the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine recommends around 30 to 60g of carbohydrate per hour for endurance efforts beyond roughly 60 to 90 minutes, alongside sodium and electrolyte replacement for long or hot sessions [4]. That reflects a large body of applied sports science, because running out of accessible carbohydrate, "hitting the wall," is one of the most well-characterised and preventable failure modes in distance events.
The wider point the same guidance makes is one this article keeps returning to: day-to-day energy and carbohydrate availability across a training week matters more than any single race-day product [4]. A runner who consistently under-fuels training will not out-supplement that gap. Carbohydrate and sodium during long efforts are less exciting than a branded recovery powder, but they have the evidence, and they are cheap.
This is where the evidence gets genuinely unflattering for a large part of the market. The NIH Office of Dietary Supplements' review of exercise and athletic performance supplements describes most marketed ergogenic aids, BCAAs in people already eating adequate protein and ketone ester supplements among them, as having weak or inconsistent evidence of a real benefit, and flags contamination as a live concern in a poorly regulated category [6]. If you are already hitting a sensible daily protein target from food, separate BCAAs are largely redundant, since whole protein already contains them alongside every other amino acid the body needs for repair.
Antioxidant megadosing is arguably worse than merely ineffective. A randomised trial found that high-dose vitamin C and E supplementation blunted some of the cellular adaptations that endurance training is meant to produce [5]. That is a genuinely counterintuitive finding: the free radicals generated by hard exercise are part of the signal that drives the body to adapt, and swamping that signal with high-dose antioxidants can interfere with the training effect itself. A cited null result like this is exactly the kind of honest evidence a runner deserves before spending money and possibly undermining their own training block.
Start with a blood test, not a shopping basket. If you are a distance runner, especially a menstruating woman or someone eating a largely plant-based diet, and you are dealing with persistent fatigue, ask your GP for a ferritin test alongside a full blood count before assuming iron is the answer or ruling it out. Supplement only to a confirmed gap, follow the dose your prescriber recommends, and expect less fatigue, not a jump in race times, if the deficiency was mild.
For the rest of the shelf, spend where the evidence points. If training includes hill repeats, sprint finishes or gym-based strength work, 3 to 5g of creatine monohydrate a day is a reasonable, well-tolerated addition. During any run over roughly 60 to 90 minutes, plan for 30 to 60g of carbohydrate per hour and replace sodium on long or hot efforts, through gels, drinks or real food. Skip BCAAs if total protein intake is already adequate, be sceptical of ketone ester marketing, and avoid high-dose vitamin C or E around hard training blocks, since the evidence suggests they may blunt the adaptation you are training for. Ask a prescriber, not a forum, before starting iron, and a sports dietitian before building a fuelling plan.
Iron deserves the most caution on this list. It should never be taken speculatively: supplementing without a documented deficiency gives no benefit and carries real risk, including gut irritation and reduced zinc absorption, and it is a hard contraindication for anyone with haemochromatosis, an inherited condition affecting roughly 1 in 200 white adults in its homozygous form [2]. Iron tablets are also a leading cause of fatal accidental poisoning in young children, so store them out of sight and out of reach [2]. Creatine is well tolerated in healthy adults, but anyone with kidney disease should discuss it with a clinician first, and high-dose antioxidants are worth avoiding around key training blocks given the adaptation-blunting signal in the trial evidence [5]. Sports supplements as a category carry a real contamination risk, so anyone competing under anti-doping rules should choose independently batch-tested products.
Pregnant, breastfeeding, or on medication? Check with a healthcare professional first.
None of this article is trying to sell you a stack. The honest list for runners is short: a blood test before iron, carbohydrate and sodium during long efforts, creatine if your programme includes strength or hill work, and scepticism toward everything else. Where PlantRx can help is in cutting through the marketing rather than adding to it. Our Remedy Library grades the evidence behind individual ingredients honestly, including where it is weak or mixed, so you are not left guessing which claims on a label are backed by trial data and which are copywriting.
If you want a second opinion before buying anything, Remy can talk you through the evidence grade for a specific product or ingredient in plain language, weighed against what your training and diet already provide. The most useful outcome of an article like this one is not a new purchase; it is a smaller, more honest basket.
Runners are trained to chase marginal gains, and that instinct is not wrong, it is just aimed at the wrong shelf most of the time. The single most valuable habit a distance runner can build is not a supplement stack but a standing question: is there any actual evidence this product does what the label claims, for someone in my situation, at this dose? Ask that of iron before you take it, and of everything else before you buy it, and you will spend less on more useful things.
1. Houston BL, et al. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of 18 randomised controlled trials. BMJ Open. 2018. Found reduced self-reported fatigue but no significant improvement in objective physical capacity, including VO2max. 2. NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. 2023. Basis for RDA figures, the heme/nonheme absorption gap, the Tolerable Upper Intake Level, haemochromatosis as a contraindication, and the child poisoning warning. 3. Kreider RB, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition. 2017. Basis for creatine dosing and the strength/power versus endurance distinction. 4. Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. Joint position stand of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine. Medicine & Science in Sports & Exercise. 2016. Basis for carbohydrate and electrolyte guidance during endurance efforts. 5. Paulsen G, et al. Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. The Journal of Physiology. 2014. Found high-dose antioxidant supplementation blunted some training adaptations. 6. NIH Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals. 2023. Basis for the weak or inconsistent evidence behind BCAAs, ketone supplements and most marketed ergogenic aids.
Only if testing shows you are actually deficient. Endurance runners, especially menstruating women and vegetarians, are among the highest-risk groups for low iron, and correcting a documented deficiency has been shown to reduce fatigue, though not to improve measured physical capacity such as VO2max. Taking iron without a confirmed deficiency gives no benefit and carries real risk of harm.
Heme iron comes from animal sources such as red meat and is absorbed more efficiently by the body. Nonheme iron comes from plant sources like lentils and spinach and is absorbed less readily, which is why vegetarians and vegans need roughly 1.8 times more dietary iron to meet the same target.
Creatine has strong, consistent evidence for short, high-intensity efforts such as sprint finishes, hill repeats and gym-based strength sessions that support a running programme. For pure steady-state endurance performance, the evidence is limited and the benefit, if any, is marginal.
For most runners already eating enough total protein, no. Branched-chain amino acids are already present in whole protein, and taken alone they have weak, inconsistent evidence of improving recovery beyond what adequate total protein and carbohydrate already provide.
For efforts beyond roughly 60 to 90 minutes, the sports nutrition consensus recommends around 30 to 60 grams of carbohydrate per hour, plus sodium replacement on long or hot sessions. This has more evidence behind it for in-race performance than any single supplement.
Yes. Unnecessary iron supplementation can irritate the gut and reduce zinc absorption, and in people with the inherited condition haemochromatosis it can cause serious organ damage. Iron tablets are also a leading cause of fatal accidental poisoning in young children, so they need to be stored well out of reach.
Evidence is mixed to negative. A randomised trial found that high-dose vitamin C and E supplementation actually blunted some of the cellular adaptations that endurance training is meant to produce, which is the opposite of what "recovery" marketing implies.
Get a blood test for iron status if you have symptoms of fatigue, prioritise sleep and total calorie and carbohydrate intake around training, and treat any supplement as a small top-up on those foundations rather than a substitute for them.
No. The trial evidence shows correcting a genuine iron deficiency reduces self-reported fatigue but does not improve measured physical capacity like VO2max. It is a repletion product for a documented gap, not a vitality or performance booster.
Sports nutrition guidance for efforts over roughly 60 to 90 minutes is 30 to 60 grams of carbohydrate per hour, alongside sodium replacement in long or hot conditions.
It can cause a small early increase in body water and weight, which some endurance runners weigh against the strength and power benefits it offers for hill work and finishing speed.
Not strongly. Government and sports-nutrition reviews class ketone supplements alongside BCAAs as having weak or inconsistent evidence of a genuine performance benefit for most athletes.