SYNTH-97 · Guide

Iron

Tiredness alone is no reason to take it: test first (ferritin, blood count, transferrin saturation, CRP), rested and not in the days after a race or during an illness; read ferritin as a range with a clinician, treat only a measured deficiency, expect tiredness to ease more surely than speed; the cause, dose, schedule and any infusion are the clinician's

Main caveat: Transfer. The A-level sources are in adults with anaemia, non-athlete women, women with fatigue whose athletic status was not characterised (the fatigue trials…

limited evidenceMostly from recreational to elite athletes

The question it answers

An athlete is tired, flat or stalled and asks about iron — or brings a ferritin result, a prescription, a plan to buy tablets, an infusion offer or an altitude camp. Is iron a likely cause, what should be tested and when, what does the number mean, does treating it help, how is it taken if a clinician agrees, and when is it a doctor's question?

Short answer: tiredness is not a diagnosis; test before any iron, rested and away from races and illness; read ferritin as a range, with CRP, haemoglobin, transferrin saturation and the athlete's own earlier values, with a clinician; treat only a measured deficiency, on the clinician's prescription; expect stores to rise slowly and tiredness perhaps to ease, not a faster race.

The assistant never diagnoses, never names a cause, never sets a dose, never suggests an infusion and never reads one ferritin as a verdict.

What this guide owns and what it leaves

Owns (decided here)	Leaves to
When to suspect iron, what to test and when, how to read the result, what treating it does and does not do	Altitude camps — the pre-camp target, the camp dose, the re-test during the camp and whether the camp goes ahead → SYNTH-06
The no-test-no-iron rule for every athlete, and what dose, schedule, timing and vitamin C do if a clinician prescribes iron	Supplements in general and the food-first order → SYNTH-67
When iron is a doctor's question, and what the assistant must never do	Heavy periods and the cycle → SYNTH-37; the REDs screen and weight loss → SYNTH-54; the teenage athlete → SYNTH-81; plant-based eating → SYNTH-69
What the base shows for each special group, and where it is thin	A stall or persistent tiredness as a load question → SYNTH-29; illness and the return → SYNTH-36; low readiness → SYNTH-30

Default — the decision, in order (what to prescribe first)

Not from a study; the base's own practice for the order of steps, the list of reasons to suspect iron, the test conditions and the special-group calls; the sources say what each step rests on:

Tiredness is not a diagnosis, and iron is never taken on a hunch. Ask first about load, sleep, fuelling, illness and life stress: overtraining is a diagnosis of exclusion with iron deficiency among the things to exclude (REF-206, A), and no single marker diagnoses fatigue (PHYS-44, established).

Suspect iron when tiredness or a stall outlasts a lighter week and those causes, or when a risk is present: heavy periods (reported by 36% of London Marathon runners and 37% of elite athletes, REF-573, B), a female endurance athlete (deficiency in about 15-35% of female and 5-11% of male athlete cohorts, REF-340, B+), low energy availability (REF-341, A), a planned altitude camp (SYNTH-06), a plant-based diet, a past deficiency, blood donation, a growth spurt or gut symptoms (practice).

Test before any iron. Ferritin with a full blood count (haemoglobin, MCV, MCH) and CRP (REF-1410, A-), plus transferrin saturation (the base's practice from the start; the consensus adds it in unclear cases, the BSG when a falsely normal ferritin is suspected, REF-1411); ferritin is the single most useful marker (REF-1411, A).

Why test rather than take iron "to be safe": among 170 amateur marathoners, once raised CRP was excluded, 15.0% of the men had ferritin above 200 µg/L and 1.6% were depleted, against 28.0% of the women depleted (REF-1419, B+, abstract); oral iron showed no benefit when ferritin was normal (REF-1409, A-); and long-term iron with normal or high ferritin is "not recommended and may be harmful" (REF-1410).

Test rested, away from races and illness. A morning sample after an easy or rest day, repeated under the same conditions and in the same laboratory (practice).

Not in the days after a marathon or longer race: ferritin rose with CRP after a marathon and after a 130-km ultra and was still raised at a follow-up within 5 days (REF-1414, B-, abstract, 15 runners, 14 of them men); in the studies the consensus cites, ferritin after exercise stayed normal or rose 27% and settled within a day, and after ultramarathons it doubled and took 6 days to return (REF-1410).

So the caution is sized by the session: about a week after a long race is the base's practice, and so is an easy day before an ordinary test; the consensus names training sessions among the acute-phase reactions to exclude, and the size of the effect after ordinary training is not measured in the base.

Not during an illness or until it has cleared, since infection raises ferritin too (REF-1410; the interval is practice).

Read it as a range, with a clinician (table below). No ferritin cut-off is a law; the lines differ because they answer different questions (PHYS-36, contested — ferritin cut-offs).

A raised CRP means ferritin can look normal while stores are low: with inflammation a value up to 100 µg/L may still reflect deficiency (a line from its inflammatory bowel disease section), and above 150 absolute deficiency is unlikely (REF-1411).

A low haemoglobin with normal ferritin, MCV and MCH in a high-volume athlete may be plasma dilution, seen in about 10-15% of mainly endurance athletes (REF-1410) — the clinician's reading, never the assistant's.

A measured deficiency is the clinician's to explain and treat. With iron deficiency, investigation is considered at any level of anaemia, and the threshold to investigate low iron without anaemia is low in men and postmenopausal women: about a third of men and postmenopausal women with iron-deficiency anaemia have an underlying lesion, most often in the gut, and coeliac disease is found in 3-5% (REF-1411, A; transfer from adults with anaemia).

Oral iron is the first line; the dose and schedule are the prescriber's, and what the options do is in the supplementation table.

Re-test after 6-8 weeks of treatment (REF-1410): courses of 4 weeks or less did not raise ferritin in the trials, while 6-8 weeks did (REF-1409, A-). Stores can slip back once a course ends: 3 months after a 3-month daily course 11.4% were iron-deficient, against 5.5% at its end (REF-1416, A, non-athlete women).

Expect tiredness to ease more surely than speed. In iron-deficient adults without anaemia, iron eased self-reported fatigue (SMD −0.38, moderate certainty, women largely from clinics) but not VO2max (SMD 0.11) or timed performance (REF-1412, A); in oral-iron trials in active people VO2max showed a non-significant trend (SMD 0.49) and ferritin rose clearly only from very low starts (REF-1409, A-); a VO2max gain was pooled in iron-deficient endurance athletes (REF-1413, A-, abstract).

A faster race is possible when stores are very low, not shown, and never promised.

Not deficient → no iron, and look elsewhere: fuelling (SYNTH-54), illness (SYNTH-36), load (SYNTH-29), readiness (SYNTH-30).

When and how to test

Marker or step	What it tells	What fools it	Source (level)
Ferritin	Iron stores; the single most useful marker; under 15 µg/L is a very specific sign of absent stores	Raised by inflammation, infection and long races for days (Default 3); fell by about a third over an altitude camp in athletes who took no iron (REF-969, B, observational; no non-altitude block in the base)	REF-1411 A; REF-1410 A-; REF-1414 B-
CRP	Whether today's ferritin can be read: an acute-phase reaction from training or infection is excluded by the history and CRP (normal under 3 mg/L)	—	REF-1410 A-
Haemoglobin, MCV, MCH (full blood count)	Anaemia, and whether red cells are iron-limited	Plasma volume: dilution in about 10-15% of mainly endurance athletes, especially above about 10 h of training a week; haemoglobin fell from about 15.4 to 14.7 g/dL across a season in elite cyclists (REF-095, B) and fell with 14 days of low energy availability in 12 men (REF-545, B); in a trained athlete the total haemoglobin mass, not the concentration, is the quantity to track (REF-464, B+, the entry's inference)	REF-1410; REF-1411
Transferrin saturation	Iron supply to the marrow; 20% or below supports deficiency, and it helps when a falsely normal ferritin is suspected	Falls with inflammation (10-20% without deficiency); rose after a marathon and fell after an ultra	REF-1410; REF-1411; REF-1414
When	A morning sample after an easy or rest day; about a week after a marathon or longer race; not during or soon after an illness; repeats at the same time of day and laboratory	The measured window is "within 5 days" in 15 runners after races; the week, the easy day and the same-conditions rule are practice	Practice built on REF-1414 and REF-1410
How often	6-8 weeks after starting treatment; then about twice a year when stores keep running low (the consensus says twice a year in one place and two to three times a year for high-level endurance athletes in another)	A re-test before 6 weeks of treatment may show no change yet	REF-1410; REF-1409

Reading ferritin — the lines and what each is for

Ferritin in µg/L, the same as ng/mL. Show the athlete the range and what each line is for, never one line as a verdict; the reading is the clinician's.

Line	Who uses it	What it is for	Source
Under 15	WHO (as the consensus cites it), the BSG guideline (specificity 0.99) and Swiss sports medicine	Absent or empty stores: WHO's line, cited without its purpose, and a very specific clinical sign	REF-1410; REF-1411
16-32	Bone-marrow staining studies, as the consensus cites them	Absent marrow iron on the gold-standard staining, a line set on red-cell production alone; the consensus says it may sit slightly higher if fatigue is counted	REF-1410
30	The BSG (under 30 "generally indicative of low body iron stores"; laboratories' lower limits sit between 15 and 30) and Swiss sports medicine (the cut-off for adults and from age 15; 20 at 12-15 years, 15 at 6-12)	A clinical cut-off for low stores, chosen by the authors ("seems to be most plausible")	REF-1411; REF-1410
35 and 20	Sport-science staging: under 35 depleted stores, under 20 limited supply to the marrow	Research staging, not validated against outcomes	REF-1409 (cites it)
40 and 50	Trial entry lines: under 40 in a review of female athletes, under 50 in runner trials, 15 to 50 across the trials pooled for fatigue and capacity	Who was studied, not a diagnosis	REF-1420; REF-1417; REF-1418; REF-1412
45	Gastroenterology: the BSG (the American Gastroenterological Association's guideline uses the same line; it is not an entry in the base)	A trade-off (specificity 0.92) below which the guideline considers looking for gut disease in anaemia	REF-1411
50 before altitude	Swiss sports medicine, for adult elite athletes only; the base's own practice is 40-50 (SYNTH-06)	A pre-altitude aim, expert-set	REF-1410
Up to 100 with inflammation; above 150	The BSG	With inflammation a value up to 100 may still reflect deficiency (its inflammatory bowel disease section); above 150 absolute deficiency is unlikely even with inflammation	REF-1411
Above 200	One study's overload definition in amateur runners, raised CRP excluded	Not a diagnosis; a high ferritin is a clinician's work-up	REF-1419
Haemoglobin	Anaemia below 130 g/L in men and 120 in non-pregnant women (the BSG, aligned with WHO); Swiss sports medicine puts men's normal above 140 and women's above 120	The laboratory's range and the clinician's reading; dilution in high-volume athletes	REF-1411; REF-1410

Why they differ: absent stores (15, WHO's line as the consensus cites it), the range from bone-marrow staining studies (16-32), a clinical cut-off for low stores (30), research staging (35 and 20), who a trial enrolled (40, 50), when to look for gut disease (45), a pre-altitude aim (50) and how far inflammation can hide a deficiency (100-150). The athlete's answer is a range, read with CRP, transferrin saturation, haemoglobin and their own earlier values, by a clinician.

Does treating it help — what to expect

Stores rise slowly, and most from the lowest start. In 13 small RCTs (449 participants, 432 of them women aged about 18-28) oral iron raised ferritin (SMD 1.27, 95% CI 0.44 to 2.10, moderate certainty) — about 10 µg/L over up to 8 weeks against 1.5 in controls (pooled means, the entry's arithmetic) — clearly only in studies whose mean starting ferritin was 12 µg/L or lower; haemoglobin and transferrin saturation did not change significantly, and ferritin was still below 35 µg/L after supplementation in 10 of 13 studies (REF-1409, A-).In 31 runners with ferritin under 50 µg/L it rose 19.7 µg/L over 8 weeks on either schedule, with men's ferritin running 21.2 µg/L higher than women's (REF-1417, B-, abstract).

Tiredness may ease. Fatigue SMD −0.38 (95% CI −0.52 to −0.23; 4 trials, 714 women; I² 0%; moderate certainty), in women with fatigue (two of the four trials recruited from clinics) whose athletic status was not characterised; in the one trial with a subgroup, the benefit was in those with ferritin ≤15 ng/mL or transferrin saturation ≤20% (REF-1412, A). A national guideline calls treating deficiency without anaemia "reasonable" if symptomatic (REF-1411).

Faster: possible, not shown. VO2max SMD 0.11 (−0.15 to 0.37; 9 trials, 235 participants) and no gain in 15-km time trials or time to exhaustion, low certainty (REF-1412, A); VO2max SMD 0.49 (−0.09 to 1.07, p = 0.086, low certainty) in oral-iron RCTs (REF-1409, A-); Hedges' g 0.61 (0.40 to 0.82) across 17 studies of iron-deficient non-anaemic endurance athletes, a pool that mixed oral iron with intramuscular injections (REF-1413, A-, abstract).In high-level female athletes with ferritin under 40 µg/L, deficient rowers were 3-4% slower in two cross-sectional studies, and iron's effect on VO2max (−3% to +15%) did not differ from placebo, the 6-15% gains being within-group changes in three studies (significant in one, none different from placebo) that the review places at ferritin criteria under 16 µg/L, one listed at under 25 in its table (REF-1420, B+).

An infusion refills stores fast but did not raise capacity. In 26 recreationally active non-anaemic iron-deficient women, IV iron raised ferritin, serum iron and transferrin saturation with a highly variable response, left VO2peak, lactate threshold and haemoglobin mass no different from placebo at 4 weeks, improved running economy against placebo, and improved fatigue scores within the iron group, with no between-group test reported (REF-1422, B+, abstract; two authors report ties to IV-iron makers).

Our reading: benefit concentrates where stores are emptiest (study means ≤12 µg/L in REF-1409; ≤15 in the fatigue subgroup of REF-1412; within-group VO2max gains at criteria under 16, significant in one of three studies and none beating placebo, in REF-1420); for an athlete whose ferritin is low-normal, iron is unlikely to change a race, and the inference is the base's, not a tested threshold.

Supplementation — what the options do (the prescription stays the clinician's)

Only when a clinician has confirmed a deficiency and agrees to treat it: correcting a measured deficiency is a different purpose from a performance supplement (REF-358, A). The assistant reports what the studies show and never sets a dose, a schedule or a product.

Question	What the evidence says	Source (level)	Label
Without a test?	No. Among amateur marathoners deficiency was rare in men (1.6%) while 15.0% had ferritin above 200 µg/L; no benefit with normal ferritin; long-term iron with normal or high ferritin "may be harmful"	REF-1419 B+; REF-1409 A-; REF-1410 A-	Evidence and consensus
How much	The usual treatment dose is about 40-100 mg of elemental iron once a day: 40-60 mg in the Swiss consensus; 50-100 mg once daily, fasting (for example one ferrous sulphate 200 mg tablet), in the BSG guideline; 16-100 mg a day in the trials that raised ferritin, with the dose not moderating the effect. Doses of 60 mg or more raise hepcidin for 24 h and cut the next day's absorption by 35-45%, and six times the dose (40 to 240 mg) gave about three times the iron absorbed (6.7 to 18.1 mg). 100-200 mg a day is the UCI's figure for treating deficiency or supporting altitude blocks, and athletes at altitude camps took 105 or 210 mg a day	REF-1410; REF-1411 A; REF-1409; REF-1415 B+ (absorption, non-athlete women); REF-748 A; REF-969 B	The prescriber's
Daily or every other day	At the same total dose, alternate days gave the same ferritin (median 43.8 vs 44.8 µg/L) with gut side effects on dosing days 1.56 times as frequent on daily iron, but took twice as long: ferritin was higher on daily iron at 46 and 93 days. In runners both schedules raised ferritin equally over the same 8 weeks, the alternate-day group on half the iron by REF-1416's account of the trial, with 6 severe gut complaints on daily iron against 1 on alternate days. Per dose, absorption is higher on alternate days (21.8% vs 16.3%). The guideline's step when daily iron is not tolerated	REF-1416 A (non-athlete women); REF-1417 B- (abstract); REF-970 B+; REF-1411	A tolerability tool, not a faster route
Twice a day?	No gain: a third dose within 24 h added nothing significant, and 120 mg split in two was absorbed no better than once	REF-1415; REF-970	Evidence (absorption)
Time of day, around training	Iron in breakfast 30 min after a 90-min morning run was absorbed better than at rest and than at dinner, although hepcidin rose 3 h after the run; iron in a meal 2 h after a ~99-min run was absorbed 36% less than at rest (11.8% vs 18.5%), with hepcidin 51% higher; hepcidin rises 1.5-2.5-fold after exercise, peaking about 3-6 h later	REF-1418 B+; REF-1421 B; PHYS-36 well supported	Two single-meal studies of meal iron, not tablets
Which day?	On an alternate-day schedule, the morning of an easy day or soon after a morning session, not in the hours after a long or hard one	—	Practice built on the row above
Food, tea, coffee, calcium	Fasting or between meals (taken with meals, iron salts can lose up to 75% of their bioavailability) and tea kept away, its effect fading within about 60 min (the BSG); coffee and calcium kept away too (the Swiss consensus; the BSG does not name them)	REF-1411 A; REF-1410 A-	Guideline and consensus
Vitamin C	Not needed with a tablet: the BSG does not recommend it, a large trial having found no gain, while the Swiss consensus's therapy line says to consider enhancers such as vitamin C (expert opinion); vitamin C-rich juice with an iron-containing meal helps absorption	REF-1411 A; REF-1410 A-	The sources disagree; the base follows the BSG because a trial backs it; folded into SYNTH-06 and REF-340
Gut side effects	Ferrous sulphate about doubled the odds of gut side effects against placebo (OR 2.32, 95% CI 1.74 to 3.08), with no dose-effect over 50-400 mg a day; fewer on alternate days	REF-1411; REF-1416	Evidence
How long	Judge after 6-8 weeks, not sooner; for anaemia, continue about 3 months after haemoglobin is normal	REF-1409; REF-1410; REF-1411	Evidence and guideline
Food first?	Meat, poultry or fish and vitamin C-rich food with plant meals help; iron-deficiency anaemia "cannot be corrected by nutrition alone"; sweat losses are minor with a normal diet	REF-1410; REF-1353 B+	Consensus; no food trial in the base
An infusion (IV iron)	A physician's decision: oral failure or intolerance, a need for rapid restoration, a concomitant disease or low stores before an altitude camp; only where resuscitation skills are available, with 30 min of observation, and under anti-doping rules (the 2015 wording is quoted; the current rule was not checked); hypophosphataemia in 58% after ferric carboxymaltose; in active women it did not raise VO2peak	REF-1410; REF-1411; REF-1422 B+	Doctor only; the assistant never suggests it

Special groups

Group	What the base shows	What to do	How thin
Women with periods, above all heavy ones	Heavy menstrual bleeding in 36% of London Marathon runners and 37% of elite athletes, and only 22% had sought advice (REF-573, B, survey); 28.0% of female amateur marathoners depleted (REF-1419, B+); nearly all iron trials are in young women (REF-1409, REF-1412, REF-1416); a combined pill cut menstrual loss by about 44% in 20 non-athletes (REF-577, B; a physician's conversation); in premenopausal women with deficiency but no anaemia gut investigation is generally not needed, while anaemia out of proportion to periods is investigated (REF-1411)	Ask about heavy periods at intake; heavy bleeding → a doctor (SYNTH-37); test, never self-treat	The base's best-supplied group
Vegetarians and vegans	No study of vegetarian or vegan athletes' iron in the base (SYNTH-69); the consensus advises citrus juice instead of tea or coffee with an iron-containing meal and recommends that vegetarian athletes prevent deficiency with intermittent iron (40-60 mg three times a week, or at least 14-28 mg a day, in place of meat) without making a test the condition — expert opinion the base does not follow, since only a test tells deficiency from excess (REF-1410; REF-1419); 32-44% of the women in one schedule trial were vegetarian (REF-1416, not a vegetarian comparison)	The same rule: test when tired or at risk, food first, iron only on a prescription	Thin: consensus only
Altitude camp	Haemoglobin mass rose 1.1%, 3.3% and 4.0% with 0, 105 and 210 mg of iron a day, and ferritin fell 33.2% without iron (REF-969, B, observational); a pre-altitude aim of 50 µg/L for adult elite athletes, with low stores before a camp among a physician's reasons to consider IV iron (REF-1410); the UCI's 100-200 mg/day (REF-748, A)	Test well before the camp and decide with a physician; SYNTH-06 owns the target, the timing and whether the camp waits, and there the sources disagree with the house practice	Thin in amateurs; no randomised dose in the base
Teenagers	Ferritin cut-offs of 15 µg/L at 6-12 years, 20 at 12-15 and 30 from 15 (REF-1410, expert-set); iron is among the nutrients to watch, with no numbers (REF-1067, A); no adolescent prevalence figure in the base	A parent and a clinician; no self-prescribed iron (SYNTH-81)	Thin
Men, and masters men and postmenopausal women	About a third of men and postmenopausal women with iron-deficiency anaemia have an underlying lesion, and the threshold to investigate even non-anaemic deficiency is low (REF-1411, A, not athletes); 15.0% of male amateur marathoners had ferritin above 200 µg/L (REF-1419)	Low iron is a doctor's question, never "just training"; never iron "to be safe"	No iron study in masters athletes in the base
Under-fuelled or losing weight	Low iron status, raised hepcidin, lower iron absorption and lower haemoglobin are among the consequences of low energy availability, and poor or declining iron studies are a potential indicator (REF-341, A; PHYS-32, established for the hormonal effects); 14 days at about 22 kcal/kg fat-free mass lowered haemoglobin in 12 trained men (REF-545, B)	Screen by SYNTH-54; fuel first; test, never self-prescribe	The consensus and one small study
High-volume endurance athletes	Dilution lowers haemoglobin in about 10-15% of mainly endurance athletes; the consensus names gut micro-ischaemia as the main exercise-related loss, hepcidin bursts as a block on absorption and foot-strike haemolysis as real but costing no iron (REF-1410); sweat losses are minor with a normal diet (REF-1353, B+)	A low haemoglobin goes to the clinician to read; never call it "sports anaemia"	No runner blood-loss study in the base

Red flags — when it is a doctor's question

Practice unless a source is given; each means a clinician, never a diagnosis:

Anaemia on any test, above all in a man or a postmenopausal woman: with iron deficiency, investigation is considered at any level of anaemia (REF-1411).

Low iron in a man, a postmenopausal woman, or anyone with gut symptoms or a family history of gut disease, even without anaemia (REF-1411: a low threshold for investigation; coeliac disease in 3-5% of iron-deficiency anaemia).

Black or bloody stools, persistent abdominal pain, diarrhoea or unexplained weight loss (practice).

Heavy periods, or anaemia out of proportion to periods (REF-573; REF-1411; SYNTH-37).

No rise when re-tested after 6-8 weeks of prescribed iron, or a ferritin that keeps falling (practice built on REF-1410).

A very high ferritin (one study's line is above 200 µg/L, REF-1419), above all with joint pain or a family history of iron overload (practice).

Breathlessness out of proportion, chest pain, palpitations, dizziness or fainting → a doctor now; it is not an iron question (SYNTH-36).

Pregnancy or trying to conceive → the obstetric team (SYNTH-95; practice: the base holds no iron entry for pregnancy).

Any wish for an infusion (REF-1410; REF-1411).

A teenager who is persistently tired → a parent and a clinician (SYNTH-81).

Key Findings — what the evidence shows

Deficiency and excess both occur in amateurs, unevenly by sex: 1.6% of male and 28.0% of female amateur marathoners were depleted, and 15.0% of the men had ferritin above 200 µg/L (REF-1419, B+, abstract); deficiency is reported in about 15-35% of female and 5-11% of male athlete cohorts (REF-340, B+).

Ferritin is the most useful single marker, and inflammation fools it: it rose after a marathon and after a 130-km ultra and remained high at a follow-up within 5 days (102.7 after the ultra vs 74.6 after the marathon, units not given in the abstract), with CRP higher after the ultra (REF-1414, B-, abstract); the consensus reads it with CRP (REF-1410, A-), the BSG allows deficiency up to 100 µg/L with inflammation in inflammatory bowel disease (REF-1411, A).

Cut-offs are expert-set and serve different purposes (the table above; PHYS-36, contested — ferritin cut-offs).

Oral iron refills low stores slowly, most from the lowest starts, and needs 6-8 weeks (REF-1409, A-; REF-1410).

Fatigue may ease; capacity is not shown to rise (REF-1412, A; REF-1409, A-), against a pooled VO2max gain in an older meta-analysis that mixed routes (REF-1413, A-, abstract); within-group gains in female athletes without a placebo difference (REF-1420, B+); IV iron with VO2peak no different from placebo (REF-1422, B+).

Dose and schedule: about 40-100 mg once a day is the usual treatment (REF-1410, REF-1411); more milligrams are absorbed less efficiently (REF-1415, B+); alternate days give the same ferritin at the same total dose with fewer gut effects, more slowly (REF-1416, A); in runners the gain was equal over the same 8 weeks with fewer severe gut complaints, on what REF-1416 describes as half the iron (REF-1417, B-).

Timing around training rests on two single-meal studies: better 30 min after a morning run (REF-1418, B+), worse 2 h after a long one (REF-1421, B).

When it is a doctor's question: anaemia, and low iron in men and postmenopausal women, because a third of them with anaemia have a lesion (REF-1411, A).

It does not show: a ferritin cut-off validated against performance or symptoms in athletes; how long to rest before a blood test beyond one small study of races; that timing a tablet around training changes ferritin over weeks; anything on iron in masters athletes, in vegetarian or vegan athletes or on its prevalence in adolescents; IV iron in anaemic athletes; or which tired athlete is deficient before the test.

Conflicts and how they are resolved

Does treating deficiency without anaemia make athletes faster? REF-1413 pooled a VO2max gain (g 0.61, abstract, oral and intramuscular iron together); REF-1409 found a non-significant trend in oral-iron RCTs (SMD 0.49) and REF-1412 none (SMD 0.11) while fatigue eased; REF-1420 reports within-group gains without a placebo difference; REF-1422 found VO2peak no different from placebo after IV iron.Resolved: tiredness may ease (moderate certainty, women with fatigue of uncharacterised athletic status); a performance gain is possible when stores are very low and is never promised. PHYS-36 already carries the question as contested.

Which ferritin line? Fifteen, 30, 35/20, 45, 50, 100-150 and 200 come from different sources for different purposes. Resolved: the table, each line with its purpose; the clinician reads it.

Vitamin C with the tablet. The house practice in SYNTH-06 and REF-340 said to take iron "with vitamin C"; the BSG does not recommend it with a tablet (REF-1411), the Swiss consensus advises citrus juice instead of tea or coffee with iron-containing meals and, in its therapy recommendations after the oral dose, says to consider enhancers such as vitamin C (REF-1410), and REF-1416 gave citrus juice to both arms as a design choice.Resolved, against the consensus's line: not needed with a tablet, because the BSG's position rests on a large trial with no gain; vitamin C-rich food or juice with an iron-containing meal helps absorption; folded into SYNTH-06 and REF-340.

The treatment dose. SYNTH-67 quoted 100-200 mg/day "when treating deficiency" (REF-748); the usual treatment dose in REF-1410, REF-1411 and the trials of REF-1409 is about 40-100 mg, and REF-1415 shows why more is less efficient per milligram.Resolved: both stated with their sources — about 40-100 mg the usual treatment, 100-200 mg/day the UCI's figure for treating deficiency and supporting altitude blocks; the dose is the prescriber's; folded into SYNTH-67.

What alternate days buy. SYNTH-06 said alternate-day iron was "absorbed better" (REF-970, absorption only). The outcome trials: the same ferritin at the same total dose, fewer gut effects, twice the time (REF-1416); equal gains over the same 8 weeks with fewer severe complaints in runners, the alternate-day group on half the iron by REF-1416's account (REF-1417). Resolved: a tolerability tool, not a faster route; folded into SYNTH-06.

Iron soon after training, or away from it? Absorption was best 30 min after a moderate morning run (REF-1418) and 36% lower 2 h after a long run (REF-1421); the studies differ in feeding time, run, iron form and design, so they do not contradict each other. Resolved: morning, soon after a morning session or well away from a long one (practice built on both); SYNTH-06's inference "not in the hours after a hard session" fits REF-1421.

Preparing iron for altitude. SYNTH-06's house practice is a panel 8-12 weeks before, ferritin 40-50 and "the camp waits" when it is low; the Swiss consensus aims for 50 µg/L in adult elite athletes and lists low stores before a camp among a physician's reasons to consider IV iron (REF-1410); a re-test about 4 weeks into the camp falls inside the window in which oral-iron trials saw no rise (REF-1409).Resolved: not settled here; SYNTH-06 now states the disagreement, keeps its practice labelled, and leaves the decision to the physician.

How long after training before a test. The measured rise is after races (REF-1414); after exercise the consensus cites a normal value or a 27% rise settling within a day, and after ultras a doubling for 6 days (REF-1410). Resolved: avoid about a week after a marathon or longer race and any illness, and take an easy day before an ordinary test (practice, sized by session).

The anaemia line for men. 130 g/L in the BSG, 140 in the Swiss consensus. Resolved: the laboratory's range and the clinician's reading; the assistant quotes neither as a verdict.

The re-test interval. 6-8 weeks after starting treatment (REF-1410; REF-1409) against SYNTH-06's re-test about 4 weeks into a camp (practice). Resolved: a course is judged at 6-8 weeks; the camp re-test stays SYNTH-06's practice, now flagged there as possibly too early to show a change.

Practical Application — how the assistant says it

"I'm always tired — could it be iron?" It could be, and so could load, sleep, fuelling or an illness that has not quite gone; we look at those first.If the tiredness persists, or you have heavy periods or a big endurance load, ask your doctor for ferritin, a blood count, transferrin saturation and CRP, taken in the morning after an easy day and not in the week after a race. Don't start iron before the test: among amateur marathoners deficiency was rare in the men while one in six had high ferritin (REF-1419, B+).

"My ferritin is 28 — is that low?" Sources draw different lines: under 15 means empty stores, under 30 low stores for the British gut specialists and the Swiss sports doctors, 45 is where gastroenterologists start looking for a gut cause in anaemia, and 50 is a Swiss aim before an elite altitude camp (REF-1411, A; REF-1410, A-).So 28 sits in the low-stores band on two of those lines; what it means for you depends on your CRP, haemoglobin, transferrin saturation and earlier values, and that reading is your doctor's.

"Should I take iron just to be safe?" No. Without a test you cannot know which way you are off: in amateur marathoners 15% of the men had ferritin above 200 while under 2% were depleted (REF-1419, B+), iron brought no benefit with normal ferritin (REF-1409, A-), and long-term iron with normal or high ferritin may be harmful (REF-1410, A-).

"Will fixing my iron make me faster?" If you are truly deficient you may feel less tired — that was the clearest effect in trials (REF-1412, A); a faster race is possible when stores are very low but has not been shown (REF-1409, A-).

"My doctor prescribed iron — when do I take it?" Their schedule decides.What the studies show: fasting or between meals and away from tea (REF-1411, A), and away from coffee and calcium too (REF-1410, A-); vitamin C is not needed with the tablet (REF-1411); in runners, iron in breakfast soon after a morning run was absorbed best and iron 2 h after a long run worst (REF-1418, B+; REF-1421, B).If it upsets your stomach, alternate days are an option to ask about: the same ferritin in the end, with fewer side effects, but slower (REF-1416, A).

"Should I get an iron infusion?" That is your doctor's decision, usually when tablets fail or are not tolerated or stores must be restored fast (REF-1410, REF-1411); in active women without anaemia an infusion refilled stores but did not raise VO2peak within 4 weeks (REF-1422, B+).

"I'm a man over 50 and my iron came back low." That is a question for your doctor, not your training: about a third of men and postmenopausal women with iron-deficiency anaemia have a cause that needs finding, most often in the gut (REF-1411, A).

"My ferritin is 350." That is not something iron or diet advice fixes; take it to your doctor, who will look at it with CRP and transferrin saturation (REF-1411; REF-1419).

Say what the base cannot judge: a ferritin cut-off tested against performance, the best rest before a blood test, iron in masters or vegetarian athletes, supplement brands and forms; say so rather than improvise a number.

What NOT to do

Diagnose iron deficiency or anaemia, or name a cause (REF-1411: finding the cause is the clinician's work).

Suggest iron without a test, or "a little iron to be safe" (REF-1419; REF-1410; REF-1409).

Set a dose, a schedule, a product or a stop date; report what the studies show and send the choice to the prescriber.

Suggest or recommend IV iron (REF-1410; REF-1411; REF-1422).

Read one ferritin as a verdict, quote one cut-off as the law, or read a ferritin drawn in the days after a race or during an illness as a store measurement (REF-1414; PHYS-44, established).

Promise a faster race or a higher VO2max (REF-1412; REF-1409).

Call a trained athlete's low haemoglobin "sports anaemia", or dismiss it (REF-1410).

Tell a man or a postmenopausal woman that low iron is "just training" (REF-1411).

Present vitamin C tablets or juice as needed with the iron tablet (REF-1411).

Treat tiredness as iron deficiency before load, sleep, fuelling and illness have been asked about (REF-206; SYNTH-29).

Evidence gaps

No ferritin cut-off validated against performance or symptoms in athletes; every line in the table is a surveillance, clinical, research or trial-entry line (REF-1410, REF-1411, REF-1409).

When to test: one small study of races, 14 of 15 runners men, one follow-up sample within 5 days (REF-1414); nothing in the base after ordinary training, across a training block or after an illness; the week of rest is practice.

Timing around training: two single-meal absorption studies of meal iron (REF-1418, REF-1421); no trial of tablet timing with a ferritin or performance outcome.

Schedules in athletes: one small abstract-only trial without a performance outcome (REF-1417).

Performance: the athlete meta-analyses disagree (REF-1413, REF-1409); IV iron in athletes is one trial of 26 women over 4 weeks (REF-1422).

Who is studied: mostly young women; no iron study in masters athletes or in vegetarian or vegan athletes, and no adolescent prevalence figure, in the base; men appear in REF-1414, REF-1417, REF-1418, REF-1419 and REF-1421.

Not in the base at all: food-first trials; supplement forms and patches; overload and haemochromatosis criteria; gut blood loss and haemolysis in runners; the current anti-doping wording on infusions; heat blocks and iron.

Limitations

Transfer. The A-level sources are in adults with anaemia (REF-1411), non-athlete women (REF-1416), women with fatigue whose athletic status was not characterised (the fatigue trials of REF-1412) or mostly young women of mixed iron status (REF-1409); athletes are the sample in small studies, most read from the abstract.

Abstract-only: REF-1413, REF-1414, REF-1415, REF-1417, REF-1418, REF-1419, REF-1421 and REF-1422, and among the older entries REF-970 and REF-969; their designs, doses and interests could not all be checked.

Expert opinion inside high levels. REF-1410's cut-offs and its 50 µg/L altitude aim are the authors' choice, with no method described; REF-1411's level is the document's, its statements range from high to very low evidence, and its thresholds serve gastroenterology.

Women and men. 432 of the 449 participants in REF-1409 and the populations of 15 of 18 trials in REF-1412 are women; REF-1414 is 14 men of 15; the sex effects in REF-1417 and REF-1421 are noted, not settled.

Errors in the sources: REF-1409's trial counts differ between table and text, and its abstract and key points disagree on VO2max; REF-1412 prints its ferritin difference in µmol/L; REF-1410 gives two re-test frequencies and two maintenance schedules; REF-1420's abstract calls intramuscular injections "parenteral"; REF-1418's absorption differences are printed as percentages.

Interests: two of 12 authors of REF-1411 report advisory roles with IV-iron companies and one leads trials of a ferric maltol tablet; two authors of REF-1422 report fees or grants from IV-iron makers, stated as outside the work; REF-1413's record prints no competing-interest statement, and later papers' interests are not carried back to it; REF-1416 was funded by the Swiss National Science Foundation, its capsules made by a company; REF-1409, REF-1410 and REF-1412 report no funding.

Practice, not evidence: the order of steps; the list of reasons to suspect iron; the test conditions (a morning sample, an easy day before, about a week after a long race, after an illness has cleared, the same laboratory); the which-day placement; the red-flag list as a screen and every flag without a source; the special-group calls; the assistant's rules.

Created 2026-10-28: new framework for the tired or stalled athlete who asks about iron (scope approved by the owner the same day), built from the batch REF-1409 to REF-1422 with REF-340, REF-573, REF-577, REF-969, REF-970, REF-341, REF-748, REF-206, REF-1067, REF-1353, REF-464, REF-545, REF-095, REF-358, REF-529 and PHYS-36, PHYS-44, PHYS-32.Folds the same day: vitamin C with a tablet is not needed and helps food iron (SYNTH-06, REF-340); the usual treatment dose is about 40-100 mg, with 100-200 mg/day the UCI's figure for treatment and altitude blocks (SYNTH-67); alternate days give the same stores with fewer gut effects, more slowly (SYNTH-06, REF-970); SYNTH-06's altitude preparation is stated as sources disagreeing with its house practice; PHYS-36 gets refIds beside its bare DOIs and its heat-block test labelled practice; REF-340's Pedlar 2018 DOI is corrected; REF-970's tag is fixed; iron joins SYNTH-54's REDs screen; and the gap lines in SYNTH-69 and SYNTH-81 point to what now exists.Level B.

How this entry was checked and changed

Why this level

(the direction — ask before blaming iron, test before any iron, read ferritin with CRP as a range, treat only a measured deficiency with oral iron first, re-test after 6-8 weeks, expect fatigue rather than capacity to change, and send anaemia and low iron in men and postmenopausal women to a doctor — rests on a national guideline REF-1411 (A), a sports-medicine consensus REF-1410 (A-, expert-set cut-offs, no method described), meta-analyses REF-1412 (A), REF-1409 (A-) and REF-1413 (A-, abstract), a registered RCT of 150 women REF-1416 (A) and a B+ cross-sectional study of 170 amateur marathoners REF-1419. Below B+ because the A-level evidence is mostly in women who are not athletes or whose athletic status was not characterised, or in anaemic adults, so it transfers; every ferritin line is expert-set and none is validated against an athlete's performance or symptoms; the athlete-specific evidence on when to test and when to take iron is B- to B+ and mostly read from abstracts (REF-1414, REF-1418, REF-1421, REF-1417, REF-1422); and whether treatment makes athletes faster is contested. Above B- because the default does not depend on the contested part and its numbers are sourced (the panel, the re-test interval, the usual dose range, what the schedules do), leaving as practice the order of steps, the list of reasons to suspect iron, the rest before a test, the which-day placement and the special-group calls)