Ulcerative Colitis Fatigue and Low Iron Explained

Ulcerative colitis drains iron in two ways at once. Bleeding from the inflamed colon loses it, and the inflammation itself raises a hormone called hepcidin that blocks iron absorption and locks the iron you already have inside storage cells. Low iron is one of the few causes of fatigue in colitis that a blood test can find and a clinician can treat, so it is worth ruling in or out properly. But it is rarely the whole story, the standard test is distorted by the very inflammation that causes the problem, and after treatment the deficiency often comes back within months.

This post is about that iron question specifically: which numbers to look at, why they mislead, why tablets can make a colitic gut feel worse, and what happens after the infusion. For the wider picture of fatigue in IBD, including sleep, mood and deconditioning, we wrote a separate guide on fatigue that persists in remission, and most of it applies to ulcerative colitis too.

Is it the colitis or the iron? Usually both are on the table

The honest answer to "is this exhaustion the colitis itself, or my iron being low?" is that you cannot tell from how it feels, and the two are not mutually exclusive. The blood test is what separates them.

Fatigue is one of the most common symptoms of IBD by any measure. A 2022 meta-analysis of 20 studies put the pooled prevalence at 47% (95% CI 41% to 54%), rising to 72% in active disease and 47% in remission, with very high heterogeneity between studies (I² 98%). Sleep disturbance, anxiety, depression and anaemia were the risk factors most commonly reported (D'Silva 2022). Note that the remission figure happens to equal the overall pooled figure. That is not a typo in our reading; the pooled estimate across all studies and the remission-only subgroup landed on the same number.

Anaemia is common too. In an individual patient data meta-analysis of 2,192 European IBD patients, mostly from tertiary centres, anaemia was present in 21% (95% CI 15% to 27%) of people with ulcerative colitis, compared with 27% in Crohn's disease, and 57% of the anaemic patients overall were iron deficient. Active disease more than doubled the odds of anaemia (odds ratio 2.72) (Filmann 2014).

The UC-specific figure that stopped us is from the United States. In a nationwide Veterans Affairs cohort of 836 people newly diagnosed with ulcerative colitis, 585 (70%) developed anaemia over a median of eight years. Only 401 of those 585 (68.6%) had iron studies done at all. Of the 401 tested, 251 (62.6%) had iron deficiency anaemia, and 191 of those (76.1%) received oral iron. None received intravenous iron (Khan 2016). The authors conclude that more than a third of anaemic UC patients were never tested for iron deficiency. Two caveats: a VA population is mostly male and older than the average UC clinic, and news coverage of the paper reported that the lead author had received an unrestricted grant from Luitpold Pharmaceuticals, a maker of intravenous iron (Healio). The arithmetic in the abstract checks out: 585 of 836 is 70.0%, 401 of 585 is 68.5% (the paper reports 68.6%), 251 of 401 is 62.6%, and 191 of 251 is 76.1%.

So the realistic position for most people asking this question is: fatigue is expected in colitis, anaemia is common, iron deficiency is frequently missed, and checking properly costs one blood draw.

Why a colon disease empties your iron, when iron is absorbed higher up

One of the most upvoted replies in an r/UlcerativeColitis thread about recurring anaemia made a correct point and drew the wrong conclusion from it: iron is absorbed in the duodenum and upper small intestine, ulcerative colitis does not involve the small intestine, so how can UC cause malabsorption (r/UlcerativeColitis thread). The first half is right. The answer is that inflammation acts on the whole body, not just the bowel wall it sits in.

Mechanism one: bleeding. The European Crohn's and Colitis Organisation (ECCO) consensus on anaemia in IBD lists continuous blood loss from the ulcerated bowel surface as a cause of iron deficiency, alongside reduced intake and impaired uptake (Dignass 2015). You do not have to see blood for this to be happening, which is why one person in that same thread asked about "micro bleeding" months after visible bleeding had stopped.

Mechanism two: hepcidin. The consensus describes it plainly. In active inflammation, cytokines drive the liver to make more hepcidin. Hepcidin shuts down ferroportin, the channel that lets iron out of cells. Iron gets trapped in the macrophages that recycle old red cells, transferrin saturation falls, and the bone marrow is starved of iron even though the body is not empty. The same rise in hepcidin reduces iron absorption from the duodenum (Dignass 2015). That is how a disease confined to the colon cuts absorption higher up.

This is not only theory. A study that gave 73 adults with IBD and 22 healthy controls a test dose of oral iron and measured how much reached the blood found absorption depended on inflammation and on the type of anaemia, not on whether someone had Crohn's disease or ulcerative colitis, or where their disease was located. CRP, interleukin-6, ferritin and hepcidin all correlated negatively with the rise in serum iron (Aksan 2020). We covered what that means for eating during a flare in what to eat during a UC flare; the short version is that red meat and spinach are working against a closed door while you are inflamed.

The British Society of Gastroenterology guideline puts the two together for IBD: roughly a third of patients with active IBD are estimated to have iron deficiency, while vitamin B12 and folate deficiency, marrow suppression from anaemia of chronic disease, and overt blood loss can all contribute to the anaemia (Snook 2021). In other words, "anaemic" in colitis is frequently a mixture, and the mixture is what makes the blood tests confusing.

Reading the numbers: ferritin, transferrin saturation, haemoglobin and CRP together

Ferritin is the test most people are given and the one most likely to mislead in colitis, because ferritin is an acute phase protein. Inflammation pushes it up whatever your iron status. Read on its own against the laboratory's normal range, it will call some iron-deficient colitis patients "fine".

A recurring theme in UK threads is exactly this. One person described two years of a slowly sliding haemoglobin, a ferritin of 112 that two doctors read as reassuring, and a transferrin saturation of 11% once someone finally ordered full iron studies (r/UlcerativeColitis thread). Whether that particular person's result means true deficiency or iron locked away by inflammation is a question the thresholds below help answer, and it is a question for their team, but the complaint about ferritin being read alone is well founded.

Here is what the two main guidelines actually say, in their own terms:

• ECCO, statement 1D: without clinical, endoscopic or biochemical evidence of active disease, a serum ferritin below 30 micrograms per litre is an appropriate criterion for iron deficiency. In the presence of inflammation, a ferritin up to 100 may still be consistent with iron deficiency (Dignass 2015).
• ECCO, statement 1E: with inflammation present, a ferritin above 100 plus a transferrin saturation below 20% points to anaemia of chronic disease. A ferritin between 30 and 100 suggests a combination of true iron deficiency and anaemia of chronic disease.
• ECCO, statement 1B: screening should use a full blood count, ferritin and CRP, every 6 to 12 months in remission or mild disease, and at least every 3 months in outpatients with active disease.
• BSG: a ferritin below 15 is highly specific for absent iron stores (specificity 0.99). A cut-off of 45 gives a specificity of 0.92 and is suggested as the best practical trade-off. A ferritin above 150 is unlikely with absolute iron deficiency, even with inflammation. A transferrin saturation below 20% indicates iron restriction (Snook 2021).

Two further lines in the ECCO document matter specifically to people with ulcerative colitis, and we have not seen either on the pages that rank for this topic.

First, disease activity is not always associated with a rise in acute phase proteins, "particularly in ulcerative colitis", and may not come with symptoms, so endoscopy may be needed to judge activity when CRP is low. That undercuts the usual advice to "read ferritin next to CRP". A normal CRP in UC does not prove there is no inflammation propping your ferritin up.

Second, for up to eight weeks after intravenous iron, ferritin correlates poorly with body iron stores, because the infusion itself stimulates ferritin synthesis. A sky-high ferritin a fortnight after an infusion does not tell you how full the tank really is.

The table below is a translation of those rules, not a diagnostic tool. The same result can mean different things in different people, and the interpretation belongs to whoever ordered the test.

Serum iron on its own is the least useful number of the lot. One person posted a serum iron of 94 alongside a ferritin of 6 and a raised transferrin and iron binding capacity (r/UlcerativeColitis thread). The single "iron" figure looked normal while every storage marker said the tank was empty. The BSG guideline lists low serum iron among the markers of deficiency but builds its recommendations around ferritin, transferrin saturation and, where ferritin may be falsely normal, the other markers (Snook 2021).

If your laboratory offers them, ECCO also names reticulocyte haemoglobin, percentage of hypochromic red cells and soluble transferrin receptor as tests that help in uncertain cases, noting that soluble transferrin receptor is not affected by chronic inflammation the way ferritin is (Dignass 2015). Many laboratories do not run them routinely, so this is a question to ask, not a demand.

"My haemoglobin is normal, so why am I told I am iron deficient?"

Because haemoglobin and iron stores are measuring different things, and the body defends haemoglobin first. Iron deficiency develops in stages: stores run down (ferritin falls), then the supply to the marrow becomes restricted (transferrin saturation falls), and only after that does haemoglobin drop far enough to count as anaemia. You can sit in the first two stages for a long time.

The ECCO consensus treats this state as clinically meaningful in IBD. It says iron deficiency can cause symptoms and impair quality of life before anaemia develops, lists reduced physical performance and cognitive function, fatigue, headache, sleep problems, loss of libido and restless legs among the possible symptoms, and makes a line that deserves more attention than it gets: it is common to find iron deficiency as the only sign of disease activity in IBD patients. On whether to treat iron deficiency without anaemia, it is explicitly non-committal: the decision "may depend on the clinical scenario and the patient's preference" (Dignass 2015).

That last phrasing is important if you are in the position of the people in the threads who have been told their iron is "low but not low enough". The guideline does not say treatment is mandatory. It does say your preference counts, which gives you standing to ask.

For how iron deficiency and ferritin show up in hair shedding, which is one of the more visible early signs people notice, see our post on Crohn's disease hair loss; the ferritin trap it describes is the same one in UC.

Does correcting iron relieve the fatigue? The evidence is weaker than you would hope

This is the part most pages skip, and it matters, because people pin a lot of hope on an infusion.

In IBD specifically, the evidence is thin. A 2025 systematic review found only seven eligible studies (1,425 people) looking at iron deficiency and fatigue in IBD, with methods too varied to pool. Two of three studies comparing iron deficiency without anaemia to iron-replete patients found a difference, which the authors summarise as "a slight increase in fatigue levels", and they state it does not explain all fatigue in IBD (Sartain 2025). Our remission post flagged an unresolved wording problem in that abstract and the review's funding and conflicts; we will not repeat it here beyond saying the direction of one result is ambiguous in the abstract.

Outside IBD, there are randomised trials, and they are instructive. None of them studied people with colitis, so read them as evidence about iron and fatigue in general, not about UC.

• Vaucher 2012: 198 menstruating women in French primary care with unexplained fatigue, a ferritin below 50 and normal haemoglobin were randomised to oral iron or placebo for 12 weeks. Fatigue scores fell by 47.7% with iron and by 28.8% with placebo. The difference attributable to iron is therefore 18.9 percentage points (95% CI 3.2 to 34.5), not 47.7. Quality of life, depression and anxiety did not differ (Vaucher 2012).
• Krayenbuehl 2011: 90 premenopausal women with fatigue, ferritin at or below 50 and normal haemoglobin were randomised to intravenous iron or intravenous placebo. Over six weeks fatigue fell by 1.1 points with iron versus 0.7 with placebo on a 0 to 10 scale, p = 0.07, which is not statistically significant. The effect appeared in the subgroup with ferritin at or below 15 (1.8 versus 0.4 points, p = 0.005). The abstract's conclusion that intravenous iron "improved fatigue" in these women is broader than its own primary result supports; the defensible reading is that benefit was confined to those with the emptiest stores (Krayenbuehl 2011). We could not find a funding or conflict statement for this trial in PubMed.
• Houston 2018: a systematic review of 18 trials (1,170 people) in iron-deficient adults without anaemia found iron reduced self-reported fatigue (standardised mean difference -0.38, 95% CI -0.52 to -0.23, from only 4 trials and 714 people) but did not improve objective physical capacity such as maximal oxygen uptake (Houston 2018). A small oddity: the abstract reports the ferritin change in micromoles per litre, which is not the unit ferritin is normally measured in; it does not affect the fatigue result.

Put together: correcting a real iron deficiency is reasonable and often helps, the effect on fatigue over placebo is modest on average, and it is concentrated in people whose stores are genuinely depleted. The placebo arms are the part to sit with. A large share of the improvement people feel after starting iron in these trials happened with no iron at all, which is not a reason to dismiss the treatment, and is a reason not to expect it to be the whole answer.

In IBD, fatigue has other contributors that iron will not touch. The meta-analysis above lists sleep disturbance, anxiety and depression alongside anaemia (D'Silva 2022). The threads say the same thing in their own words: several people describe normal iron, normal B12, normal vitamin D, a clean sleep study and still needing ten hours a night (r/UlcerativeColitis thread). If that is you, the next place to look is covered in our fatigue guide, and explaining fatigue to other people deals with the practical fallout.

Why iron tablets can make a colitic gut feel worse

"Iron supplements made my BMs worse" is one of the most common experiences in the anaemia threads, alongside people who could not tolerate tablets at all, people who got diarrhoea rather than the constipation they expected, and one person who described bleeding and cramps whenever they took tablets on consecutive days (r/UlcerativeColitis thread, diarrhoea thread, consecutive days thread). The research is more mixed than either the threads or the guideline language suggests.

The mechanism argument. ECCO notes that most swallowed iron is not absorbed, so unabsorbed iron is exposed to the ulcerated intestinal surface; that mucosal harm has been described in IBD; and that in animal models of IBD, luminal iron may exacerbate disease activity and alter the gut microbiota (Dignass 2015). Those are animal findings, and the consensus labels them as such.

What happened when people were actually studied.

• In a retrospective review of 277 IBD patients and 24 non-IBD controls, intolerance to oral iron was reported in 25% of IBD patients and 17% of controls, a difference that was not statistically significant. In only two of eight adequately monitored iron-intolerant IBD patients was iron associated with a rise in inflammatory markers (de Silva 2003).
• In a prospective study from the same group, iron intolerance occurred in about a quarter of patients in each group. Two of 33 IBD patients (6%) relapsed during four weeks of oral iron. Symptoms worsened in the ulcerative colitis patients, but not in Crohn's or non-IBD patients, while inflammatory markers, sigmoidoscopy scores, biopsy scores and oxidative markers did not change, and the UC patients' quality-of-life questionnaire scores actually improved (de Silva 2005).

That second study contradicts itself on the surface, and we think the contradiction is the most useful thing in it. UC patients felt worse on their symptom diaries, their quality of life scores improved (plausibly because their anaemia was being corrected), and their colon looked no different. So a tablet can make you feel worse in the gut without making the colitis worse, which is exactly the distinction people in the threads are trying to make. The study was small, though, and its authors' own conclusion, that oral iron is "equally efficacious and well tolerated" in IBD, sits oddly next to their finding that UC symptoms worsened.

• In a randomised trial of 46 IBD patients, intractable gastrointestinal side effects forced permanent withdrawal in 5 of 24 on oral iron sulfate (20.8%) versus 1 of 22 on intravenous iron sucrose (4.5%) (Schröder 2005). We should flag that the abstract, in both Europe PMC and PubMed, reports the median haemoglobin increase as 0.25 g/L versus 0.21 g/L. An increase of 0.25 g/L would be clinically negligible, a fortieth of the 10 g/L response the BSG uses to define success, so the unit or the timeframe in the abstract is almost certainly mis-stated. We could not resolve it without the full text and are not relying on that figure.
• Pooling across trials, the Cochrane review found withdrawals due to adverse events may be more common with oral than intravenous iron: 31 of 373 on oral versus 15 of 554 on intravenous, a pooled risk ratio of 0.39 (95% CI 0.20 to 0.74), rated low certainty (Gordon 2021). The crude percentages are 8.3% and 2.7%; the pooled ratio differs from the crude one because it is weighted across trials.

The microbiome finding. An open-label randomised trial in 31 Crohn's, 22 UC and 19 non-inflamed iron-deficient controls found oral and intravenous iron both corrected iron deficiency, with higher ferritin after intravenous iron. Changes in disease activity were independent of which route people received. But oral iron sulfate was associated with fewer Faecalibacterium prausnitzii and several other bacteria usually regarded as beneficial (Lee 2017). That is a real biological difference between routes. It is not evidence that oral iron worsens colitis in people, and the trial did not show that.

So the defensible summary: oral iron is poorly tolerated by a meaningful minority, tolerability in IBD may be no worse than in other people on some measures, and in UC it can worsen how the gut feels without measurable damage. Relapse clearly linked to tablets appears uncommon. None of that means you should push through side effects, or quietly stop, without telling your team. It means there are options.

The folk fixes in the threads: alternate days, vitamin C, taking it with food

The threads are full of practical advice, and some of it has better evidence than you might expect. None of the trials below included people with IBD, so treat them as background for a conversation with your team, not as a regimen.

Alternate days. Several people describe being told to take tablets every other day "for better absorption", and one described the reason well: the body makes a protein that blocks iron absorption after each dose, and a day's gap lets it subside (r/UlcerativeColitis thread). That protein is hepcidin, and the explanation is correct. In iron-depleted young Swiss women, serum hepcidin was higher with daily dosing than with alternate-day dosing, and cumulative fractional absorption was 21.8% with alternate-day dosing versus 16.3% with consecutive-day dosing, and splitting a dose into two a day raised hepcidin without improving absorption (Stoffel 2017). A follow-up in 19 women with iron deficiency anaemia, whose authors note that an oral dose raises hepcidin for about 24 hours, found absorption on the alternate day was 40% to 50% higher than on the second consecutive day (Stoffel 2020). The BSG guideline cites the same physiology (Snook 2021).

Two cautions. These were healthy women without colitis, measured over days and weeks, and absorption is not the same as haemoglobin recovery. And in active colitis, hepcidin is already raised by inflammation, which is the reason guidelines lean toward intravenous iron in active disease in the first place. Whether alternate-day dosing helps enough in inflamed UC to matter has, as far as our searching found, not been tested. Timing and frequency of any medicine are for your prescriber to set.

Vitamin C. This is the most repeated tip in the threads, including from dietitians and GIs. The best trial we found says it adds little. In a randomised equivalence trial of 440 adults with newly diagnosed iron deficiency anaemia in Shanghai, haemoglobin rose by 2.00 g/dL with iron plus vitamin C and 1.84 g/dL with iron alone at two weeks, a difference of 0.16 g/dL (95% CI -0.03 to 0.35) that met the prespecified equivalence margin. Ferritin at eight weeks and side effect rates (20.9% versus 20.5%) did not differ (Li 2020). The population was 96.8% women without IBD, so it does not settle the question for colitis, but it does suggest vitamin C is not the missing ingredient when tablets are not working.

Cast iron pans, liquid iron, eating more red meat. People report all three. We did not find trials of any of them in IBD, and the absorption study above suggests that during active inflammation the limiting factor is hepcidin, not supply.

Tablets versus infusions: what the guidelines and trials say

ECCO's position is that intravenous iron should be considered first line in people with clinically active IBD, previous intolerance to oral iron, haemoglobin below 10 g/dL, or those needing erythropoiesis-stimulating drugs (statement 2C), and that oral iron is effective and may be used in mild anaemia when the disease is clinically inactive and the person has not previously been intolerant (statement 2E) (Dignass 2015). The BSG guideline adds that failure to raise haemoglobin by at least 10 g/L after two weeks of oral iron strongly predicts that it will not work, and that intravenous iron should then be considered (Snook 2021).

This is where the frustration in the threads comes from. People describe being told to "try every other day, then every third day", being refused an infusion, and feeling looked at "like I'm crazy" for asking. The guideline criteria give you a concrete question to ask: "My colitis is active, or I have been intolerant of tablets, or my haemoglobin is below 10: does that put me in the group ECCO suggests considering intravenous iron for?" That is a request about criteria, not about preference, and it tends to get a better answer.

On what the trials found:

• Intravenous versus oral overall. The Cochrane review of 11 trials (1,670 people) found intravenous iron probably produces more responders than oral iron, 368 of 554 versus 205 of 373, a risk ratio of 1.17 (95% CI 1.05 to 1.31), number needed to treat 11, at low certainty. Most other comparisons were too sparse to conclude anything (Gordon 2021).
• Two intravenous products compared. The FERGIcor trial found a fixed-dose ferric carboxymaltose regimen produced more haemoglobin responders than individually calculated iron sucrose (Evstatiev 2011). We could not read a funding or conflict statement for it.
• A newer oral product that was supposed to close the gap. In a trial sponsored by Shield Therapeutics, the maker of ferric maltol, ferric maltol was compared with intravenous ferric carboxymaltose in 250 IBD patients. At week 12, 67% of the ferric maltol group responded versus 84% of the ferric carboxymaltose group, and the trial missed its primary non-inferiority endpoint. Haemoglobin gains were similar by weeks 24 and 52. Treatment-emergent adverse events occurred in 59% on the oral product versus 36% on the infusion, with discontinuation in 10% versus 3% (Howaldt 2022). Credit to the sponsor for publishing a miss plainly. Note also that the per-protocol analysis included only 78 of 125 on ferric maltol and 88 of 125 on the infusion, so a sizeable share of the oral arm did not complete per protocol.

The infusion is not the finish line: iron deficiency comes back

This is the question behind "why am I still wiped out months after the bleeding stopped?" and "why is my ferritin dropping again when I feel fine?", and the answer is that in IBD, iron deficiency is often a recurring condition rather than a one-off event.

The key study followed 88 IBD patients for five years after a course of intravenous iron sucrose, some with erythropoietin. Anaemia recurred at a median of 10 months (95% CI 8 to 12), and iron deficiency at a median of 19 months (95% CI 11 to 28). The total iron dose made no difference. What did matter was the ferritin reached after treatment: iron deficiency returned at a median of 4 months when post-treatment ferritin was below 100, 11 months when it was 100 to 400, and 49 months when it was above 400 (Kulnigg 2009). The effect of post-treatment ferritin applied to recurrence of iron deficiency, not anaemia, which the abstract states explicitly and which is easy to blur when summarising.

ECCO built its monitoring advice on this. Statement 3A: monitor for recurrent iron deficiency every 3 months for at least a year after correction, then every 6 to 12 months. Statement 3E: after successful intravenous treatment, re-treat as soon as ferritin falls below 100 or haemoglobin below 12 or 13 g/dL depending on sex. And statement 3B, which is the one that turns a lab result into a question about your colitis: recurrent anaemia may indicate persistent intestinal disease activity even if there is clinical remission and inflammatory markers such as CRP are normal. ECCO rates the evidence for 3B as expert opinion (level 5), so it is a sensible prompt rather than a proven rule (Dignass 2015).

The threads show the pattern clearly. One person posted six ferritin values over three years: 11, then 116 after an infusion, then 65, 58, 83, and 22, with a faecal calprotectin of 40 and no symptoms at the time of the last one (r/UlcerativeColitis thread). Another had a ferritin of 166 after an infusion, flared for four months, reached remission, and found it back at 45 with deficiency symptoms returning (r/UlcerativeColitis thread). Both are exactly what the Kulnigg curves predict. A low calprotectin is reassuring, and our guide to what calprotectin levels mean in UC explains how much weight a single result can bear; it does not account for menstrual losses, dietary intake or slow losses elsewhere, and ECCO's statement 3B is a reason to mention a falling ferritin even when you feel well. Our guide on how to tell if your UC is flaring covers the other signals worth checking at the same time.

Can maintenance treatment prevent it? The FERGImain trial took 204 non-anaemic patients who had completed FERGIcor and gave ferric carboxymaltose or placebo whenever ferritin fell below 100. Anaemia recurred in 26.7% versus 39.4%, a hazard ratio of 0.62 with a 95% confidence interval of 0.38 to 1.00, p = 0.049 (Evstatiev 2013). A confidence interval whose upper limit is exactly 1.00 means the data are only just compatible with a real effect. The abstract concludes that the drug "prevents recurrence of anemia"; that is a stronger statement than a p value of 0.049 on a single-blind trial supports. Quality of life and disease activity changes were comparable between groups, adverse events were reported in 59.0% versus 50.5%, and we could not read a funding statement. For what it is worth, the ECCO consensus describes the same trial's quality of life result as "a positive trend", while the trial abstract says the changes were comparable. Those are the same data described with different enthusiasm.

Tired after an infusion: the phosphate question

"I feel exhausted the day of the infusion, then slowly better" is a common description, and for most people that is simply the day. But there is one specific, measurable reason fatigue can recover more slowly after certain intravenous irons, and it is worth knowing about.

Some intravenous iron preparations lower blood phosphate. In the PHOSPHARE-IBD trial, 97 adults with IBD and iron deficiency anaemia were randomised, double blind, to ferric carboxymaltose or ferric derisomaltose. Low phosphate occurred in 51.0% (25 of 49) with ferric carboxymaltose versus 8.3% (4 of 48) with ferric derisomaltose. Both products corrected the anaemia. Fatigue scores improved in both groups, but more slowly and to a lesser extent with ferric carboxymaltose, and slower fatigue improvement was associated with a greater fall in phosphate (Zoller 2023).

Three things to weigh alongside that. The trial was funded by Pharmacosmos, the maker of ferric derisomaltose, which also participated in design, data collection and analysis; the lead author declares payments from the makers of both products. Recruitment ended early because of the COVID-19 pandemic, short of the planned 60 per group, and the fatigue scale was a prespecified exploratory outcome, not the primary endpoint. And the abstract describes enrolment as "49 FDI, 48 FCM" but reports results as 4 of 48 for FDI and 25 of 49 for FCM; the full text repeats the results-line denominators, and we could not tell which line has the arms swapped. The percentages match the results line.

An earlier independent observational study points the same way. In 106 IBD patients analysed per protocol, moderate to severe low phosphate at week 2 occurred in 56.9% after ferric carboxymaltose versus 5.7% after iron isomaltoside, and some cases persisted at week 6 (Detlie 2019).

The practical point is not to choose your own product. It is that if you are unusually wiped out or develop bone or muscle pain in the weeks after an infusion, asking whether your phosphate was checked is a reasonable, specific question.

"My haemoglobin is rising but my ferritin keeps falling"

This question comes up often and has a simple answer (r/UlcerativeColitis thread). When iron arrives, the marrow uses it to make red cells before anything is put into storage. A rising haemoglobin with a falling ferritin means the iron is going where it is needed most urgently. It also means stores are not yet being rebuilt, and possibly that iron is still being lost faster than it is being replaced.

The guidelines reflect this. ECCO statement 2B says the goal of iron treatment is to normalise both haemoglobin and iron stores, and gives an increase of at least 2 g/dL within four weeks as an acceptable speed of response (Dignass 2015). The BSG recommends continuing oral iron for around three months after haemoglobin normalises to replete stores (Snook 2021). So "your haemoglobin is normal now" is not the same as "you are done", and it is fair to ask what the plan is for the ferritin.

What to track, and how to show your GI the fatigue is not improving

The question behind this section came straight from the queue of reader questions: how do I show my GI that the fatigue is not improving when my scope is better? The answer is to bring numbers that were collected the same way every time, rather than adjectives.

The lab timeline. Keep one running table, with dates, of haemoglobin, ferritin, transferrin saturation, CRP, and calprotectin if you have it, plus a column for anything that happened in between: an infusion, a flare, a course of steroids, a period that was heavier than usual. Six rows of that table explain more than any description of how you feel. It is also the only way to spot the slow slide in ferritin that both of the threads above describe.

A fatigue measure that is not a vibe. The IBD-Fatigue (IBD-F) scale was developed with 567 people with IBD across five phases. It has five questions on frequency and severity of fatigue and 30 on its impact, and it showed acceptable to excellent test-retest stability (Czuber-Dochan 2014). You do not need to score it formally. Using the same five severity questions every two weeks gives you a line on a chart rather than "still tired".

The things iron does not explain. Note sleep hours and whether you woke, mood, and activity. If your ferritin climbs and your fatigue score does not move, that is useful information: it points the next conversation toward sleep, mood, medication or deconditioning rather than another round of iron.

Menstrual losses, if they apply to you. Heavy periods can be a second, easily overlooked drain on the same stores. Our post on periods and Crohn's disease covers the iron side of that in more detail.

For how to package all of this into something a clinician will actually read in a short slot, see symptom tracker for doctor, and for bowel activity scoring that sits alongside the labs, the PRO-2 score in ulcerative colitis. If you want the lab values held in the same place as your bowel log, Clairop's one-page GI visit report includes an activity score, bowel pattern, medication adherence and labs.

A worked example: two people with the same haemoglobin

Two people with ulcerative colitis each have a haemoglobin of 11.5 g/dL and describe the same crushing tiredness. These are illustrative composites, not real patients.

Person A had a left-sided flare that settled on treatment three months ago. No visible blood for eight weeks. CRP normal. Ferritin 18, transferrin saturation 9%. Reading the ECCO criteria, a ferritin below 30 with no evidence of active disease meets the iron deficiency definition. The obvious next question is why stores are still this low eight weeks after bleeding stopped: were they ever rebuilt after the flare, is there ongoing loss, and has calprotectin been checked? How to replace the iron is their team's decision; whether replacing it eases the fatigue is a separate question, worth revisiting with a fatigue score six to eight weeks later.

Person B is mid-flare with six bloody stools a day. CRP raised. Ferritin 140, transferrin saturation 14%. On the lab's range, ferritin looks fine. On the ECCO criteria, a ferritin above 100 with a transferrin saturation below 20% during inflammation points to anaemia of chronic disease, with iron trapped by hepcidin rather than absent. Oral iron is likely to be poorly absorbed while the colitis is this active, and the consensus says that in anaemia of chronic disease, treatment of the IBD should be optimised in combination with anaemia-specific treatment (statement 4F). Person B's most important task is not iron. It is getting the flare controlled, and if the bleeding and stool count are at that level, contacting their IBD team now. Our guide on when to go to hospital for a UC flare covers the thresholds.

Same haemoglobin, same fatigue, different problem. That is the whole argument for full iron studies read alongside inflammation.

Myths worth retiring

"Your ferritin is normal, so your iron is fine." Not in inflamed colitis. The ECCO consensus accepts a ferritin up to 100 as possibly deficient when inflammation is present, and the BSG guideline says apparently normal ferritin can occur with iron deficiency in inflammatory disease.

"UC is in the colon, so it cannot affect iron absorption." It can, through hepcidin, which acts on the whole body. Absorption tracked inflammation rather than disease location in the one study that measured it directly.

"No bleeding means no iron loss." Slow losses you cannot see, menstrual losses and low intake all continue. ECCO also treats recurrent anaemia as a possible sign of disease activity even when CRP is normal.

"One infusion and you are sorted." Median time to recurrence of anaemia in the key follow-up study was 10 months.

"If you fix the iron, the fatigue will go." Sometimes. The trials outside IBD show a real but modest benefit over placebo, concentrated in people with the lowest stores, and fatigue in IBD has other drivers.

"Iron tablets cause flares." The human data show symptom worsening in some UC patients without measurable inflammatory change, and relapse in a small minority. That is a reason to report side effects, not proof that iron inflames the colon.

"People with UC should never take oral iron." One person in the threads was told exactly that at diagnosis. ECCO says oral iron is effective and may be used in mild anaemia with clinically inactive disease and no prior intolerance. Which route suits you is your team's call.

When to see a doctor promptly

Contact your doctor or IBD team promptly, rather than waiting for the next routine blood test, if you have:

• Breathlessness at rest or on minimal effort, chest pain, palpitations, or fainting or near-fainting. These can mean severe anaemia and need same-day assessment; if they are severe, seek emergency care.
• Heavy or increasing rectal bleeding, especially with six or more bloody stools a day, fever, a fast pulse or feeling unwell.
• Black, tarry stools, particularly with dizziness or weakness. Do not assume it is the iron you are taking; get it assessed.
• Unexplained weight loss, night sweats, or new symptoms after the age of 50.
• A haemoglobin or ferritin that keeps falling despite treatment, or anaemia that returns quickly after an infusion.
• Fatigue that is new, severe, or getting steadily worse, even if your colitis seems quiet.

These are not diagnoses. They are reasons to be seen rather than to wait.

The honest bottom line

Ulcerative colitis takes iron through bleeding and through inflammation, and the test most people are given is distorted by the same inflammation. The answer to that is not a better supplement. It is asking for full iron studies read against inflammation, and, in UC specifically, remembering that a normal CRP does not prove the colitis is quiet.

Treating a confirmed deficiency is reasonable and often helps. It does not reliably end the fatigue, the evidence for that effect is modest, and the deficiency frequently comes back. The most useful habit is a dated lab table and a consistent fatigue score, because a slowly falling ferritin in someone who feels in remission is exactly the signal the ECCO consensus says to take seriously, and it is invisible without a record.