Folate (Vitamin B9)

Why it happens in Crohn’s disease

Folate is absorbed in the jejunum, which proximal Crohn’s disease can involve directly. Three further mechanisms stack on top. Inflamed, rapidly proliferating mucosa consumes folate at an elevated rate, raising requirements exactly when intake is lowest. Sulfasalazine competitively inhibits the intestinal folate transporter and is a documented cause of deficiency. Methotrexate, used as a Crohn’s immunomodulator, is a dihydrofolate reductase inhibitor — folate antagonism is its mechanism of action, not a side effect. Diet during flares, typically low in leafy greens and legumes, completes the picture.

Who is most at risk

• Sulfasalazine therapy — inhibits folate absorption directly
• Methotrexate therapy — pharmacological folate antagonism
• Extensive jejunal disease or resection
• Active inflammation with high mucosal turnover
• Pregnancy or planning pregnancy, where requirements rise sharply

Symptoms of deficiency

• Macrocytic anaemia — indistinguishable from B12 deficiency on a blood film
• Fatigue, weakness, breathlessness
• Mouth ulcers and glossitis
• Low mood and irritability
• Raised homocysteine, an independent thrombotic risk factor in a population already prone to venous thromboembolism
• In pregnancy, neural tube defects in the fetus

Many of these overlap with Crohn’s disease itself, which is exactly why deficiencies are missed. Fatigue attributed to active disease is frequently anaemia, and neuropathy attributed to medication is sometimes B12.

How it is tested

Serum folate reflects recent intake and fluctuates; red cell folate better represents tissue stores over the preceding months. Always measure B12 at the same time. The order matters clinically: giving folate to someone with unrecognised B12 deficiency corrects the anaemia while allowing the neurological damage to progress unnoticed, which is a well-documented and avoidable harm. Patients on long-term sulfasalazine or methotrexate should be monitored as a matter of routine.

Replenishment protocols

Standard supplementation is 1 mg/day of folic acid for documented deficiency. Patients on sulfasalazine are routinely supplemented as a preventive measure. Patients on methotrexate are given folic acid 1–5 mg/day on non-methotrexate days, or folinic acid 24 hours after the dose — this reduces mucositis, nausea and hepatotoxicity without abolishing efficacy, and is standard practice rather than optional. Anyone with Crohn’s who may become pregnant should take at least 400 mcg/day, and higher doses are often advised given malabsorption. Observational data suggest folate supplementation may also be associated with lower colorectal cancer risk in long-standing IBD, though this is not established as a treatment indication.

Amounts reflect what gastroenterology literature and guidelines describe. The right dose for you depends on your test results, absorption and surgical history, so test first and work with your care team to personalise it.

Medication interactions

Sulfasalazine and methotrexate are the two major interactions and both are common in Crohn’s care. Note that Mesalamine (5-ASA) does not share sulfasalazine’s folate-blocking effect — the sulfapyridine moiety is responsible, which is one reason patients are often switched. Trimethoprim, phenytoin and long-term alcohol use also antagonise folate.

Sources

1. 1

   Associations between folate and vitamin B12 levels and inflammatory bowel disease: a meta-analysis

   Pan Y, Liu Y, Guo H, et al. · Nutrients · 2017 · PMID 28422077

2. 2

   Sulfasalazine inhibits the absorption of folates in ulcerative colitis

   Halsted CH, Gandhi G, Tamura T. · New England Journal of Medicine · 1981

3. 3

   Does folic acid protect patients with inflammatory bowel disease from complications?

   Burr NE, Hull MA, Subramanian V. · Nutrients · 2021