Vitamin D

Vitamin D is made in the skin in sunlight and obtained from a few foods and from supplements. Observational studies have reported that low vitamin D levels are common in people with IBS, and the proposed mechanisms run through immune activation, low-grade inflammation, and mood PMID 27154424 PMID 35396764. Whether raising vitamin D levels changes bowel symptoms is a separate question, and the randomised trials give different answers.

The most-cited positive trial randomised 90 Iranian patients to 50,000 IU of vitamin D3 or placebo every two weeks for six months. The vitamin D group reported greater improvement in abdominal pain, distension, flatulence, and overall symptoms, with the IBS Severity Scoring System falling by 53.8 points versus 16.9 on placebo and a larger gain in quality of life PMID 27154424. The largest trial to date pointed the other way. In Sheffield, 135 people with IBS recruited from the community in winter received 3,000 IU of vitamin D or placebo daily for 12 weeks; 60% were deficient or insufficient at baseline, and blood levels rose by 45.1 nmol/L on vitamin D versus 3.1 on placebo, but symptom severity fell by a similar amount in both arms (62.5 versus 75.2 points) and quality of life did not differ. The authors concluded there was no case for using vitamin D to manage IBS symptoms, while noting that deficiency was common enough to justify supplementation for general health; the study was part-funded by a supplement supplier PMID 34328539.

Two meta-analyses published in 2022 pooled overlapping sets of these trials and disagreed. One, covering eight studies and 685 patients, found a standardised mean difference in symptom severity of -0.77 favouring vitamin D, with heterogeneity of 91% and no significant change in quality of life, and flagged small samples, young populations, limited ethnic diversity, and varied dosing as limitations PMID 35396764. The other, restricted to six randomised trials in 616 patients, found no significant difference in symptom severity (mean difference -45.8 points, confidence interval -93.6 to 2.0) but a small significant gain in quality of life PMID 35807798. The difference comes down to which trials were included and how much weight the heterogeneous positive studies carry.

Points clinicians commonly raise: checking and correcting deficiency is reasonable on general health grounds whatever it does for the bowel; the regimens tested range from daily 3,000 IU to fortnightly 50,000 IU, which are not interchangeable, and high-dose regimens are not something to self-prescribe; and the best-controlled trial in a Western population was negative, so an improvement after starting vitamin D is not evidence that vitamin D caused it.

Talk to a clinician before starting or stopping anything.