Vitamin D and IBD (Crohn's, ulcerative colitis): should you monitor your level in winter?

Published October 19, 2026

The starting point

Werner Masseglia, the creator of We IBD, lives in Belgium with IBD, diagnosed five years ago (his story). This topic comes from him:

“Over the years I have noticed that the months with little light are harder. After some research, it turned out that the link between vitamin D and IBD exists.”

His observation is his own: he is not saying that a lack of light makes the disease worse. As for the link, studies do show an association between vitamin D and IBD, solid on some points, still uncertain on others. Here are which.

Vitamin D comes mostly from the sun

Our skin makes vitamin D under the effect of the sun's UVB rays. But the sun has to rise high enough in the sky. In 1988, a Boston team exposed human skin to sunlight on clear days in the laboratory, at different latitudes [1]. In Boston (42° N), the skin produced no previtamin D, the precursor of vitamin D, from November to February. In Edmonton, Canada (52° N), this period without production ran from October to March.

Belgium lies between 49.5 and 51.5° N, much closer to Edmonton than to Boston. At this latitude, skin production of vitamin D can be strongly limited during several winter months. This study does not measure the blood level of Belgians: a winter poor in UVB does not mean that everyone becomes deficient. The blood level also depends on the body's reserves, diet, any supplements and other individual factors.

Less sun, more Crohn's disease?

Several large studies have compared how common IBD is according to sun exposure.

• In the United States, more than 175,000 nurses were followed for years [2]. Those who lived in the south of the country at age 30 had about half the risk of developing Crohn's disease, and about a third lower risk for ulcerative colitis, than those in the north.
• In France, one study compared the 94 departments of metropolitan France [3]. The least sunny regions had more new cases of Crohn's disease. No link was found for ulcerative colitis.
• In the same cohort of American nurses, another team estimated the vitamin D level of 72,719 women from their sun exposure, the sunshine in their region, their diet and their supplements [4]. The quarter of women with the highest estimated level had almost half the risk of Crohn's disease of the quarter with the lowest level. For ulcerative colitis, the decrease observed was not significant.

What these studies do not say: they observe populations, they do not run an experiment. From one region to another, many things other than the sun change. The authors say so themselves: more studies are needed to know whether the link is a cause [3].

With IBD, vitamin D deficiency is more common

A meta-analysis pooled 14 studies comparing 938 people with IBD to 953 people without IBD [5]. Patients had a 64% higher risk of being deficient (deficiency defined in these studies as a blood level of 20 ng/mL or less). For ulcerative colitis, this risk was more than doubled.

The authors point out one limit: these figures are raw, with no adjustment to take other factors into account.

A low level often goes with more active disease

Another meta-analysis brought together 27 studies and 8,316 patients [6]. In people with a low vitamin D level, active disease, inflammation of the intestinal lining, a poorer quality of life and, slightly more often, a relapse in the following months were seen more frequently. This was true for Crohn's disease as for ulcerative colitis.

Here again, this type of study does not say which way the link goes. A low level can accompany active disease without being its cause. The authors in fact speak of a marker of disease activity.

And vitamin D supplements?

To know whether vitamin D really helps, we need trials that compare patients who take it with patients who receive a placebo. Their results do not all point the same way.

• A Danish trial followed 94 patients with Crohn's disease in remission for one year [7]. There were fewer relapses in the vitamin D group (13%) than in the placebo group (29%), but the difference was not statistically significant. The authors called for larger studies.
• A 2024 meta-analysis pooled the placebo-controlled trials [8]. Across 458 patients, the risk of relapse was lower with vitamin D, especially in people with Crohn's disease in remission. For ulcerative colitis, only 67 patients were included: too few to conclude.
• Another meta-analysis, of 17 trials and 1,127 patients, finds that supplements do raise the blood level of vitamin D and lower CRP, a marker of inflammation, a little, but with no significant decrease in disease activity or in relapses [9].

In summary: the available data suggest that vitamin D could reduce the risk of relapse in some situations, notably in people with Crohn's disease in remission. But the trials remain limited, and their results are not solid enough to recommend supplementation to all patients with the aim of preventing flares, particularly in ulcerative colitis.

Two things must be distinguished. Correcting a deficiency found by a blood test can be medically indicated: this is what the European recommendation presented below provides for. Taking vitamin D in the hope of preventing flares is something else: the studies do not allow recommending it to everyone.

What the guidelines say

The European Society for Clinical Nutrition and Metabolism (ESPEN) recommends monitoring the blood level of 25(OH) vitamin D in adults and children with IBD who have active disease, who are treated with corticosteroids, or in whom a lack of vitamin D is suspected. If necessary, a calcium and vitamin D supplement should be prescribed to prevent a decrease in bone density [10]. This is its recommendation 20, grade B, adopted with strong consensus (96%). It therefore provides neither for systematic testing in all patients, nor for supplementation independent of the result and the clinical context.

Winter is not only about vitamin D

The dark months change many other things: we often move less, colds and flu circulate more, fatigue and stress can weigh more. The studies presented here do not allow us to separate the share of each of these factors.

In practice

If you live with IBD, talk to your gastroenterologist, who can have your level measured. It is a simple blood test, and it is up to them to say whether a supplement is useful for you, and at what dose.

A log to prepare consultations

We IBD lets you add your vitamin D results to your lab tracking and find them in the report intended for your doctor.

Since version 2.1.1, on iPhone, you can also choose to add to your log the weather of your area (full version, off by default) and the time spent in daylight measured by the Apple Watch. This information is viewed alongside your symptoms, without the app deducing any cause-and-effect relationship from it, and it never enters its indicator. For the weather, your location, rounded to about 10 km, is sent to Apple Weather (details in our privacy policy).

These are tracking tools, not diagnostic tools: they do not measure your vitamin D level and do not make it possible to say whether light influences your disease.

This article summarises published studies. It does not replace medical advice.

Sources

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2. Khalili H et al. Gut, 2012;61(12):1686-92. PubMed 22241842
3. Nerich V et al. Aliment Pharmacol Ther, 2011;33(8):940-5. PubMed 21332762
4. Ananthakrishnan AN et al. Gastroenterology, 2012;142(3):482-9. PubMed 22155183
5. Del Pinto R et al. Inflamm Bowel Dis, 2015;21(11):2708-17. PubMed 26348447
6. Gubatan J et al. Aliment Pharmacol Ther, 2019;50(11-12):1146-58. PubMed 31647134
7. Jørgensen SP et al. Aliment Pharmacol Ther, 2010;32(3):377-83. PubMed 20491740
8. Valvano M et al. Inflamm Bowel Dis, 2024;30(2):281-91. PubMed 36579768
9. Guo Y et al. Food Funct, 2021;12(17):7588-606. PubMed 34231596
10. Bischoff SC et al. ESPEN guideline on clinical nutrition in inflammatory bowel disease. Clin Nutr, 2023;42(3):352-79, recommendation 20. PubMed 36739756 · full text (ESPEN)