Ulcerative colitis and vitamin D

I live in the Netherlands. If you've ever spent a Dutch winter here, you know what that means: grey skies from October through March, temperatures that keep you indoors, and daylight that starts fading by half past four. It's not exactly a climate that fills your body with sunshine.

And that's exactly the problem. Because vitamin D, the substance your skin produces when UVB light hits it, turns out to be one of the most important factors in how your immune system behaves. When your levels drop, your immune system becomes less regulated. For someone with ulcerative colitis, that can mean the difference between a calm gut and a full flare.

Vitamin D is not really a vitamin

This is something most people don't realise. Vitamin D functions as a hormone in your body, not as a simple nutrient like vitamin C. Your skin synthesises it when exposed to UVB light, and your liver and kidneys then convert it into its active form. That active form doesn't just help with calcium and bones — it directly influences how your immune cells behave.

Vitamin D receptors are present on nearly every cell in your body, including the cells that line your entire gut. These receptors act as switches. When vitamin D binds to them, it tells your immune system to calm down, to stop overreacting, to distinguish between a genuine threat and your own tissue. When there isn't enough vitamin D to activate those receptors, your immune system loses that regulation. And in UC, the immune system is already attacking the gut lining. Remove the brake, and the damage accelerates.

Research from Penn State University demonstrated this clearly in a controlled study. Mice with colitis that were vitamin D deficient had significantly worse inflammation than those with normal levels. When the deficient mice received vitamin D supplementation for just two weeks, their symptoms improved measurably. The inflammation reduced, the gut lining began to recover.

The vicious cycle most people don't know about

If you have UC and you're vitamin D deficient, there's a good chance you're stuck in a cycle that feeds itself. Inflammation in the gut damages the lining where nutrients are absorbed. A damaged gut lining absorbs less vitamin D from food. Lower vitamin D levels mean less immune regulation. Less regulation means more inflammation. More inflammation means more damage to the gut lining. And so it continues.

This isn't theoretical. Studies consistently show that people with inflammatory bowel disease have lower vitamin D levels than the general population. Some researchers argue this is simply a consequence of the disease — you're inflamed, so you absorb less. But the evidence increasingly suggests it works both ways. Deficiency isn't just a side effect of IBD. It's a driver of it.

What makes this particularly relevant is the geographical pattern. The incidence of ulcerative colitis increases the further you move from the equator. Countries with less sunlight — the UK, Scandinavia, the Netherlands, Canada — have consistently higher rates of autoimmune conditions, including UC. Countries closer to the equator, where people are exposed to stronger UVB light year-round, have significantly lower rates. That's not a coincidence.

Why your symptoms get worse in winter

If you've noticed that your UC flares tend to come in autumn and winter, you're not imagining things. I tracked this pattern in myself for years. Summer: more energy, better digestion, fewer symptoms. Winter: achy joints, fatigue, stomach pain, and that familiar heaviness that makes everything harder.

I see the same pattern reflected in the traffic on this website. Visitor numbers climb from October onwards and peak in January and February. People search for answers when they're struggling, and they struggle more when the days are short.

The reason is straightforward. In northern countries during winter, the sun sits too low in the sky for UVB rays to penetrate the atmosphere effectively. Even if you stand outside for an hour in December in Amsterdam or London, your skin produces almost no vitamin D. Your body relies on whatever stores it built up during summer, and for most people those stores run out by November.

Seasonal Affective Disorder follows the same pattern, and research suggests vitamin D plays a role there too. The fatigue, low mood, and general malaise that come with SAD overlap heavily with what UC patients experience during winter flares. It may not all be the same mechanism, but the vitamin D connection is hard to ignore.

Vitamin D helps maintain your gut barrier

Beyond immune regulation, vitamin D has a direct structural role in your gut. The cells lining your intestine are held together by tight junctions — protein structures that control what passes through the gut wall and what stays out. When these tight junctions loosen, you get what's commonly called leaky gut: partially digested food particles and bacteria pass through the gut wall into the bloodstream, triggering immune reactions and inflammation.

Vitamin D helps maintain those tight junctions. It supports the production of the proteins that keep the gut barrier intact. When your levels are low, the barrier weakens. This connects directly to the inflammation cycle: a leaky gut wall lets more irritants through, which triggers more immune activity, which causes more damage to the wall, which lets more irritants through.

What to do about it

Get tested first. Before you start supplementing, ask your doctor for a 25-hydroxyvitamin D blood test. This is the standard marker for vitamin D status. The general population is told that 30 ng/mL (75 nmol/L) is sufficient, but many researchers working with IBD patients suggest that 40 to 60 ng/mL is a better target for people with autoimmune conditions. Know your number before deciding on a dose.

Supplement with D3, not D2. Vitamin D comes in two forms: D2 (ergocalciferol, plant-derived) and D3 (cholecalciferol, the form your skin produces naturally). D3 is absorbed more effectively and raises blood levels more reliably. Most studies on vitamin D and IBD use D3. If your levels are low, a daily supplement of 1,000 to 4,000 IU is common, but higher doses may be needed initially under medical supervision. Vitamin D is fat-soluble, which means it builds up in your body — so more is not automatically better. Get tested, supplement, retest after three months.

Take it with vitamin K2. This is something that often gets overlooked. Vitamin D increases calcium absorption, but calcium needs to go to the right places — your bones and teeth — and not accumulate in your arteries. Vitamin K2 (specifically the MK-7 form) directs calcium where it belongs. If you're supplementing with vitamin D over a longer period, adding K2 is a sensible step. Many quality supplements now combine D3 and K2 in a single capsule.

Get sunlight when you can. In summer, 20 to 30 minutes of sun exposure on your arms and face can produce significant amounts of vitamin D. No sunscreen during that window (but don't burn). In winter, this becomes nearly impossible in northern Europe, which is why supplementation matters more from October through March. Some people use tanning beds with UVB lamps during winter. If you go that route, limit the time and frequency — you want UVB exposure, not skin damage.

Eat vitamin D-rich foods. Oily fish like salmon, mackerel, and sardines are the best dietary sources. Eggs, particularly the yolks, contain smaller amounts. You won't get enough from food alone to correct a deficiency, but it contributes. The Dutch Gezondheidsraad recommends supplementation for large parts of the population precisely because dietary intake and winter sunlight are not enough in this climate.

What we do in our practice

Vitamin D is one of the first things we look at when someone comes to us with ulcerative colitis or Crohn's disease. Not because it's a magic fix — it's not — but because it's one of the foundational pieces that needs to be in place before the gut can begin to recover. You can work on nutrition, gut bacteria, and stress management, but if your immune system is dysregulated because of a basic deficiency, you're working against yourself.

I had Crohn's disease myself, diagnosed in 2015. I recovered without medication in six months and have been symptom-free for more than eight years. Vitamin D was part of that process. Not the whole answer, but a piece I'm glad I didn't overlook.

If you're dealing with UC and you've never had your vitamin D levels checked, that's a good place to start. It's a simple blood test, and the result might explain more than you expect.