Before I was diagnosed with Crohn's disease in 2015, I wasn't paying much attention to what I ate. Processed meals, biscuits with coffee, fizzy drinks when I was thirsty. I wasn't unusual in that regard — most people in the Netherlands get around 61% of their daily calories from ultra-processed food, and in the UK and US the numbers are similar. I was just eating what was normal, and my gut was paying the price without me realising it.
When I started investigating the connection between diet and gut health after my diagnosis, sugar was one of the first things I looked into. Not because someone told me to avoid it, but because the science was so clear about what excess sugar does to a gut that's already inflamed. In our practice, reducing sugar intake is typically one of the first dietary changes we recommend, and there are three specific reasons why.
Sugar feeds the wrong bacteria
Your gut contains trillions of bacteria, and the balance between beneficial and harmful species matters enormously for anyone with colitis. When you consume excess sugar, you're providing the preferred fuel source for several types of harmful bacteria that thrive on simple carbohydrates. These bacteria ferment the sugar rapidly, producing large volumes of gas and toxic byproducts in the process.
This is why excessive, foul-smelling flatulence is often one of the first signs that something is off in your gut. The fermentation creates bloating, abdominal pain, and further irritation of an already inflamed gut lining. Over time, the harmful bacteria that feed on sugar begin to outnumber the beneficial ones, creating a state called dysbiosis — an imbalance in your gut bacteria that makes inflammation harder to control and flare-ups more likely.
It's a self-reinforcing cycle. The more sugar you eat, the more these bacteria proliferate. The more they proliferate, the more they crowd out the protective species that would normally help keep inflammation in check. And because an inflamed gut absorbs nutrients poorly, you end up craving more sugar for quick energy, which feeds the cycle all over again.
Glycation: how sugar directly damages your proteins
Beyond feeding harmful bacteria, sugar causes a chemical reaction in your body called glycation. When there's too much sugar in your bloodstream, the glucose and fructose molecules bind to proteins and fats without any enzyme controlling the process. This produces compounds called Advanced Glycation End Products, or AGEs.
AGEs are essentially damaged, misshapen proteins that your immune system doesn't recognise as normal tissue. Your body treats them as foreign invaders and mounts an inflammatory response to deal with them. For someone with colitis, where the immune system is already overactive in the gut, this is like throwing fuel on a fire that's already burning.
Both sucrose (table sugar) and fructose drive this process, and fructose is actually worse at producing AGEs than glucose. That matters because fructose is the primary sweetener in many fizzy drinks, fruit juices, and processed sweets. A single can of a popular soft drink can contain 30 to 40 grams of fructose, and your body has to deal with all of it.
Insulin resistance drives chronic inflammation
When you consistently consume more sugar than your body can process, your cells gradually become less responsive to insulin. This is insulin resistance, and it doesn't just affect your blood sugar — it creates a state of low-grade, systemic inflammation throughout your body. Your liver produces more inflammatory signalling molecules, your fat tissue becomes metabolically active in ways that promote inflammation, and your immune system stays in a heightened state.
For someone without gut issues, this chronic low-level inflammation might show up as fatigue, joint stiffness, or slow wound healing. But for someone with ulcerative colitis or Crohn's disease, it compounds the inflammation that's already present in the gut. Your baseline level of inflammation rises, which means it takes less to trigger a flare-up and more to bring one under control.
The hidden sugar problem: ultra-processed food
Most people think of sugar as the white stuff they spoon into their tea, and they assume they can manage their intake by simply not adding sugar to things. But the reality is that the majority of sugar in a modern diet comes from sources you wouldn't expect. Around 70% of products in an average supermarket qualify as ultra-processed food, and many of these contain added sugars under names most people wouldn't recognise: maltodextrin, dextrose, high-fructose corn syrup, rice syrup, agave nectar, and dozens of others.
Ultra-processed food doesn't just deliver hidden sugar, though. Research consistently shows that it disrupts the gut microbiome independently of its sugar content. A 2026 study in children demonstrated measurable changes in gut bacteria composition within weeks of a diet high in ultra-processed products. The emulsifiers, preservatives, and artificial additives in these foods damage the mucous layer that protects your gut lining, which is exactly the tissue that's already compromised in colitis.
This is why simply switching from sugary food to "sugar-free" processed alternatives rarely solves the problem. You may reduce the sugar, but you're still consuming the other ingredients that harm your gut.
Artificial sweeteners are not the answer
One of the most common substitutions people make is switching from sugar to artificial sweeteners, assuming that zero calories means zero impact on the gut. The evidence suggests otherwise. A 2022 study published in Cell showed that common artificial sweeteners — including saccharin, sucralose, and aspartame — significantly alter the composition of gut bacteria in humans, not just in laboratory mice. More recently, a 2026 intergenerational study found that these effects can persist across generations, meaning the impact on your microbiome may be more lasting than anyone initially thought.
In our practice, we see this regularly. Clients who have replaced sugar with sweeteners in their tea, their yoghurt, and their soft drinks, but whose gut symptoms haven't improved at all. The taste may be similar, but the effect on your gut bacteria is different rather than absent.
Meal timing matters too
There's another aspect of sugar consumption that often gets overlooked: when and how often you eat it. Your gut has a built-in cleaning mechanism called the Migrating Motor Complex, or MMC. It's a wave of muscular contractions that sweeps through your digestive system between meals, clearing out leftover food particles, dead bacteria, and debris. Think of it as a cleaning crew that only works when the restaurant is closed.
The MMC needs roughly four hours between meals to complete its cycle. Every time you eat something — even a small snack, a biscuit, a sweetened coffee — the cycle resets. In a culture of constant snacking, many people's MMC never gets a chance to finish its job. The uncleared debris feeds bacterial overgrowth, particularly in the small intestine, which can lead to a condition called SIBO (small intestinal bacterial overgrowth). SIBO causes bloating, gas, and abdominal pain that can look and feel very similar to a colitis flare.
Sugary snacks between meals are the worst combination: they disrupt the cleaning wave and simultaneously provide fuel for the wrong bacteria.
What to do instead
Reducing sugar isn't about willpower or perfection. It's about understanding where the sugar is coming from and making practical changes that your gut will actually respond to.
Start by reading ingredient labels on the processed foods you buy regularly. You'll likely find added sugar in bread, pasta sauces, breakfast cereals, yoghurts, and salad dressings — products most people don't think of as sweet. Replace these with whole-food alternatives where possible: cook your own sauces, choose plain yoghurt and add fresh fruit yourself, and switch to bread with short, recognisable ingredient lists.
Space your meals properly. Aim for three meals a day with roughly four to five hours between them, and avoid snacking in between. Your gut needs that quiet time to clean itself. If you need something between meals, a glass of water or herbal tea won't reset the MMC the way food will.
When you want something sweet, whole fruit is a better choice than fruit juice or dried fruit, because the fibre in whole fruit slows down the absorption of fructose and feeds beneficial gut bacteria. And be cautious with artificial sweeteners — they're not the neutral alternative they were once thought to be.
These aren't dramatic changes, but they address the specific mechanisms that make sugar problematic for an inflamed gut. In our practice, we guide people through these adjustments step by step, taking into account what's realistic for their situation, because sustainable changes always work better than strict rules that last a week.
References
- Suez, J. et al. (2022) — Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell, 185(18), 3307–3328.
- Monteiro, C.A. et al. (2019) — Ultra-processed foods: what they are and how to identify them. Public Health Nutrition, 22(5), 936–941.
- Deloose, E. et al. (2012) — The migrating motor complex: control mechanisms and its role in health and disease. Nature Reviews Gastroenterology & Hepatology, 9, 271–285.
- Vlassara, H. & Striker, G.E. (2011) — AGE restriction in diabetes mellitus: a paradigm shift. Nature Reviews Endocrinology, 7, 526–539.
- Rauber, F. et al. (2020) — Ultra-processed food consumption and indicators of obesity in the United Kingdom population. PLOS ONE, 15(5).