How Refined Carbs and Added Sugar Affect Your Gut

What is the problem with sugar and refined carbs?

Refining can remove fibre and change how quickly a carbohydrate food is digested, although the response depends on the whole food, portion and meal. Repeated dietary patterns matter more than one isolated ingredient or meal.

High-fructose corn syrup (HFCS) is especially problematic. Unlike glucose, which every cell in your body can metabolise, fructose goes almost exclusively to the liver. There it is converted directly into fat through a process called de novo lipogenesis. This is not a slow, subtle effect. In controlled studies, fructose-sweetened beverages increased visceral fat and worsened insulin resistance within just ten weeks (Stanhope et al., J Clin Invest 2009, doi:10.1172/JCI37385).

UK adults consume roughly 50g of free sugars daily -- above the NHS recommendation of 30g (NDNS 2019-2023). Free sugars can appear in products that do not taste especially sweet, so the nutrition panel and portion are more informative than taste alone. These concerns apply to added and free sugars, not the fructose naturally present in whole fruit.

An important distinction: these concerns apply to added and free sugars, not the fructose naturally present in whole fruit. Whole fruit delivers fructose within a fibre matrix that slows absorption and feeds beneficial gut bacteria. Research suggests fruit intake is actually associated with lower intestinal permeability.

How might a high-sugar dietary pattern affect gut health?

Researchers are studying three connected pathways. The evidence is not equally strong for each, and much of the direct barrier and microbiome work is experimental rather than proof of an effect in every person.

Gut barrier research. Reviews describe animal and mechanistic studies in which high-sugar diets altered tight-junction proteins or permeability. These findings support a hypothesis; they do not show that sugar in one food directly caused an individual's symptoms (Arnone et al., Clin Gastroenterol Hepatol 2022, doi:10.1016/j.cgh.2021.12.011).

Microbiota dysbiosis. Your beneficial gut bacteria thrive on dietary fibre, fermenting it into short-chain fatty acids (SCFAs) like butyrate that nourish the gut lining and regulate immune function. Refined carbohydrates deliver energy without fibre, effectively starving these protective species. As fibre-fermenting bacteria decline, opportunistic and inflammatory species expand to fill the gap. The resulting imbalance -- dysbiosis -- further weakens the gut barrier and reduces the anti-inflammatory compounds your intestine depends on for repair.

Metabolic inflammation. Fructose-driven fat accumulation does not stay confined to the liver. Visceral fat -- the deep fat surrounding your organs -- is metabolically active tissue that secretes inflammatory cytokines. This creates a feedback loop: visceral fat drives inflammation, inflammation increases gut permeability, and a leaky gut allows more bacterial endotoxins into circulation, which in turn promotes further fat storage. The cycle is difficult to break once established.

Common in

• White bread
• Breakfast cereals
• Soft drinks
• Biscuits
• Sweets

Read the related label guides