Fructose, not just sugar and salt, is the real culprit in ultra-processed foods, scientists have warned. A comprehensive review by researchers at the University of Colorado Anschutz, published in Nature Metabolism, has identified the sweetener as a key metabolic trigger that pushes the body to produce and store fat in ways fundamentally different from glucose. Lead author Richard Johnson said fructose “acts as a metabolic signal that promotes fat production and storage in ways that differ fundamentally from glucose,” adding that it is “not just another calorie.”
How fructose rewires the body’s metabolism
Unlike glucose, which enters a tightly regulated energy pathway, fructose bypasses critical regulatory steps. The body metabolises it almost entirely in the liver, where it drives fat production, oxidative stress and inflammation, according to the review. “Fructose is metabolised almost entirely in the liver, where it promotes fat production, oxidative stress and inflammation,” said Dr Edmond Hakimi, medical director at Wellbridge in Calverton.
The process depletes cellular energy stores, known as ATP, and leads to the accumulation of compounds linked to metabolic dysfunction. This unique mechanism means that even when total calorie intake is controlled, fructose can still stimulate fat storage. The researchers noted that under modern conditions of overnutrition, “chronic excess fructose drives features of metabolic syndrome” — a cluster of conditions that includes obesity, insulin resistance and heightened cardiovascular risk.
Importantly, the body can produce fructose internally from glucose. High-sugar and high-salt diets, as well as alcohol consumption, can stimulate this internal production, meaning that fructose’s role in disease may extend beyond what is directly consumed. Dr Johnson’s team highlighted that this endogenous fructose production could make the sweetener a more pervasive problem than previously thought.
The health toll: from metabolic syndrome to dementia
The review warned that prolonged overconsumption of fructose drives metabolic syndrome, but the dangers do not stop there. Emerging scientific evidence links high fructose intake to an increased risk of both cancer and dementia. Studies using data from the UK Biobank have examined associations between fructose consumption and conditions such as hyperandrogenism in women, while separate research has suggested a link with all-cause dementia and Alzheimer’s disease. One hypothesis proposes that fructose may trigger an ancient “survival switch” in the brain that promotes overeating and fat storage, potentially contributing to the development of Alzheimer’s.
In the UK, fructose is a common ingredient in processed foods and sugar-sweetened beverages. Research shows that ultra-processed foods contribute 56.8% of total energy intake and 64.7% of total free sugars in the UK diet. While high-fructose corn syrup is widely used in the US, its use in the UK and Europe is more restricted; beet sugar is the more common sweetener. However, table sugar (sucrose) itself is composed of glucose and fructose, meaning the sweetener remains pervasive.
The UK government advises that free sugars should not exceed 5% of total energy intake — no more than 30g per day for adults. Yet current average intakes exceed this recommendation by 50% to 150% across all age groups, according to the UK National Diet and Nutrition Survey. To tackle this, the government introduced the Soft Drinks Industry Levy in 2018 and has pursued advertising restrictions for high-fat, salt and sugar foods, alongside a voluntary sugar reduction programme for manufacturers.
Critics point out that some research suggests the evidence linking fructose directly to the global obesity epidemic is insufficient, and that attributing obesity to a single factor oversimplifies a complex issue. The review is a synthesis of existing evidence, not new experimental findings, which limits the ability to draw definitive conclusions about cause and effect.
What you can do: alternatives and the bigger picture
For those looking to reduce their fructose intake, several alternatives exist. Natural sweeteners such as stevia and monk fruit contain no fructose whatsoever, making them suitable for people managing diabetes or obesity. Maple syrup contains less fructose than honey, while rice malt syrup and glucose syrup are entirely fructose-free.
Whole fruits are a sensible choice because their fibre promotes fullness and moderates blood sugar responses — a critical distinction between fructose naturally present in whole foods and added fructose in processed items. The primary concern lies with “free sugars,” which include added sugars and those naturally present in juices and smoothies.
Artificial sweeteners like aspartame and sucralose are low in calories, but some studies suggest they may stimulate appetite and potentially contribute to weight gain. The World Health Organisation has linked sweeteners to an increased risk of Type 2 diabetes and heart disease, although more research is needed.
The review underscores that fructose’s metabolic disruption is not limited to dietary intake alone. Because the body can manufacture fructose internally from glucose — driven by high-sugar, high-salt and alcohol consumption — its influence on fat production and storage may be even more widespread than what enters the mouth.