Natural sugar found in healthy fruits may actually fuel metabolic diseases like diabetes and liver disease, a study suggests.
Fructose is a simple sugar naturally found in fruits, vegetables and honey. It also makes up about half of table sugar and can be converted into high fructose corn syrup in ultra-processed foods like soda and candy.
Fructose has long been seen as a 'healthier' alternative to added sugar because it has a lower glycemic index, meaning blood sugar does not spike as much as it does with glucose, which helps maintain energy levels without crashes.
But researchers in Colorado have found that fructose may increase fat synthesis, depleting the body of energy and fueling metabolic diseases.
Using data from cell, animal and human studies, the researchers found that unlike glucose, which gets converted into fuel that the body uses as energy, fructose bypasses key steps in the body's energy-processing pathways.
Instead, the team believes fructose tells the body to store fat and conserve energy, which was the result of evolution to ward off famines.
But in modern times, that extra fat storage raises cholesterol, triggers inflammation and increases weight.
The researchers are now arguing that these mechanisms contribute to increasing rates of metabolic diseases in the US such as diabetes, liver disease and obesity.
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'Fructose is not just another calorie,' Dr Richard Johnson, lead study author and professor at the University of Colorado Anschutz, said. 'It acts as a metabolic signal that promotes fat production and storage in ways that differ fundamentally from glucose.'
The body breaks down glucose to generate adenosine triphosphate (ATP), the primary energy source for all living cells that is used for metabolism, muscle contraction and nervous system regulation.
Fructose, however, is absorbed by the small intestine and sent to the liver, where it's rapidly processed into fructose-1-phosphate (F1P). This process consumes ATP quickly rather than helping to produce it, leading to fructose being converted into fat.
The researchers also wrote in the journal Nature Metabolism that the metabolism of fructose increases the enzyme malonyl-CoA, which causes the body to produce more fat and burn less of it.
The team wrote that even if calories are controlled, fructose can increase fat around the liver, promote insulin resistance and increase fats in the blood called triglycerides.
This may lead to metabolic syndrome, a cluster of conditions such as obesity, high blood pressure and high cholesterol, which raise the risk of heart disease, diabetes and stroke.
The researchers in the new study noted that fructose may contribute to metabolic disease, a cluster of conditions including obesity (stock image)
The CDC estimates 40 percent of Americans have metabolic syndrome.
'This review highlights fructose as a central player in metabolic health,' Johnson said. 'Understanding its unique biological effects is critical for developing more effective strategies to prevent and treat metabolic disease.'
The researchers particularly sounded the alarm about fructose in beverages such as soda, as these lack the nutritional value of fruits with fructose, which also have fiber and a variety of vitamins. Sugary drinks are also easier to overdo than whole fruits.
The review has several limitations, including relying primarily on animal studies, meaning the findings may not translate to humans.