Fructose
A simple sugar (monosaccharide) naturally found in fruit and honey, but also added in large amounts as glucose-fructose syrup in industrial products.
Fructose has a very low glycaemic index (about 15 to 20) because it does not directly stimulate insulin. You might think it is « healthy »… it is more complicated.
An original angle (liver metabolism): unlike glucose (used by all cells), fructose is metabolised almost exclusively by the liver. In moderate amounts (whole fruit) there is no problem. But in excess (fizzy drinks, industrial syrups), the liver converts it into fat (de novo lipogenesis), contributing to non-alcoholic fatty liver disease, insulin resistance and high triglycerides.
The crucial difference: the fructose in a whole fruit comes with fibre, water and vitamins and is released slowly. The fructose in a fizzy drink arrives as a concentrated bolus without fibre. Same molecule, totally different effect depending on the matrix.
Another nuance: fructose does not trigger leptin (the satiety hormone) as glucose does. The result: liquid calories sweetened with fructose do not fill you up and encourage overconsumption.
Fructose is also a common cause of intolerance (fructose malabsorption), often mistaken for IBS, causing bloating and diarrhoea.