Fructose and glucose are both simple sugars (monosaccharides) with the identical chemical formula C6H12O6, but they differ in molecular structure, sweetness, food sources, and how the body metabolizes them.
Structure: Glucose is an aldose, meaning its carbonyl group forms an aldehyde and it has a six-membered (pyranose) ring structure. Fructose is a ketose, meaning its carbonyl group forms a ketone, and it typically forms a five-membered (furanose) ring. This structural difference is what makes their chemical behavior and biological processing distinct despite sharing the same formula.
Sweetness: Fructose is considerably sweeter than glucose to the human palate — often cited as roughly twice as sweet — which is why it's a key component of high-fructose corn syrup and contributes heavily to the sweetness of fruit and honey.
Natural sources: Glucose is the most abundant simple sugar in nature and is the primary energy currency circulating in blood ("blood sugar"). It's found in starchy foods, fruits, and vegetables, often bound into larger molecules like starch. Fructose is found naturally in fruits, some vegetables, and honey, and is one component (along with glucose) of table sugar (sucrose).
Metabolism: This is the most biologically significant difference. Glucose can be used directly by almost every cell in the body for energy and is regulated by insulin. Fructose, by contrast, is metabolized almost exclusively by the liver. There, it can be converted into glucose, glycogen, or triglycerides (fat). Because fructose metabolism bypasses some of the regulatory steps that control glucose use, high intake of added fructose (especially from processed foods and sugary drinks) has been linked in research to increased liver fat, elevated triglycerides, and metabolic concerns — though naturally occurring fructose in whole fruit, eaten alongside fiber, is generally not considered a health concern in typical amounts.
In short: same formula, different shape, different taste intensity, and notably different metabolic pathways in the body.