Sugar contains both glucose and fructose. What does each one do to your body?
The Short Answer
Table sugar (sucrose) is half glucose and half fructose, but your body handles them completely differently. Glucose goes everywhere: every cell can use it, insulin regulates it, and excess gets stored as glycogen. Fructose goes primarily to the liver, bypasses insulin regulation, and readily converts to fat. When you eat sugar, you’re getting both hits at once.
Something to Sit With
Sugar isn’t just “empty calories.” It’s two different molecules with two different metabolic fates: one regulated, one not. The fructose half bypasses the systems your body uses to manage energy intake, heading straight to the liver where it becomes fat.
Understanding this changes how you see sugar. It’s not just about calories. It’s about what those calories do once they’re inside you.
Learn More
What glucose does. Glucose is the body’s universal fuel. Every cell in your body can use it directly for energy. When you eat glucose:
Insulin responds. Your pancreas releases insulin, which signals cells throughout your body to open up and take in glucose. This is a well-regulated system: insulin rises, glucose gets distributed, blood sugar comes back down.
It goes everywhere. Muscles take it up for immediate energy or store it as glycogen. The brain uses it constantly (about 120 grams per day). The liver stores some as glycogen for later release.
Excess gets stored, then burned. When glycogen stores are full, excess glucose can be converted to fat, but this process (de novo lipogenesis) is relatively inefficient with glucose. The body prefers to burn it or store it as glycogen first.
Glucose is what carbohydrate metabolism is designed for. The system works.
What fructose does. Fructose follows a completely different path. It’s metabolized primarily by the liver, and the process looks more like alcohol metabolism than glucose metabolism.[1]
Insulin doesn’t regulate it. Fructose doesn’t trigger much insulin release, and its metabolism doesn’t require insulin.[1] This sounds like it might be good, but it means there’s no feedback system telling the liver to slow down.
It goes mostly to the liver. Unlike glucose, which largely escapes first-pass liver processing and circulates to the rest of the body, fructose is extracted at 50-70% on that first pass, so the liver deals with most of it, though not all.[1]
It readily becomes fat. The liver converts fructose to fat through de novo lipogenesis at a much higher rate than glucose. Studies show that fructose rapidly increases liver fat when substituted for other carbohydrates at equal calories.[2] This isn’t a small effect.
It promotes fatty liver. The fat created from fructose doesn’t all leave the liver. Some accumulates there, contributing to non-alcoholic fatty liver disease (NAFLD). Fatty liver, in turn, drives insulin resistance, creating a vicious cycle.[1]
The double hit. When you eat table sugar, you get both sugars at once. The glucose spikes your blood sugar and triggers insulin. The fructose bypasses that system entirely and loads the liver with substrate for fat production.
This is worse than either sugar alone, though the study most often cited for it needs both of its conditions stated. In mice, fructose uniquely increased fat synthesis genes and reduced liver insulin signaling while glucose at the same calorie level improved it, and only on a high-fat diet. On ordinary chow there was no major difference between the two sugars.[3] It is a real finding about a mechanism; it is not a measurement of what sucrose does in a person.[2]
The combination (found in sugar, high-fructose corn syrup, honey, agave, and fruit juice) delivers a metabolic one-two punch. Your blood sugar spikes (glucose), your liver gets overwhelmed (fructose), and over time, insulin resistance develops from both directions.
Why whole fruit is different. Fruit contains fructose, but in a different context. Fiber may slow absorption (the review’s own word is potential), the water adds volume, and the total fructose dose is typically much lower than from sweetened beverages or processed foods. An apple carries roughly a third of the sugar of a can of soda, and brings fiber and water with it, though check your own labels rather than trust remembered figures, since fruit varies and formulations change. The human evidence of harm comes from fructose-sweetened beverages at high doses, and those trials did not test fruit.[4]
Further reading:
- Fung J, The Obesity Code[5] (2016) — Chapters on sugar and insulin resistance
- Lustig RH, Fat Chance[6] (2012) — Detailed explanation of fructose metabolism
- Softic S et al., “Divergent effects of glucose and fructose on hepatic lipogenesis and insulin signaling”[7] — Journal of Clinical Investigation, 2017
- Stanhope KL et al., “Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans”[8] — Journal of Clinical Investigation, 2009
- Basaranoglu M et al., “Fructose as a key player in the development of fatty liver disease”[9] — World Journal of Gastroenterology, 2013
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