What Is Sugar, Exactly?

Sugar is a carbohydrate, one of the three major dietary nutrients called macronutrients (the other two being fat and protein). Sugar is easy to vilify, but it’s a macronutrient for a reason — and our biological requirement for it demands nuanced understanding and respect.

Sugar is either a monosaccharide or a disaccharide. A monosaccharide, such as glucose, is the smallest unit of sugar. A disaccharide is a combination of two monosaccharides. Sucrose, commonly known as table sugar, is a disaccharide made up of glucose and fructose. It’s typically refined from sugarcane or sugar beets.

“Glucose is so essential that if you don’t eat it, your body turns other things into glucose just so you have it.”

Glucose fuels each one of the body’s approximately 30 trillion cells. “Glucose is so essential that if you don’t eat it, your body turns other things into glucose just so you have it,” says pediatric endocrinologist Robert Lustig, MD, MSL, author of Metabolical: The Lure and the Lies of Processed Food, Nutrition, and Modern Medicine.

Still, when it comes to table sugar, glucose is only half of the picture. The other half is fructose.

Until the 1960s, most of the fructose in the American diet came from fruit, which is why fructose is also called “fruit sugar.” Yet as the price of refining sugar rose, food makers turned to corn for a cheaper, sweeter alternative, and high-fructose corn syrup entered the marketplace for use mostly in sodas and ultraprocessed foods.

Unlike glucose, fructose plays no essential role in the body, Lustig says. And in high quantities, it can be addictive, contribute to cellular aging, and damage the liver. (More on fructose and the liver in question No. 3.)