There are monosaccharides, meaning single molecules, and then there are disaccharides, meaning combinations of two molecules together. Monosaccharides are the basic units of biologically important carbohydrates, the simplest form of sugar, and the three most important are: glucose, galactose, and fructose.
Glucose is the energy of life. Your body will make glucose if you don’t take it in dietarily because it’s so important. Every cell in your body—in fact every cell on the planet—can metabolize glucose for energy. Glucose is, for lack of a better word, necessary.
Then you have galactose. Galactose is the monosaccharide (the single molecule) found in milk sugar. Your liver will turn galactose to glucose in about a nanosecond. So galactose is essentially glucose for the overwhelming majority of the population.
And then there’s this other third molecule called fructose. And fructose is the sweet part of table sugar. It’s the molecule we seek. We love fructose. We think that any food that has sugar (fructose) in it is a safe food to eat, and that is actually built into our DNA. It is a Darwinian precept because there is no foodstuff on the planet that is both sweet and acutely poisonous.