Sugar is the name commonly applied to table sugar, or sucrose. Sucrose consists of two sugars, glucose and fructose, that are bonded together. Glucose is the primary sugar used in our body for fuel, while fructose is the primary sugar present in fruit. In the body some glucose can be converted to fructose, while fructose can also be converted to glucose.
Historically glucose and fructose were both viewed as carbohydrates with similar functions. However, our work has shown that the two sugars are very different. Glucose is the immediate fuel that increases the energy in the cell which animals like, and it tends to cause some pleasure, fullness, and is the preferred fuel for the brain.
In contrast, fructose has very different effects, likely because it tends to lower the energy in the cell. This leads to persistent hunger and thirst, the foraging for food, and the activation of processes to store fat even if fat stores are already present. It is as if fructose is the fuel one eats to prepare oneself for a time when food is not available. For example, it stimulates insulin resistance. This is a condition in which the uptake of glucose into the muscle is reduced. In essence the muscle is starved despite glucose levels being high in the blood, as the muscles are resistant to the effect of insulin that is key to taking the glucose up into the muscle. This might be viewed as a bad response, but it keeps the glucose levels high so that the brain can preferentially use it. This is important during crises in which there is not enough food available, as it allows the brain to get the critical fuel it needs.
While fructose is a great way to prepare for a calamity, it is not good for us, as our western diet is high not only in sugar, but also high fructose corn syrup (HFCS), another added sugar that contains fructose. Today the dramatic intake of sugar and HFCS is resulting in even more marked intake of fructose, and the consequence is an increase in a wide variety of diseases.