All-too-common fructose — found in table sugar and high fructose corn syrup, used widely in everyday foods like ketchup — has long been considered a major reason why Americans pack on the pounds.

Now, scientists say they’re closer to figuring out exactly what makes the suspect sweetener such a sticky business for those struggling to stay fit.

Researchers at the University of Colorado are floating a theory that fructose lowers and blocks the body’s adenosine triphosphate (ATP), a compound that naturally provides energy for cells.

One of the results of that reduction is a decreased metabolism, which is needed to burn off energy from food that has been consumed.

That shift — essentially initiated by fructose — can then lead to weight gain.

“Fructose is what triggers our metabolism to go into low power mode,” explained Dr. Richard Johnson, a researcher at the Anschutz Medical Campus in Aurora, Colo., explained in a news release.

“Fructose is unique in resetting ATP levels to a lower level in the cell,” according to the study, published in the journal “Obesity,” which noted that the sugar is also known for “suppressing” mitochondria, which generate energy to power cells.

In turn, the body deals with subsequent hunger by triggering cravings for less-than-healthy energy sources, including carbs and fatty foods that would ultimately be stored as fat that the body would not burn up.

“The low intracellular ATP levels result in carbohydrate-dependent hunger” — or carb-craving — ” … and metabolic effects that result in the increased intake of energy-dense fats,” according to the research.

Thus begins a cycle of consuming more food that gets stored as fat, and our bodies then “lose our control of appetite, but fatty foods become the major source of calories that drive weight gain,” according to Johnson.

“Obesity is a disorder of energy metabolism, in which there is low usable energy (ATP) in the setting of elevated total energy,” according to the study, concluding that “excess energy drives weight gain.”

Previous research from the University of Colorado School of Medicine also identified fructose as a sugar tied to unhealthy weight issues, noting that key characteristics of the negative process are “hunger, thirst, foraging, weight gain, fat accumulation, insulin resistance, systemic inflammation and increased blood pressure.”

According to Johnson, who worked on both projects, the research offers a “full argument for how a particular carbohydrate, fructose, might have a central role in driving obesity and diabetes.”

He also found that sugar causes sluggish, hibernation-like behavior in humans — something well-documented in bears and other animals.

“We can trace it back to our ancestors, as well as learn from hibernating animals, exactly how fructose causes this ‘switch’ within us.”